17
Heat: Energy in transit EFEU May 2009 Renilde Nihoul 1 H.U.Brussel Heat: Energy in Transit Renilde Nihoul B BRUSSEL05
Heat Energy in transit EFEU May 2009
Renilde Nihoul 1 HUBrussel
Heat Energy in Transit
Renilde Nihoul B BRUSSEL05
Heat Energy in transit EFEU May 2009
Renilde Nihoul 2 HUBrussel
1 Introduction Motivating pupils in science can be a challenge for teachers This is especially true as the pupils get older and science no longer has the novelty value that it had when they were younger Enhancing pupil‟s motivation is not a simple process One of the most important factors is creating a clear purpose for classroom activities This can be done in a variety of ways including explaining daily life problems A lot of misconceptions about physical phenomena exist including about heat Pupils (often) don‟t see the relation between the theoretical principles of physical phenomena and situations of daily live If we are able to give an answer at daily live questions by using the theoretical principles they learned in physics lessons they might be more interested in these lessons Examples of such questions could be
Will a snowman melt faster with a coat on or without a coat If you have a cup of coffee which is too hot to drink should you
add cream to it immediately to cool it or let it stay black and sit for a while before adding cream (The object is to get it cool enough to drink in the shortest possible time)
Heat flow is normally from a high temperature toward a low temperature region How do you manage to cool your body on a July day when the temperature is 39degC (compared to 37degC normal body temperature)
Will hot water freeze into ice cubes faster than cold water in your freezer
Is a metal teapot better than a porcelain one to keep the tea hot
Providing objective information alone is not enough The pupils should investigate these phenomenon‟s themselves to find out (and understand) the correct concepts behind the observations First they have to discuss the (little) problem and try to formulate workhypotheses Then they set up a little experiment to find out in a scientific way which workhypothesis is correct In this way we not only stimulate the interest of the pupils but also their scientific skills
Before explaining how to tackle this the main principles of heat transfer are given
2 Basics of Heat Transfer
Heat may be defined as energy in transit from a high temperature object to a lower temperature object If you place a hot object (fi a cup of coffee) or a cold object (a glass of ice water) in an environment at ordinary room temperature the object will tend toward thermal equilibrium with its environment That is the coffee gets colder and the ice water gets warmer The temperature of each approaches the temperature of the room
Heat Energy in transit EFEU May 2009
Renilde Nihoul 3 HUBrussel
Therefore we need some sort of exchange of energy between the system and its environment The following definition of heat can be used Heat (symbol Q) is energy that flows between a system and its environment by virtue of a temperature difference between them1 Figure 1 summarizes this view
(a) TS lt TE (b) TS = TE (c) TS gt TE
Figure 1 (a) If the temperature of a system is less than the temperature of its environment heat flows into the
system until thermal equilibrium is obtained as in (b) (c) If the temperature of a system is greater than that of
its environment heat flows out of the system
If the temperature TS of a system is less than the temperature of its environment TE heat flows into the system By convention is Q positive in this case It is a process by which the internal energy of the system is increased When TS gt TE heat flows out of the system in which case Q is taken to be negative Since heat is a form of energy its units are those of energy namely the joule (J) in the SI system Other units are still in use today The ldquocalorierdquo is in common use as a measure of nutrition 1 cal = 4186 J The ldquoBritish thermal unitrdquo (Btu) is still commonly found as a measure of the ability of an air conditioner to transfer energy (as heat) from a room to the outside environment A typical room air conditioner rated at 10 000 Btuh can therefore remove about 107 J from a room every hour and transfer it to the outside environment 1 Btu = 1055 J
3 Misconceptions about heat
Heat is similar to work in that both represent a means for the transfer of energy The following figure of the interchange ability of heat and work as agents for adding energy to a system can help to dispel some misconceptions about heat
1 Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Environment
Q gt O
Environment
Q = 0
Environment
Q lt 0
System System
System
Heat Energy in transit EFEU May 2009
Renilde Nihoul 4 HUBrussel
Figure 2 The interchange ability of heat and work as agents for adding energy to a system 2
If you are presented with a high temperature gas you can‟t tell whether it reached that high temperature by being heated or by having work done on it or a combination of the two To describe the energy that a high temperature object has it is not a correct use of the word heat to say that the object ldquopossesses heatrdquo It is better to say that it possesses internal energy as a result of its molecular motion Neither heat nor work is an intrinsic property of a system A system doesn‟t ldquocontainrdquo a certain amount of work Instead we say that it can transfer a certain amount of energy as heat or work under certain specified conditions The word heat is reserved to describe the process of transfer of energy from a high temperature object to a lower temperature one You can take an object at low internal energy and raise it to higher internal energy by heating it But you can also increase its internal energy by doing work on it and since the internal energy of a high temperature object resides in random motions of the molecules you can‟t tell which mechanism was used to give it that energy Some of the confusion about the precise meaning of heat results from the popular usage of the term Often heat is used when what is really meant is temperature or internal energy When we hear about heat in relation to weather or when cooking instructions indicate ldquoheat at 200 degreesrdquo it is temperature that is being discussed When someone talks about the ldquoheat generatedrdquo by the brake linings of an automobile or by briskly rubbing the palms of your ands together it is usually internal energy that is meant When you rub your hands together they do work on one another thereby increasing their internal energy and raising their temperature This excess energy can then be transferred to the environment as heat because the hands are at a higher temperature than the environment So don‟t refer to the ldquoheat in a bodyrdquo or say ldquothis object has twice as much heat as that bodyrdquo Avoid the use of the rather vague term ldquothermal energyrdquo and the use of the word
2 Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 5 HUBrussel
ldquoheatrdquo as a verb since this feeds the misconceptions Introduce and use the concept of internal energy as quickly as possible
4 The mechanical equivalent of heat
In a classic experiment in 1850 James Joule showed the energy equivalence of heating and doing work by using the change in potential energy of falling masses to stir an insulated container of water with paddles
Figure 3 Joules arrangement for measuring the mechanical equivalent of heat The falling weights turn paddles
that stir the water in the container thus raising the temperature3
The mechanical work done by the falling weights produces a measurable temperature rise of the water At that time calories were the accepted unit of heat It was originally defined as the quantity of heat necessary to raise the temperature of 1 g of water from 145 to 155 degC From the measured temperature increase of the water Joule was able to deduce the amount of calories of heat Q that would have produced the same temperature increase The work done on the water by the falling weights (in joules) therefore produced a temperature rise equivalent to the absorption by the water of a certain heat (in calories) and from this equivalence it is possible to determine the relationship between the calorie and the joule This experiment is noteworthy not only for the skill ingenuity and its precision but also for the direction it provided in showing that heat like work could properly be regarded as a means of transferring energy In the 18th century it was believed that a material fluid called caloric was exchanged between bodies at different temperatures
5 Internal energy and the first law of thermodynamics
3 httpenwikipediaorgwikiImageJoule27s_heat_apparatusJPG
Heat Energy in transit EFEU May 2009
Renilde Nihoul 6 HUBrussel
Internal energy is defined as the energy associated with the random disordered motion of molecules It is separated in scale from the macroscopic ordered energy associated with moving objects it refers to the invisible microscopic energy on the atomic and molecular scale For example a glass of water at room temperature standing on a table has no apparent energy either potential or kinetic But on the microscopic scale it is a seething mass of high speed molecules travelling at hundreds of meters per second
Figure 4 Internal energy of a glass of water at room temperature4
U is the most common symbol used for internal energy The change in internal energy between equilibrium states 1 and 2 is ΔU = U2 ndash U1
The value of U1 depends only on the coordinates of the state 1(only on temperature for an ideal gas) Similarly U2 depends only on the coordinates of point 2 Such a function is called a state function it depends only on the state of a system and not at all on how the system arrived at that state This brings us to the first law of thermodynamics which can be stated as follows In any thermodynamic process between equilibrium states 1 and 2 the quantity Q ndash W has the same value for any path between state 1 and 2 This quantity is equal to the change in the value of a state function called the internal energy Mathematically the first law is ΔU = Q ndash W The change in internal energy of a system is equal to the heat added to the system minus the work done by the system In other textbooks you may find the first law written as ΔU = Q + W It is the same law namely the thermodynamic expression of the conservation of energy principle In this case W is defined as the work done on the system instead of work done by the system For many processes (eg an internal combustion engine) the common
4 httphyperphysicsphy-astrgsueduhbasethermointenghtmlc2
Heat Energy in transit EFEU May 2009
Renilde Nihoul 7 HUBrussel
scenario is one of adding heat to a volume of gas and using the expansion of that gas to do work Therefore we prefer to use the first notation
6 The transfer of heat
The transfer of heat between a system and its environment can take place by three mechanisms conduction convection and radiation
61 Conduction
If you leave a metal poker in a fire for any length of time its handle will become hot Energy is transferred from the fire to the handle by conduction along the length of the metal shaft The atoms at the hot end by virtue of the high temperature at that end are vibrating with large amplitude These large vibrational amplitudes are passed along the shaft from atom to atom by interactions between adjacent atoms In this way a region of rising temperature travels along the shaft to your hand The energy will be transferred because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones The rate of conduction heat transfer between two plane surfaces can be calculated Consider a thin slab of homogeneous material of thickness d and cross-sectional area A The temperature is T + ΔT (Thot) on one face and T (Tcold) on the other The rate of heat flow through the slab is directly proportional to A (the more area available the more heat can flow per unit time) inversely proportional to d (the thicker the slab the less heat can flow per unit time) directly proportional to ΔT (the larger the temperature difference the more heat can flow per unit time) [To minimize the loss of heat from your house in winter make the surface area smaller (a two-story house is more efficient than a one-story house of the same total floor area) use thick walls filled with insulation and perhaps most important move to a warmer climate] Mathematically we can summarize this as
dTA
tQ=
With Q = heat transferred in time = Δt κ = thermal conductivity of the barrier A = area ΔT = temperature difference d = thickness of the barrier A substance with a large value of κ is a good heat conductor one with a small value of κ is a poor conductor or a good insulator In the case of solids the properties of materials that make them good electrical conductors (namely the ability of electrons to move
Heat Energy in transit EFEU May 2009
Renilde Nihoul 8 HUBrussel
relatively easily throughout the bulk of the material) also make them good thermal conductors Table 1 shows some representative values of κ
Material Conductivity κ (Wm middot K)
Metals
Stainless steel 14
Lead 35
Aluminium 235
Copper 401
Silver 428
Gases
Air 0026
Helium 015
Hydrogen 018
Building materials
Polyurethane foam 0024
Rock wool 0043
Fiberglas 0048
White pine 011
Window glass 10
Table 1 Some thermal conductivities5
Over the range of temperatures we normally encounter we can regard κ as a constant but over wide temperature ranges it does show a slight variation with T Gases transfer heat by direct collisions between molecules and as would be expected their thermal conductivity is low compared to most solids since they are dilute media
62 Convection
If you look at the flame of a candle or a match you are watching heat energy being transported upward by convection Heat transfer by convection occurs when a fluid such as air of water is in contact with an object whose temperature is higher than that of its surroundings The temperature of the fluid that is in contact with the hot object increases and (in most cases) the fluid expands Being less dense than the surrounding cooler fluid it rises because of buoyant forces
5 Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Heat Energy in transit EFEU May 2009
Renilde Nihoul 9 HUBrussel
Figure 5 Heat convection6
Convection can also lead to circulation in a liquid as in heating a pot of water over a flame Heated water expands and becomes more buoyant Cooler denser water near the surface descends and patterns of circulation can be formed
Figure 6 Convection currents7
Atmospheric convection plays a fundamental role in determining the global climate patterns and in our daily weather variations Glider pilots and condors alike seek the convective thermals that rising from the warmer Earth beneath keep them aloft Huge energy transfers take place within the oceans by the same process The outer region of the sun called the photosphere contains a vast array of convection cells that transport energy from the interior to the solar surface and give the surface a granulated appearance with a typical dimension of a granule being 1000 kilometres
6 httphyperphysicsphy-astrgsueduhbasethermoheatrahtmlc2 7 httpwwwphysicsarizonaedu
Heat Energy in transit EFEU May 2009
Renilde Nihoul 10 HUBrussel
Figure 7 The photosphere of the sun8
Heat-induced fluid motion in initially static fluids is known as free convection For cases where the fluid is already in motion heat conducted into the fluid will be transported away chiefly by fluid convection These cases known as forced convection require a pressure gradient to drive the fluid motion as opposed to a gravity gradient to induce motion through buoyancy It is difficult to quantify the effects of convection since it inherently depends upon small no uniformities in an otherwise fairly homogeneous medium
63 Radiation
Energy is carried from the sun to us by electromagnetic waves that travel freely through the near vacuum of the intervening space If you stand near a bonfire or an open fireplace you are warmed by the same process
8 httpsolarsciencemsfcnasagovimagesgranulesjpg
Heat Energy in transit EFEU May 2009
Renilde Nihoul 11 HUBrussel
Figure 8 The sun at 5800K and a campfire at 800K9
The sun at 5800 K and a hot campfire at perhaps 800 K give off radiation at a rate proportional to the 4th power of the temperature All objects emit such electromagnetic radiation because of their temperature and also absorb some radiation that falls on them from the objects The higher the temperature of an object the more it radiates The energy is carried by photons of light in the infrared and visible portions of the electromagnetic spectrum When temperatures are uniform the radiative flux between objects is in equilibrium and no net thermal energy is exchanged The balance is upset when temperatures are not uniform and thermal energy is transported from surfaces of higher to surfaces of lower temperatures
7 Heat Transfer Examples
71 With focus on radiation the Greenhouse effect
The greenhouse effect refers to circumstances where the short wavelengths of visible light from the sun pass through a transparent medium and are absorbed but the longer wavelengths of the infrared re-radiation from the heated objects are unable to pass through that medium The trapping of the long wavelength radiation leads to more heating and a higher resultant temperature Besides the heating of an automobile by sunlight through the windshield and the namesake example of heating the greenhouse by sunlight passing through sealed transparent windows the greenhouse effect has been widely used to describe the trapping of excess heat by the rising concentration of carbon dioxide
9 httphyperphysicsphy-astrgsueduhbasethermostefanhtmlc2
Heat Energy in transit EFEU May 2009
Renilde Nihoul 12 HUBrussel
in the atmosphere The carbon dioxide strongly absorbs infrared and does not allow as much of it to escape into space
Figure 9 The greenhouse effect10
A major part of the efficiency of the heating of an actual greenhouse is the trapping of the air so that the energy is not lost by convection Keeping the hot air from escaping out the top is part of the practical greenhouse effect but it is common usage to refer to the infrared trapping as the greenhouse effect in atmospheric applications where the air trapping is not applicable
The action of carbon dioxide and other greenhouse gases in trapping outgoing infrared energy from the Earth thereby warming the planet is called the greenhouse effect Those gas molecules in the Earths atmosphere with three or more atoms are called greenhouse gases The greenhouse gases include water vapour (H2O) ozone (O3) carbon dioxide (CO2) and methane (CH4) Also trace quantities of chlorofluorocarbons (CFCs) can have a disproportionately large effect Current analysis suggests that the combustion of fossil fuels is a major contributor to the increase in the carbon dioxide concentration It may measurably increase the overall average temperature of the Earth which could have disastrous consequences Because the potential consequences of global warming in terms of loss of snow cover sea level rise change in weather patterns etc are so great it is a major societal concern On the other hand proposed measures to reduce human contributions to greenhouse gases can also have great consequences For more information httphyperphysicsphy-astrgsueduhbasehframehtml
10 httphyperphysicsphy-astrgsueduhbasehframehtml
Heat Energy in transit EFEU May 2009
Renilde Nihoul 13 HUBrussel
72 Involving all three basic heat transfer mechanisms cooling of the human body
Figure 10 Cooling of the human body11
This is a simplified model of the process by which the human body gives off heat Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism One implication of the model is that radiation is the most important heat transfer mechanism at ordinary room temperatures This model indicates that an unclothed person at rest in a room temperature of 23 Celsius would be uncomfortably cool The skin temperature of 34 degC is a typical skin temperature taken from physiology texts compared to the normal core body temperature of 37 degC What will happen if the ambient temperature is above the skin temperature Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism This becomes a problem when the ambient temperature is above body temperature because all three standard heat transfer mechanisms work against this heat loss by transferring heat into the body
Figure 11 Cooling of the human body when the ambient temperature is above the skin temperature
11 httphyperphysicsphy-astrgsueduhbasethermocoobodhtmlc1
Heat Energy in transit EFEU May 2009
Renilde Nihoul 14 HUBrussel
Our ability to exist in such conditions comes from the efficiency of cooling by the evaporation of perspiration At a temperature of 45 Celsius the evaporation process must overcome the transfer of heat into the body and give off enough heat to accomplish a 90 watt net outward flow rate of energy Because of the bodys temperature regulation mechanisms the skin temperature would be expected to rise to 37degC at which point perspiration is initiated and increases until the evaporation cooling is sufficient to hold the skin at 37degC if possible With those assumptions about the temperatures one can calculate that there will be a net input power of 109 watts to the body (by radiation) The perspiration cooling must overcome that and produce the net outflow of 90 watts for equilibrium
8 Dealing with misconceptions by training the scientific method
As mentioned before some misconceptions about heat can stubbornly persist in pupils mind In order to explain how to deal with this misconceptions by training the scientific method we illustrate this concept with an example
81 Step 1 Formulation of a problem
A problem (of a daily life situation) can be introduced by the teacher or by a pupil For instance
I want to keep the tea as hot as possible as long as possible in a teapot Therefore I better choose a thick teapot A metal teapot hellip
82 Step 2 Formulation of workhypotheses
The pupils discuss the problem in small groups (3 ndash 4 pupils) and try to formulate workhypotheses The emphasis has to lay on the formulation of the hypotheses and not on the ldquocorrectrdquo formulation As a teacher you try to stimulate the discussion by asking questions as ldquoWhat exactly do you mean Why do you think that is How could you write this downhelliprdquo Don‟t give any information with respect to content Discuss in class all the preconceptions made by the pupils It is important they realise not everyone has the same preconceptions It is also important that they can practice to express their visions They have to become aware of their own visions
Summarize all the visions of the pupils
A metal teapot is better than a porcelain one to keep the tea hot
In a thicker teapot the tea will remain longer hot It doesn‟t matter which teapot you use (the tea will chill
anyway) hellip
Heat Energy in transit EFEU May 2009
Renilde Nihoul 15 HUBrussel
83 Step 3 Experimental verification
Normally the teacher thinks out the experiment However it is important that the pupils plan their own experiment so they see the experiment as a mean to answer questions and not only as a recipe they have to follow
At first they will use their sensory perceptions to plan the experiment Let them realise that these sensory perceptions are valuable but they give only vague and superficial information Then they ought to be able to translate the problem in measurable quantities All quantities that are important for the experiment need to be listed Some of these quantities need to be kept at a constant level during the experiment Other quantities need to be measured The pupils should estimate the magnitude of the results in advance so they can select the appropriate measuring instruments They can use the following scheme to think out their experiment
Workhypothesis What is the question
Description and outline of the experiment Which experiment can answer that question
Material list Which equipment is necessary
Procedure How exactly is it done
For the experiment with the teapot this could be resolved as follows
Tea can be replaced by boiling water In stead of a real teapot we can use cups of different materials
(with the same volume) To discover the effect of the thickness we use cups of the
same material but with a different thickness We measure the temperature with a thermometer the time with
a chronometer and the volume with a pipette
Procedure Fill the cups with equal volumes of boiling water (use the
different kind of cups) Measure the temperature of the water at different points of time
(every 15 seconds) Put the results in a convenient table and try to figure out which
cup is most suitable to keep the tea hot
Heat Energy in transit EFEU May 2009
Renilde Nihoul 16 HUBrussel
84 Step 4 Reflection
When the experiment is finished they have to select the work hypothesis confirmed by the experiment and give one‟s motives for this choice All the difficulties have to be put into words If their own vision is in conflict with the outcome of the experiment the pupils will be intellectually challenged
Selection of the hypothesis confirmed by the experiment a porcelain teapot will keep the tea longer hot than a metal teapot The thickness of the teapot is important The tea will chill in every teapot but the time it takes to do so will be different
Motivation of the selection
85 Step 5 Scientific explanation
The scientific explanation should start from the concrete problem from a conceptual approach and lead then to a mathematical formulation The scientific explanation should be told in a fascinating way so they will remember it longer
The better you prevent an energy exchange between the hot tea and the surrounding air the longer the tea remains hot
A metal pot is a very bad insulator (A complete explanation is not given here)
86 Conclusion
To deal with the existing misconceptions let the pupils make their own experiments By practicing a scientific method they will be able to discover the bdquotruth‟ The link between the physical principles and situations of daily life will become clearer They might be more interested in physics lessons as they understand they can use these theoretical principles to explain daily life problems Evaluate the concept afterwards (Evalution of your practice is a powerful tool for enhancing your teaching as well as their students learning) Did the strategie capture and retain the pupil‟s attention How effective was this in encouraging their scientific skills Hopefully the pupils will perceive themselves as participants in the journey of discovering and learning and as co-operating detectives looking for and evaluating evidence Let them share your feelings of curiosity enthusiasm and critical reflection
Heat Energy in transit EFEU May 2009
Renilde Nihoul 17 HUBrussel
9 References
Balck C Cocquyt B Van Peteghem R (2005) Misvattingen fysica te lijf Syllabus Universiteit Antwerpen ndash Centrum Nascholing Onderwijs
Giancoli DC (1999) Natuurkunde voor Wetenschap en Techniek deel II Golven en Geluid Kinetische Theorie en Thermodynamica Elektriciteit en Magnetisme Licht Academic Service Schoonhoven
Hathaway D (2008) Solar Physics NASAs Marshall Space Flight Center httpsolarsciencemsfcnasagov
NaveCR (2005) HyperPhysics Departement of Physics and Astronomy Georgia State University httphyperphysicsphy-astrgsueduhbasehframehtml
Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 2 HUBrussel
1 Introduction Motivating pupils in science can be a challenge for teachers This is especially true as the pupils get older and science no longer has the novelty value that it had when they were younger Enhancing pupil‟s motivation is not a simple process One of the most important factors is creating a clear purpose for classroom activities This can be done in a variety of ways including explaining daily life problems A lot of misconceptions about physical phenomena exist including about heat Pupils (often) don‟t see the relation between the theoretical principles of physical phenomena and situations of daily live If we are able to give an answer at daily live questions by using the theoretical principles they learned in physics lessons they might be more interested in these lessons Examples of such questions could be
Will a snowman melt faster with a coat on or without a coat If you have a cup of coffee which is too hot to drink should you
add cream to it immediately to cool it or let it stay black and sit for a while before adding cream (The object is to get it cool enough to drink in the shortest possible time)
Heat flow is normally from a high temperature toward a low temperature region How do you manage to cool your body on a July day when the temperature is 39degC (compared to 37degC normal body temperature)
Will hot water freeze into ice cubes faster than cold water in your freezer
Is a metal teapot better than a porcelain one to keep the tea hot
Providing objective information alone is not enough The pupils should investigate these phenomenon‟s themselves to find out (and understand) the correct concepts behind the observations First they have to discuss the (little) problem and try to formulate workhypotheses Then they set up a little experiment to find out in a scientific way which workhypothesis is correct In this way we not only stimulate the interest of the pupils but also their scientific skills
Before explaining how to tackle this the main principles of heat transfer are given
2 Basics of Heat Transfer
Heat may be defined as energy in transit from a high temperature object to a lower temperature object If you place a hot object (fi a cup of coffee) or a cold object (a glass of ice water) in an environment at ordinary room temperature the object will tend toward thermal equilibrium with its environment That is the coffee gets colder and the ice water gets warmer The temperature of each approaches the temperature of the room
Heat Energy in transit EFEU May 2009
Renilde Nihoul 3 HUBrussel
Therefore we need some sort of exchange of energy between the system and its environment The following definition of heat can be used Heat (symbol Q) is energy that flows between a system and its environment by virtue of a temperature difference between them1 Figure 1 summarizes this view
(a) TS lt TE (b) TS = TE (c) TS gt TE
Figure 1 (a) If the temperature of a system is less than the temperature of its environment heat flows into the
system until thermal equilibrium is obtained as in (b) (c) If the temperature of a system is greater than that of
its environment heat flows out of the system
If the temperature TS of a system is less than the temperature of its environment TE heat flows into the system By convention is Q positive in this case It is a process by which the internal energy of the system is increased When TS gt TE heat flows out of the system in which case Q is taken to be negative Since heat is a form of energy its units are those of energy namely the joule (J) in the SI system Other units are still in use today The ldquocalorierdquo is in common use as a measure of nutrition 1 cal = 4186 J The ldquoBritish thermal unitrdquo (Btu) is still commonly found as a measure of the ability of an air conditioner to transfer energy (as heat) from a room to the outside environment A typical room air conditioner rated at 10 000 Btuh can therefore remove about 107 J from a room every hour and transfer it to the outside environment 1 Btu = 1055 J
3 Misconceptions about heat
Heat is similar to work in that both represent a means for the transfer of energy The following figure of the interchange ability of heat and work as agents for adding energy to a system can help to dispel some misconceptions about heat
1 Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Environment
Q gt O
Environment
Q = 0
Environment
Q lt 0
System System
System
Heat Energy in transit EFEU May 2009
Renilde Nihoul 4 HUBrussel
Figure 2 The interchange ability of heat and work as agents for adding energy to a system 2
If you are presented with a high temperature gas you can‟t tell whether it reached that high temperature by being heated or by having work done on it or a combination of the two To describe the energy that a high temperature object has it is not a correct use of the word heat to say that the object ldquopossesses heatrdquo It is better to say that it possesses internal energy as a result of its molecular motion Neither heat nor work is an intrinsic property of a system A system doesn‟t ldquocontainrdquo a certain amount of work Instead we say that it can transfer a certain amount of energy as heat or work under certain specified conditions The word heat is reserved to describe the process of transfer of energy from a high temperature object to a lower temperature one You can take an object at low internal energy and raise it to higher internal energy by heating it But you can also increase its internal energy by doing work on it and since the internal energy of a high temperature object resides in random motions of the molecules you can‟t tell which mechanism was used to give it that energy Some of the confusion about the precise meaning of heat results from the popular usage of the term Often heat is used when what is really meant is temperature or internal energy When we hear about heat in relation to weather or when cooking instructions indicate ldquoheat at 200 degreesrdquo it is temperature that is being discussed When someone talks about the ldquoheat generatedrdquo by the brake linings of an automobile or by briskly rubbing the palms of your ands together it is usually internal energy that is meant When you rub your hands together they do work on one another thereby increasing their internal energy and raising their temperature This excess energy can then be transferred to the environment as heat because the hands are at a higher temperature than the environment So don‟t refer to the ldquoheat in a bodyrdquo or say ldquothis object has twice as much heat as that bodyrdquo Avoid the use of the rather vague term ldquothermal energyrdquo and the use of the word
2 Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 5 HUBrussel
ldquoheatrdquo as a verb since this feeds the misconceptions Introduce and use the concept of internal energy as quickly as possible
4 The mechanical equivalent of heat
In a classic experiment in 1850 James Joule showed the energy equivalence of heating and doing work by using the change in potential energy of falling masses to stir an insulated container of water with paddles
Figure 3 Joules arrangement for measuring the mechanical equivalent of heat The falling weights turn paddles
that stir the water in the container thus raising the temperature3
The mechanical work done by the falling weights produces a measurable temperature rise of the water At that time calories were the accepted unit of heat It was originally defined as the quantity of heat necessary to raise the temperature of 1 g of water from 145 to 155 degC From the measured temperature increase of the water Joule was able to deduce the amount of calories of heat Q that would have produced the same temperature increase The work done on the water by the falling weights (in joules) therefore produced a temperature rise equivalent to the absorption by the water of a certain heat (in calories) and from this equivalence it is possible to determine the relationship between the calorie and the joule This experiment is noteworthy not only for the skill ingenuity and its precision but also for the direction it provided in showing that heat like work could properly be regarded as a means of transferring energy In the 18th century it was believed that a material fluid called caloric was exchanged between bodies at different temperatures
5 Internal energy and the first law of thermodynamics
3 httpenwikipediaorgwikiImageJoule27s_heat_apparatusJPG
Heat Energy in transit EFEU May 2009
Renilde Nihoul 6 HUBrussel
Internal energy is defined as the energy associated with the random disordered motion of molecules It is separated in scale from the macroscopic ordered energy associated with moving objects it refers to the invisible microscopic energy on the atomic and molecular scale For example a glass of water at room temperature standing on a table has no apparent energy either potential or kinetic But on the microscopic scale it is a seething mass of high speed molecules travelling at hundreds of meters per second
Figure 4 Internal energy of a glass of water at room temperature4
U is the most common symbol used for internal energy The change in internal energy between equilibrium states 1 and 2 is ΔU = U2 ndash U1
The value of U1 depends only on the coordinates of the state 1(only on temperature for an ideal gas) Similarly U2 depends only on the coordinates of point 2 Such a function is called a state function it depends only on the state of a system and not at all on how the system arrived at that state This brings us to the first law of thermodynamics which can be stated as follows In any thermodynamic process between equilibrium states 1 and 2 the quantity Q ndash W has the same value for any path between state 1 and 2 This quantity is equal to the change in the value of a state function called the internal energy Mathematically the first law is ΔU = Q ndash W The change in internal energy of a system is equal to the heat added to the system minus the work done by the system In other textbooks you may find the first law written as ΔU = Q + W It is the same law namely the thermodynamic expression of the conservation of energy principle In this case W is defined as the work done on the system instead of work done by the system For many processes (eg an internal combustion engine) the common
4 httphyperphysicsphy-astrgsueduhbasethermointenghtmlc2
Heat Energy in transit EFEU May 2009
Renilde Nihoul 7 HUBrussel
scenario is one of adding heat to a volume of gas and using the expansion of that gas to do work Therefore we prefer to use the first notation
6 The transfer of heat
The transfer of heat between a system and its environment can take place by three mechanisms conduction convection and radiation
61 Conduction
If you leave a metal poker in a fire for any length of time its handle will become hot Energy is transferred from the fire to the handle by conduction along the length of the metal shaft The atoms at the hot end by virtue of the high temperature at that end are vibrating with large amplitude These large vibrational amplitudes are passed along the shaft from atom to atom by interactions between adjacent atoms In this way a region of rising temperature travels along the shaft to your hand The energy will be transferred because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones The rate of conduction heat transfer between two plane surfaces can be calculated Consider a thin slab of homogeneous material of thickness d and cross-sectional area A The temperature is T + ΔT (Thot) on one face and T (Tcold) on the other The rate of heat flow through the slab is directly proportional to A (the more area available the more heat can flow per unit time) inversely proportional to d (the thicker the slab the less heat can flow per unit time) directly proportional to ΔT (the larger the temperature difference the more heat can flow per unit time) [To minimize the loss of heat from your house in winter make the surface area smaller (a two-story house is more efficient than a one-story house of the same total floor area) use thick walls filled with insulation and perhaps most important move to a warmer climate] Mathematically we can summarize this as
dTA
tQ=
With Q = heat transferred in time = Δt κ = thermal conductivity of the barrier A = area ΔT = temperature difference d = thickness of the barrier A substance with a large value of κ is a good heat conductor one with a small value of κ is a poor conductor or a good insulator In the case of solids the properties of materials that make them good electrical conductors (namely the ability of electrons to move
Heat Energy in transit EFEU May 2009
Renilde Nihoul 8 HUBrussel
relatively easily throughout the bulk of the material) also make them good thermal conductors Table 1 shows some representative values of κ
Material Conductivity κ (Wm middot K)
Metals
Stainless steel 14
Lead 35
Aluminium 235
Copper 401
Silver 428
Gases
Air 0026
Helium 015
Hydrogen 018
Building materials
Polyurethane foam 0024
Rock wool 0043
Fiberglas 0048
White pine 011
Window glass 10
Table 1 Some thermal conductivities5
Over the range of temperatures we normally encounter we can regard κ as a constant but over wide temperature ranges it does show a slight variation with T Gases transfer heat by direct collisions between molecules and as would be expected their thermal conductivity is low compared to most solids since they are dilute media
62 Convection
If you look at the flame of a candle or a match you are watching heat energy being transported upward by convection Heat transfer by convection occurs when a fluid such as air of water is in contact with an object whose temperature is higher than that of its surroundings The temperature of the fluid that is in contact with the hot object increases and (in most cases) the fluid expands Being less dense than the surrounding cooler fluid it rises because of buoyant forces
5 Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Heat Energy in transit EFEU May 2009
Renilde Nihoul 9 HUBrussel
Figure 5 Heat convection6
Convection can also lead to circulation in a liquid as in heating a pot of water over a flame Heated water expands and becomes more buoyant Cooler denser water near the surface descends and patterns of circulation can be formed
Figure 6 Convection currents7
Atmospheric convection plays a fundamental role in determining the global climate patterns and in our daily weather variations Glider pilots and condors alike seek the convective thermals that rising from the warmer Earth beneath keep them aloft Huge energy transfers take place within the oceans by the same process The outer region of the sun called the photosphere contains a vast array of convection cells that transport energy from the interior to the solar surface and give the surface a granulated appearance with a typical dimension of a granule being 1000 kilometres
6 httphyperphysicsphy-astrgsueduhbasethermoheatrahtmlc2 7 httpwwwphysicsarizonaedu
Heat Energy in transit EFEU May 2009
Renilde Nihoul 10 HUBrussel
Figure 7 The photosphere of the sun8
Heat-induced fluid motion in initially static fluids is known as free convection For cases where the fluid is already in motion heat conducted into the fluid will be transported away chiefly by fluid convection These cases known as forced convection require a pressure gradient to drive the fluid motion as opposed to a gravity gradient to induce motion through buoyancy It is difficult to quantify the effects of convection since it inherently depends upon small no uniformities in an otherwise fairly homogeneous medium
63 Radiation
Energy is carried from the sun to us by electromagnetic waves that travel freely through the near vacuum of the intervening space If you stand near a bonfire or an open fireplace you are warmed by the same process
8 httpsolarsciencemsfcnasagovimagesgranulesjpg
Heat Energy in transit EFEU May 2009
Renilde Nihoul 11 HUBrussel
Figure 8 The sun at 5800K and a campfire at 800K9
The sun at 5800 K and a hot campfire at perhaps 800 K give off radiation at a rate proportional to the 4th power of the temperature All objects emit such electromagnetic radiation because of their temperature and also absorb some radiation that falls on them from the objects The higher the temperature of an object the more it radiates The energy is carried by photons of light in the infrared and visible portions of the electromagnetic spectrum When temperatures are uniform the radiative flux between objects is in equilibrium and no net thermal energy is exchanged The balance is upset when temperatures are not uniform and thermal energy is transported from surfaces of higher to surfaces of lower temperatures
7 Heat Transfer Examples
71 With focus on radiation the Greenhouse effect
The greenhouse effect refers to circumstances where the short wavelengths of visible light from the sun pass through a transparent medium and are absorbed but the longer wavelengths of the infrared re-radiation from the heated objects are unable to pass through that medium The trapping of the long wavelength radiation leads to more heating and a higher resultant temperature Besides the heating of an automobile by sunlight through the windshield and the namesake example of heating the greenhouse by sunlight passing through sealed transparent windows the greenhouse effect has been widely used to describe the trapping of excess heat by the rising concentration of carbon dioxide
9 httphyperphysicsphy-astrgsueduhbasethermostefanhtmlc2
Heat Energy in transit EFEU May 2009
Renilde Nihoul 12 HUBrussel
in the atmosphere The carbon dioxide strongly absorbs infrared and does not allow as much of it to escape into space
Figure 9 The greenhouse effect10
A major part of the efficiency of the heating of an actual greenhouse is the trapping of the air so that the energy is not lost by convection Keeping the hot air from escaping out the top is part of the practical greenhouse effect but it is common usage to refer to the infrared trapping as the greenhouse effect in atmospheric applications where the air trapping is not applicable
The action of carbon dioxide and other greenhouse gases in trapping outgoing infrared energy from the Earth thereby warming the planet is called the greenhouse effect Those gas molecules in the Earths atmosphere with three or more atoms are called greenhouse gases The greenhouse gases include water vapour (H2O) ozone (O3) carbon dioxide (CO2) and methane (CH4) Also trace quantities of chlorofluorocarbons (CFCs) can have a disproportionately large effect Current analysis suggests that the combustion of fossil fuels is a major contributor to the increase in the carbon dioxide concentration It may measurably increase the overall average temperature of the Earth which could have disastrous consequences Because the potential consequences of global warming in terms of loss of snow cover sea level rise change in weather patterns etc are so great it is a major societal concern On the other hand proposed measures to reduce human contributions to greenhouse gases can also have great consequences For more information httphyperphysicsphy-astrgsueduhbasehframehtml
10 httphyperphysicsphy-astrgsueduhbasehframehtml
Heat Energy in transit EFEU May 2009
Renilde Nihoul 13 HUBrussel
72 Involving all three basic heat transfer mechanisms cooling of the human body
Figure 10 Cooling of the human body11
This is a simplified model of the process by which the human body gives off heat Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism One implication of the model is that radiation is the most important heat transfer mechanism at ordinary room temperatures This model indicates that an unclothed person at rest in a room temperature of 23 Celsius would be uncomfortably cool The skin temperature of 34 degC is a typical skin temperature taken from physiology texts compared to the normal core body temperature of 37 degC What will happen if the ambient temperature is above the skin temperature Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism This becomes a problem when the ambient temperature is above body temperature because all three standard heat transfer mechanisms work against this heat loss by transferring heat into the body
Figure 11 Cooling of the human body when the ambient temperature is above the skin temperature
11 httphyperphysicsphy-astrgsueduhbasethermocoobodhtmlc1
Heat Energy in transit EFEU May 2009
Renilde Nihoul 14 HUBrussel
Our ability to exist in such conditions comes from the efficiency of cooling by the evaporation of perspiration At a temperature of 45 Celsius the evaporation process must overcome the transfer of heat into the body and give off enough heat to accomplish a 90 watt net outward flow rate of energy Because of the bodys temperature regulation mechanisms the skin temperature would be expected to rise to 37degC at which point perspiration is initiated and increases until the evaporation cooling is sufficient to hold the skin at 37degC if possible With those assumptions about the temperatures one can calculate that there will be a net input power of 109 watts to the body (by radiation) The perspiration cooling must overcome that and produce the net outflow of 90 watts for equilibrium
8 Dealing with misconceptions by training the scientific method
As mentioned before some misconceptions about heat can stubbornly persist in pupils mind In order to explain how to deal with this misconceptions by training the scientific method we illustrate this concept with an example
81 Step 1 Formulation of a problem
A problem (of a daily life situation) can be introduced by the teacher or by a pupil For instance
I want to keep the tea as hot as possible as long as possible in a teapot Therefore I better choose a thick teapot A metal teapot hellip
82 Step 2 Formulation of workhypotheses
The pupils discuss the problem in small groups (3 ndash 4 pupils) and try to formulate workhypotheses The emphasis has to lay on the formulation of the hypotheses and not on the ldquocorrectrdquo formulation As a teacher you try to stimulate the discussion by asking questions as ldquoWhat exactly do you mean Why do you think that is How could you write this downhelliprdquo Don‟t give any information with respect to content Discuss in class all the preconceptions made by the pupils It is important they realise not everyone has the same preconceptions It is also important that they can practice to express their visions They have to become aware of their own visions
Summarize all the visions of the pupils
A metal teapot is better than a porcelain one to keep the tea hot
In a thicker teapot the tea will remain longer hot It doesn‟t matter which teapot you use (the tea will chill
anyway) hellip
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Renilde Nihoul 15 HUBrussel
83 Step 3 Experimental verification
Normally the teacher thinks out the experiment However it is important that the pupils plan their own experiment so they see the experiment as a mean to answer questions and not only as a recipe they have to follow
At first they will use their sensory perceptions to plan the experiment Let them realise that these sensory perceptions are valuable but they give only vague and superficial information Then they ought to be able to translate the problem in measurable quantities All quantities that are important for the experiment need to be listed Some of these quantities need to be kept at a constant level during the experiment Other quantities need to be measured The pupils should estimate the magnitude of the results in advance so they can select the appropriate measuring instruments They can use the following scheme to think out their experiment
Workhypothesis What is the question
Description and outline of the experiment Which experiment can answer that question
Material list Which equipment is necessary
Procedure How exactly is it done
For the experiment with the teapot this could be resolved as follows
Tea can be replaced by boiling water In stead of a real teapot we can use cups of different materials
(with the same volume) To discover the effect of the thickness we use cups of the
same material but with a different thickness We measure the temperature with a thermometer the time with
a chronometer and the volume with a pipette
Procedure Fill the cups with equal volumes of boiling water (use the
different kind of cups) Measure the temperature of the water at different points of time
(every 15 seconds) Put the results in a convenient table and try to figure out which
cup is most suitable to keep the tea hot
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Renilde Nihoul 16 HUBrussel
84 Step 4 Reflection
When the experiment is finished they have to select the work hypothesis confirmed by the experiment and give one‟s motives for this choice All the difficulties have to be put into words If their own vision is in conflict with the outcome of the experiment the pupils will be intellectually challenged
Selection of the hypothesis confirmed by the experiment a porcelain teapot will keep the tea longer hot than a metal teapot The thickness of the teapot is important The tea will chill in every teapot but the time it takes to do so will be different
Motivation of the selection
85 Step 5 Scientific explanation
The scientific explanation should start from the concrete problem from a conceptual approach and lead then to a mathematical formulation The scientific explanation should be told in a fascinating way so they will remember it longer
The better you prevent an energy exchange between the hot tea and the surrounding air the longer the tea remains hot
A metal pot is a very bad insulator (A complete explanation is not given here)
86 Conclusion
To deal with the existing misconceptions let the pupils make their own experiments By practicing a scientific method they will be able to discover the bdquotruth‟ The link between the physical principles and situations of daily life will become clearer They might be more interested in physics lessons as they understand they can use these theoretical principles to explain daily life problems Evaluate the concept afterwards (Evalution of your practice is a powerful tool for enhancing your teaching as well as their students learning) Did the strategie capture and retain the pupil‟s attention How effective was this in encouraging their scientific skills Hopefully the pupils will perceive themselves as participants in the journey of discovering and learning and as co-operating detectives looking for and evaluating evidence Let them share your feelings of curiosity enthusiasm and critical reflection
Heat Energy in transit EFEU May 2009
Renilde Nihoul 17 HUBrussel
9 References
Balck C Cocquyt B Van Peteghem R (2005) Misvattingen fysica te lijf Syllabus Universiteit Antwerpen ndash Centrum Nascholing Onderwijs
Giancoli DC (1999) Natuurkunde voor Wetenschap en Techniek deel II Golven en Geluid Kinetische Theorie en Thermodynamica Elektriciteit en Magnetisme Licht Academic Service Schoonhoven
Hathaway D (2008) Solar Physics NASAs Marshall Space Flight Center httpsolarsciencemsfcnasagov
NaveCR (2005) HyperPhysics Departement of Physics and Astronomy Georgia State University httphyperphysicsphy-astrgsueduhbasehframehtml
Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 3 HUBrussel
Therefore we need some sort of exchange of energy between the system and its environment The following definition of heat can be used Heat (symbol Q) is energy that flows between a system and its environment by virtue of a temperature difference between them1 Figure 1 summarizes this view
(a) TS lt TE (b) TS = TE (c) TS gt TE
Figure 1 (a) If the temperature of a system is less than the temperature of its environment heat flows into the
system until thermal equilibrium is obtained as in (b) (c) If the temperature of a system is greater than that of
its environment heat flows out of the system
If the temperature TS of a system is less than the temperature of its environment TE heat flows into the system By convention is Q positive in this case It is a process by which the internal energy of the system is increased When TS gt TE heat flows out of the system in which case Q is taken to be negative Since heat is a form of energy its units are those of energy namely the joule (J) in the SI system Other units are still in use today The ldquocalorierdquo is in common use as a measure of nutrition 1 cal = 4186 J The ldquoBritish thermal unitrdquo (Btu) is still commonly found as a measure of the ability of an air conditioner to transfer energy (as heat) from a room to the outside environment A typical room air conditioner rated at 10 000 Btuh can therefore remove about 107 J from a room every hour and transfer it to the outside environment 1 Btu = 1055 J
3 Misconceptions about heat
Heat is similar to work in that both represent a means for the transfer of energy The following figure of the interchange ability of heat and work as agents for adding energy to a system can help to dispel some misconceptions about heat
1 Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Environment
Q gt O
Environment
Q = 0
Environment
Q lt 0
System System
System
Heat Energy in transit EFEU May 2009
Renilde Nihoul 4 HUBrussel
Figure 2 The interchange ability of heat and work as agents for adding energy to a system 2
If you are presented with a high temperature gas you can‟t tell whether it reached that high temperature by being heated or by having work done on it or a combination of the two To describe the energy that a high temperature object has it is not a correct use of the word heat to say that the object ldquopossesses heatrdquo It is better to say that it possesses internal energy as a result of its molecular motion Neither heat nor work is an intrinsic property of a system A system doesn‟t ldquocontainrdquo a certain amount of work Instead we say that it can transfer a certain amount of energy as heat or work under certain specified conditions The word heat is reserved to describe the process of transfer of energy from a high temperature object to a lower temperature one You can take an object at low internal energy and raise it to higher internal energy by heating it But you can also increase its internal energy by doing work on it and since the internal energy of a high temperature object resides in random motions of the molecules you can‟t tell which mechanism was used to give it that energy Some of the confusion about the precise meaning of heat results from the popular usage of the term Often heat is used when what is really meant is temperature or internal energy When we hear about heat in relation to weather or when cooking instructions indicate ldquoheat at 200 degreesrdquo it is temperature that is being discussed When someone talks about the ldquoheat generatedrdquo by the brake linings of an automobile or by briskly rubbing the palms of your ands together it is usually internal energy that is meant When you rub your hands together they do work on one another thereby increasing their internal energy and raising their temperature This excess energy can then be transferred to the environment as heat because the hands are at a higher temperature than the environment So don‟t refer to the ldquoheat in a bodyrdquo or say ldquothis object has twice as much heat as that bodyrdquo Avoid the use of the rather vague term ldquothermal energyrdquo and the use of the word
2 Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 5 HUBrussel
ldquoheatrdquo as a verb since this feeds the misconceptions Introduce and use the concept of internal energy as quickly as possible
4 The mechanical equivalent of heat
In a classic experiment in 1850 James Joule showed the energy equivalence of heating and doing work by using the change in potential energy of falling masses to stir an insulated container of water with paddles
Figure 3 Joules arrangement for measuring the mechanical equivalent of heat The falling weights turn paddles
that stir the water in the container thus raising the temperature3
The mechanical work done by the falling weights produces a measurable temperature rise of the water At that time calories were the accepted unit of heat It was originally defined as the quantity of heat necessary to raise the temperature of 1 g of water from 145 to 155 degC From the measured temperature increase of the water Joule was able to deduce the amount of calories of heat Q that would have produced the same temperature increase The work done on the water by the falling weights (in joules) therefore produced a temperature rise equivalent to the absorption by the water of a certain heat (in calories) and from this equivalence it is possible to determine the relationship between the calorie and the joule This experiment is noteworthy not only for the skill ingenuity and its precision but also for the direction it provided in showing that heat like work could properly be regarded as a means of transferring energy In the 18th century it was believed that a material fluid called caloric was exchanged between bodies at different temperatures
5 Internal energy and the first law of thermodynamics
3 httpenwikipediaorgwikiImageJoule27s_heat_apparatusJPG
Heat Energy in transit EFEU May 2009
Renilde Nihoul 6 HUBrussel
Internal energy is defined as the energy associated with the random disordered motion of molecules It is separated in scale from the macroscopic ordered energy associated with moving objects it refers to the invisible microscopic energy on the atomic and molecular scale For example a glass of water at room temperature standing on a table has no apparent energy either potential or kinetic But on the microscopic scale it is a seething mass of high speed molecules travelling at hundreds of meters per second
Figure 4 Internal energy of a glass of water at room temperature4
U is the most common symbol used for internal energy The change in internal energy between equilibrium states 1 and 2 is ΔU = U2 ndash U1
The value of U1 depends only on the coordinates of the state 1(only on temperature for an ideal gas) Similarly U2 depends only on the coordinates of point 2 Such a function is called a state function it depends only on the state of a system and not at all on how the system arrived at that state This brings us to the first law of thermodynamics which can be stated as follows In any thermodynamic process between equilibrium states 1 and 2 the quantity Q ndash W has the same value for any path between state 1 and 2 This quantity is equal to the change in the value of a state function called the internal energy Mathematically the first law is ΔU = Q ndash W The change in internal energy of a system is equal to the heat added to the system minus the work done by the system In other textbooks you may find the first law written as ΔU = Q + W It is the same law namely the thermodynamic expression of the conservation of energy principle In this case W is defined as the work done on the system instead of work done by the system For many processes (eg an internal combustion engine) the common
4 httphyperphysicsphy-astrgsueduhbasethermointenghtmlc2
Heat Energy in transit EFEU May 2009
Renilde Nihoul 7 HUBrussel
scenario is one of adding heat to a volume of gas and using the expansion of that gas to do work Therefore we prefer to use the first notation
6 The transfer of heat
The transfer of heat between a system and its environment can take place by three mechanisms conduction convection and radiation
61 Conduction
If you leave a metal poker in a fire for any length of time its handle will become hot Energy is transferred from the fire to the handle by conduction along the length of the metal shaft The atoms at the hot end by virtue of the high temperature at that end are vibrating with large amplitude These large vibrational amplitudes are passed along the shaft from atom to atom by interactions between adjacent atoms In this way a region of rising temperature travels along the shaft to your hand The energy will be transferred because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones The rate of conduction heat transfer between two plane surfaces can be calculated Consider a thin slab of homogeneous material of thickness d and cross-sectional area A The temperature is T + ΔT (Thot) on one face and T (Tcold) on the other The rate of heat flow through the slab is directly proportional to A (the more area available the more heat can flow per unit time) inversely proportional to d (the thicker the slab the less heat can flow per unit time) directly proportional to ΔT (the larger the temperature difference the more heat can flow per unit time) [To minimize the loss of heat from your house in winter make the surface area smaller (a two-story house is more efficient than a one-story house of the same total floor area) use thick walls filled with insulation and perhaps most important move to a warmer climate] Mathematically we can summarize this as
dTA
tQ=
With Q = heat transferred in time = Δt κ = thermal conductivity of the barrier A = area ΔT = temperature difference d = thickness of the barrier A substance with a large value of κ is a good heat conductor one with a small value of κ is a poor conductor or a good insulator In the case of solids the properties of materials that make them good electrical conductors (namely the ability of electrons to move
Heat Energy in transit EFEU May 2009
Renilde Nihoul 8 HUBrussel
relatively easily throughout the bulk of the material) also make them good thermal conductors Table 1 shows some representative values of κ
Material Conductivity κ (Wm middot K)
Metals
Stainless steel 14
Lead 35
Aluminium 235
Copper 401
Silver 428
Gases
Air 0026
Helium 015
Hydrogen 018
Building materials
Polyurethane foam 0024
Rock wool 0043
Fiberglas 0048
White pine 011
Window glass 10
Table 1 Some thermal conductivities5
Over the range of temperatures we normally encounter we can regard κ as a constant but over wide temperature ranges it does show a slight variation with T Gases transfer heat by direct collisions between molecules and as would be expected their thermal conductivity is low compared to most solids since they are dilute media
62 Convection
If you look at the flame of a candle or a match you are watching heat energy being transported upward by convection Heat transfer by convection occurs when a fluid such as air of water is in contact with an object whose temperature is higher than that of its surroundings The temperature of the fluid that is in contact with the hot object increases and (in most cases) the fluid expands Being less dense than the surrounding cooler fluid it rises because of buoyant forces
5 Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Heat Energy in transit EFEU May 2009
Renilde Nihoul 9 HUBrussel
Figure 5 Heat convection6
Convection can also lead to circulation in a liquid as in heating a pot of water over a flame Heated water expands and becomes more buoyant Cooler denser water near the surface descends and patterns of circulation can be formed
Figure 6 Convection currents7
Atmospheric convection plays a fundamental role in determining the global climate patterns and in our daily weather variations Glider pilots and condors alike seek the convective thermals that rising from the warmer Earth beneath keep them aloft Huge energy transfers take place within the oceans by the same process The outer region of the sun called the photosphere contains a vast array of convection cells that transport energy from the interior to the solar surface and give the surface a granulated appearance with a typical dimension of a granule being 1000 kilometres
6 httphyperphysicsphy-astrgsueduhbasethermoheatrahtmlc2 7 httpwwwphysicsarizonaedu
Heat Energy in transit EFEU May 2009
Renilde Nihoul 10 HUBrussel
Figure 7 The photosphere of the sun8
Heat-induced fluid motion in initially static fluids is known as free convection For cases where the fluid is already in motion heat conducted into the fluid will be transported away chiefly by fluid convection These cases known as forced convection require a pressure gradient to drive the fluid motion as opposed to a gravity gradient to induce motion through buoyancy It is difficult to quantify the effects of convection since it inherently depends upon small no uniformities in an otherwise fairly homogeneous medium
63 Radiation
Energy is carried from the sun to us by electromagnetic waves that travel freely through the near vacuum of the intervening space If you stand near a bonfire or an open fireplace you are warmed by the same process
8 httpsolarsciencemsfcnasagovimagesgranulesjpg
Heat Energy in transit EFEU May 2009
Renilde Nihoul 11 HUBrussel
Figure 8 The sun at 5800K and a campfire at 800K9
The sun at 5800 K and a hot campfire at perhaps 800 K give off radiation at a rate proportional to the 4th power of the temperature All objects emit such electromagnetic radiation because of their temperature and also absorb some radiation that falls on them from the objects The higher the temperature of an object the more it radiates The energy is carried by photons of light in the infrared and visible portions of the electromagnetic spectrum When temperatures are uniform the radiative flux between objects is in equilibrium and no net thermal energy is exchanged The balance is upset when temperatures are not uniform and thermal energy is transported from surfaces of higher to surfaces of lower temperatures
7 Heat Transfer Examples
71 With focus on radiation the Greenhouse effect
The greenhouse effect refers to circumstances where the short wavelengths of visible light from the sun pass through a transparent medium and are absorbed but the longer wavelengths of the infrared re-radiation from the heated objects are unable to pass through that medium The trapping of the long wavelength radiation leads to more heating and a higher resultant temperature Besides the heating of an automobile by sunlight through the windshield and the namesake example of heating the greenhouse by sunlight passing through sealed transparent windows the greenhouse effect has been widely used to describe the trapping of excess heat by the rising concentration of carbon dioxide
9 httphyperphysicsphy-astrgsueduhbasethermostefanhtmlc2
Heat Energy in transit EFEU May 2009
Renilde Nihoul 12 HUBrussel
in the atmosphere The carbon dioxide strongly absorbs infrared and does not allow as much of it to escape into space
Figure 9 The greenhouse effect10
A major part of the efficiency of the heating of an actual greenhouse is the trapping of the air so that the energy is not lost by convection Keeping the hot air from escaping out the top is part of the practical greenhouse effect but it is common usage to refer to the infrared trapping as the greenhouse effect in atmospheric applications where the air trapping is not applicable
The action of carbon dioxide and other greenhouse gases in trapping outgoing infrared energy from the Earth thereby warming the planet is called the greenhouse effect Those gas molecules in the Earths atmosphere with three or more atoms are called greenhouse gases The greenhouse gases include water vapour (H2O) ozone (O3) carbon dioxide (CO2) and methane (CH4) Also trace quantities of chlorofluorocarbons (CFCs) can have a disproportionately large effect Current analysis suggests that the combustion of fossil fuels is a major contributor to the increase in the carbon dioxide concentration It may measurably increase the overall average temperature of the Earth which could have disastrous consequences Because the potential consequences of global warming in terms of loss of snow cover sea level rise change in weather patterns etc are so great it is a major societal concern On the other hand proposed measures to reduce human contributions to greenhouse gases can also have great consequences For more information httphyperphysicsphy-astrgsueduhbasehframehtml
10 httphyperphysicsphy-astrgsueduhbasehframehtml
Heat Energy in transit EFEU May 2009
Renilde Nihoul 13 HUBrussel
72 Involving all three basic heat transfer mechanisms cooling of the human body
Figure 10 Cooling of the human body11
This is a simplified model of the process by which the human body gives off heat Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism One implication of the model is that radiation is the most important heat transfer mechanism at ordinary room temperatures This model indicates that an unclothed person at rest in a room temperature of 23 Celsius would be uncomfortably cool The skin temperature of 34 degC is a typical skin temperature taken from physiology texts compared to the normal core body temperature of 37 degC What will happen if the ambient temperature is above the skin temperature Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism This becomes a problem when the ambient temperature is above body temperature because all three standard heat transfer mechanisms work against this heat loss by transferring heat into the body
Figure 11 Cooling of the human body when the ambient temperature is above the skin temperature
11 httphyperphysicsphy-astrgsueduhbasethermocoobodhtmlc1
Heat Energy in transit EFEU May 2009
Renilde Nihoul 14 HUBrussel
Our ability to exist in such conditions comes from the efficiency of cooling by the evaporation of perspiration At a temperature of 45 Celsius the evaporation process must overcome the transfer of heat into the body and give off enough heat to accomplish a 90 watt net outward flow rate of energy Because of the bodys temperature regulation mechanisms the skin temperature would be expected to rise to 37degC at which point perspiration is initiated and increases until the evaporation cooling is sufficient to hold the skin at 37degC if possible With those assumptions about the temperatures one can calculate that there will be a net input power of 109 watts to the body (by radiation) The perspiration cooling must overcome that and produce the net outflow of 90 watts for equilibrium
8 Dealing with misconceptions by training the scientific method
As mentioned before some misconceptions about heat can stubbornly persist in pupils mind In order to explain how to deal with this misconceptions by training the scientific method we illustrate this concept with an example
81 Step 1 Formulation of a problem
A problem (of a daily life situation) can be introduced by the teacher or by a pupil For instance
I want to keep the tea as hot as possible as long as possible in a teapot Therefore I better choose a thick teapot A metal teapot hellip
82 Step 2 Formulation of workhypotheses
The pupils discuss the problem in small groups (3 ndash 4 pupils) and try to formulate workhypotheses The emphasis has to lay on the formulation of the hypotheses and not on the ldquocorrectrdquo formulation As a teacher you try to stimulate the discussion by asking questions as ldquoWhat exactly do you mean Why do you think that is How could you write this downhelliprdquo Don‟t give any information with respect to content Discuss in class all the preconceptions made by the pupils It is important they realise not everyone has the same preconceptions It is also important that they can practice to express their visions They have to become aware of their own visions
Summarize all the visions of the pupils
A metal teapot is better than a porcelain one to keep the tea hot
In a thicker teapot the tea will remain longer hot It doesn‟t matter which teapot you use (the tea will chill
anyway) hellip
Heat Energy in transit EFEU May 2009
Renilde Nihoul 15 HUBrussel
83 Step 3 Experimental verification
Normally the teacher thinks out the experiment However it is important that the pupils plan their own experiment so they see the experiment as a mean to answer questions and not only as a recipe they have to follow
At first they will use their sensory perceptions to plan the experiment Let them realise that these sensory perceptions are valuable but they give only vague and superficial information Then they ought to be able to translate the problem in measurable quantities All quantities that are important for the experiment need to be listed Some of these quantities need to be kept at a constant level during the experiment Other quantities need to be measured The pupils should estimate the magnitude of the results in advance so they can select the appropriate measuring instruments They can use the following scheme to think out their experiment
Workhypothesis What is the question
Description and outline of the experiment Which experiment can answer that question
Material list Which equipment is necessary
Procedure How exactly is it done
For the experiment with the teapot this could be resolved as follows
Tea can be replaced by boiling water In stead of a real teapot we can use cups of different materials
(with the same volume) To discover the effect of the thickness we use cups of the
same material but with a different thickness We measure the temperature with a thermometer the time with
a chronometer and the volume with a pipette
Procedure Fill the cups with equal volumes of boiling water (use the
different kind of cups) Measure the temperature of the water at different points of time
(every 15 seconds) Put the results in a convenient table and try to figure out which
cup is most suitable to keep the tea hot
Heat Energy in transit EFEU May 2009
Renilde Nihoul 16 HUBrussel
84 Step 4 Reflection
When the experiment is finished they have to select the work hypothesis confirmed by the experiment and give one‟s motives for this choice All the difficulties have to be put into words If their own vision is in conflict with the outcome of the experiment the pupils will be intellectually challenged
Selection of the hypothesis confirmed by the experiment a porcelain teapot will keep the tea longer hot than a metal teapot The thickness of the teapot is important The tea will chill in every teapot but the time it takes to do so will be different
Motivation of the selection
85 Step 5 Scientific explanation
The scientific explanation should start from the concrete problem from a conceptual approach and lead then to a mathematical formulation The scientific explanation should be told in a fascinating way so they will remember it longer
The better you prevent an energy exchange between the hot tea and the surrounding air the longer the tea remains hot
A metal pot is a very bad insulator (A complete explanation is not given here)
86 Conclusion
To deal with the existing misconceptions let the pupils make their own experiments By practicing a scientific method they will be able to discover the bdquotruth‟ The link between the physical principles and situations of daily life will become clearer They might be more interested in physics lessons as they understand they can use these theoretical principles to explain daily life problems Evaluate the concept afterwards (Evalution of your practice is a powerful tool for enhancing your teaching as well as their students learning) Did the strategie capture and retain the pupil‟s attention How effective was this in encouraging their scientific skills Hopefully the pupils will perceive themselves as participants in the journey of discovering and learning and as co-operating detectives looking for and evaluating evidence Let them share your feelings of curiosity enthusiasm and critical reflection
Heat Energy in transit EFEU May 2009
Renilde Nihoul 17 HUBrussel
9 References
Balck C Cocquyt B Van Peteghem R (2005) Misvattingen fysica te lijf Syllabus Universiteit Antwerpen ndash Centrum Nascholing Onderwijs
Giancoli DC (1999) Natuurkunde voor Wetenschap en Techniek deel II Golven en Geluid Kinetische Theorie en Thermodynamica Elektriciteit en Magnetisme Licht Academic Service Schoonhoven
Hathaway D (2008) Solar Physics NASAs Marshall Space Flight Center httpsolarsciencemsfcnasagov
NaveCR (2005) HyperPhysics Departement of Physics and Astronomy Georgia State University httphyperphysicsphy-astrgsueduhbasehframehtml
Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 4 HUBrussel
Figure 2 The interchange ability of heat and work as agents for adding energy to a system 2
If you are presented with a high temperature gas you can‟t tell whether it reached that high temperature by being heated or by having work done on it or a combination of the two To describe the energy that a high temperature object has it is not a correct use of the word heat to say that the object ldquopossesses heatrdquo It is better to say that it possesses internal energy as a result of its molecular motion Neither heat nor work is an intrinsic property of a system A system doesn‟t ldquocontainrdquo a certain amount of work Instead we say that it can transfer a certain amount of energy as heat or work under certain specified conditions The word heat is reserved to describe the process of transfer of energy from a high temperature object to a lower temperature one You can take an object at low internal energy and raise it to higher internal energy by heating it But you can also increase its internal energy by doing work on it and since the internal energy of a high temperature object resides in random motions of the molecules you can‟t tell which mechanism was used to give it that energy Some of the confusion about the precise meaning of heat results from the popular usage of the term Often heat is used when what is really meant is temperature or internal energy When we hear about heat in relation to weather or when cooking instructions indicate ldquoheat at 200 degreesrdquo it is temperature that is being discussed When someone talks about the ldquoheat generatedrdquo by the brake linings of an automobile or by briskly rubbing the palms of your ands together it is usually internal energy that is meant When you rub your hands together they do work on one another thereby increasing their internal energy and raising their temperature This excess energy can then be transferred to the environment as heat because the hands are at a higher temperature than the environment So don‟t refer to the ldquoheat in a bodyrdquo or say ldquothis object has twice as much heat as that bodyrdquo Avoid the use of the rather vague term ldquothermal energyrdquo and the use of the word
2 Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 5 HUBrussel
ldquoheatrdquo as a verb since this feeds the misconceptions Introduce and use the concept of internal energy as quickly as possible
4 The mechanical equivalent of heat
In a classic experiment in 1850 James Joule showed the energy equivalence of heating and doing work by using the change in potential energy of falling masses to stir an insulated container of water with paddles
Figure 3 Joules arrangement for measuring the mechanical equivalent of heat The falling weights turn paddles
that stir the water in the container thus raising the temperature3
The mechanical work done by the falling weights produces a measurable temperature rise of the water At that time calories were the accepted unit of heat It was originally defined as the quantity of heat necessary to raise the temperature of 1 g of water from 145 to 155 degC From the measured temperature increase of the water Joule was able to deduce the amount of calories of heat Q that would have produced the same temperature increase The work done on the water by the falling weights (in joules) therefore produced a temperature rise equivalent to the absorption by the water of a certain heat (in calories) and from this equivalence it is possible to determine the relationship between the calorie and the joule This experiment is noteworthy not only for the skill ingenuity and its precision but also for the direction it provided in showing that heat like work could properly be regarded as a means of transferring energy In the 18th century it was believed that a material fluid called caloric was exchanged between bodies at different temperatures
5 Internal energy and the first law of thermodynamics
3 httpenwikipediaorgwikiImageJoule27s_heat_apparatusJPG
Heat Energy in transit EFEU May 2009
Renilde Nihoul 6 HUBrussel
Internal energy is defined as the energy associated with the random disordered motion of molecules It is separated in scale from the macroscopic ordered energy associated with moving objects it refers to the invisible microscopic energy on the atomic and molecular scale For example a glass of water at room temperature standing on a table has no apparent energy either potential or kinetic But on the microscopic scale it is a seething mass of high speed molecules travelling at hundreds of meters per second
Figure 4 Internal energy of a glass of water at room temperature4
U is the most common symbol used for internal energy The change in internal energy between equilibrium states 1 and 2 is ΔU = U2 ndash U1
The value of U1 depends only on the coordinates of the state 1(only on temperature for an ideal gas) Similarly U2 depends only on the coordinates of point 2 Such a function is called a state function it depends only on the state of a system and not at all on how the system arrived at that state This brings us to the first law of thermodynamics which can be stated as follows In any thermodynamic process between equilibrium states 1 and 2 the quantity Q ndash W has the same value for any path between state 1 and 2 This quantity is equal to the change in the value of a state function called the internal energy Mathematically the first law is ΔU = Q ndash W The change in internal energy of a system is equal to the heat added to the system minus the work done by the system In other textbooks you may find the first law written as ΔU = Q + W It is the same law namely the thermodynamic expression of the conservation of energy principle In this case W is defined as the work done on the system instead of work done by the system For many processes (eg an internal combustion engine) the common
4 httphyperphysicsphy-astrgsueduhbasethermointenghtmlc2
Heat Energy in transit EFEU May 2009
Renilde Nihoul 7 HUBrussel
scenario is one of adding heat to a volume of gas and using the expansion of that gas to do work Therefore we prefer to use the first notation
6 The transfer of heat
The transfer of heat between a system and its environment can take place by three mechanisms conduction convection and radiation
61 Conduction
If you leave a metal poker in a fire for any length of time its handle will become hot Energy is transferred from the fire to the handle by conduction along the length of the metal shaft The atoms at the hot end by virtue of the high temperature at that end are vibrating with large amplitude These large vibrational amplitudes are passed along the shaft from atom to atom by interactions between adjacent atoms In this way a region of rising temperature travels along the shaft to your hand The energy will be transferred because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones The rate of conduction heat transfer between two plane surfaces can be calculated Consider a thin slab of homogeneous material of thickness d and cross-sectional area A The temperature is T + ΔT (Thot) on one face and T (Tcold) on the other The rate of heat flow through the slab is directly proportional to A (the more area available the more heat can flow per unit time) inversely proportional to d (the thicker the slab the less heat can flow per unit time) directly proportional to ΔT (the larger the temperature difference the more heat can flow per unit time) [To minimize the loss of heat from your house in winter make the surface area smaller (a two-story house is more efficient than a one-story house of the same total floor area) use thick walls filled with insulation and perhaps most important move to a warmer climate] Mathematically we can summarize this as
dTA
tQ=
With Q = heat transferred in time = Δt κ = thermal conductivity of the barrier A = area ΔT = temperature difference d = thickness of the barrier A substance with a large value of κ is a good heat conductor one with a small value of κ is a poor conductor or a good insulator In the case of solids the properties of materials that make them good electrical conductors (namely the ability of electrons to move
Heat Energy in transit EFEU May 2009
Renilde Nihoul 8 HUBrussel
relatively easily throughout the bulk of the material) also make them good thermal conductors Table 1 shows some representative values of κ
Material Conductivity κ (Wm middot K)
Metals
Stainless steel 14
Lead 35
Aluminium 235
Copper 401
Silver 428
Gases
Air 0026
Helium 015
Hydrogen 018
Building materials
Polyurethane foam 0024
Rock wool 0043
Fiberglas 0048
White pine 011
Window glass 10
Table 1 Some thermal conductivities5
Over the range of temperatures we normally encounter we can regard κ as a constant but over wide temperature ranges it does show a slight variation with T Gases transfer heat by direct collisions between molecules and as would be expected their thermal conductivity is low compared to most solids since they are dilute media
62 Convection
If you look at the flame of a candle or a match you are watching heat energy being transported upward by convection Heat transfer by convection occurs when a fluid such as air of water is in contact with an object whose temperature is higher than that of its surroundings The temperature of the fluid that is in contact with the hot object increases and (in most cases) the fluid expands Being less dense than the surrounding cooler fluid it rises because of buoyant forces
5 Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Heat Energy in transit EFEU May 2009
Renilde Nihoul 9 HUBrussel
Figure 5 Heat convection6
Convection can also lead to circulation in a liquid as in heating a pot of water over a flame Heated water expands and becomes more buoyant Cooler denser water near the surface descends and patterns of circulation can be formed
Figure 6 Convection currents7
Atmospheric convection plays a fundamental role in determining the global climate patterns and in our daily weather variations Glider pilots and condors alike seek the convective thermals that rising from the warmer Earth beneath keep them aloft Huge energy transfers take place within the oceans by the same process The outer region of the sun called the photosphere contains a vast array of convection cells that transport energy from the interior to the solar surface and give the surface a granulated appearance with a typical dimension of a granule being 1000 kilometres
6 httphyperphysicsphy-astrgsueduhbasethermoheatrahtmlc2 7 httpwwwphysicsarizonaedu
Heat Energy in transit EFEU May 2009
Renilde Nihoul 10 HUBrussel
Figure 7 The photosphere of the sun8
Heat-induced fluid motion in initially static fluids is known as free convection For cases where the fluid is already in motion heat conducted into the fluid will be transported away chiefly by fluid convection These cases known as forced convection require a pressure gradient to drive the fluid motion as opposed to a gravity gradient to induce motion through buoyancy It is difficult to quantify the effects of convection since it inherently depends upon small no uniformities in an otherwise fairly homogeneous medium
63 Radiation
Energy is carried from the sun to us by electromagnetic waves that travel freely through the near vacuum of the intervening space If you stand near a bonfire or an open fireplace you are warmed by the same process
8 httpsolarsciencemsfcnasagovimagesgranulesjpg
Heat Energy in transit EFEU May 2009
Renilde Nihoul 11 HUBrussel
Figure 8 The sun at 5800K and a campfire at 800K9
The sun at 5800 K and a hot campfire at perhaps 800 K give off radiation at a rate proportional to the 4th power of the temperature All objects emit such electromagnetic radiation because of their temperature and also absorb some radiation that falls on them from the objects The higher the temperature of an object the more it radiates The energy is carried by photons of light in the infrared and visible portions of the electromagnetic spectrum When temperatures are uniform the radiative flux between objects is in equilibrium and no net thermal energy is exchanged The balance is upset when temperatures are not uniform and thermal energy is transported from surfaces of higher to surfaces of lower temperatures
7 Heat Transfer Examples
71 With focus on radiation the Greenhouse effect
The greenhouse effect refers to circumstances where the short wavelengths of visible light from the sun pass through a transparent medium and are absorbed but the longer wavelengths of the infrared re-radiation from the heated objects are unable to pass through that medium The trapping of the long wavelength radiation leads to more heating and a higher resultant temperature Besides the heating of an automobile by sunlight through the windshield and the namesake example of heating the greenhouse by sunlight passing through sealed transparent windows the greenhouse effect has been widely used to describe the trapping of excess heat by the rising concentration of carbon dioxide
9 httphyperphysicsphy-astrgsueduhbasethermostefanhtmlc2
Heat Energy in transit EFEU May 2009
Renilde Nihoul 12 HUBrussel
in the atmosphere The carbon dioxide strongly absorbs infrared and does not allow as much of it to escape into space
Figure 9 The greenhouse effect10
A major part of the efficiency of the heating of an actual greenhouse is the trapping of the air so that the energy is not lost by convection Keeping the hot air from escaping out the top is part of the practical greenhouse effect but it is common usage to refer to the infrared trapping as the greenhouse effect in atmospheric applications where the air trapping is not applicable
The action of carbon dioxide and other greenhouse gases in trapping outgoing infrared energy from the Earth thereby warming the planet is called the greenhouse effect Those gas molecules in the Earths atmosphere with three or more atoms are called greenhouse gases The greenhouse gases include water vapour (H2O) ozone (O3) carbon dioxide (CO2) and methane (CH4) Also trace quantities of chlorofluorocarbons (CFCs) can have a disproportionately large effect Current analysis suggests that the combustion of fossil fuels is a major contributor to the increase in the carbon dioxide concentration It may measurably increase the overall average temperature of the Earth which could have disastrous consequences Because the potential consequences of global warming in terms of loss of snow cover sea level rise change in weather patterns etc are so great it is a major societal concern On the other hand proposed measures to reduce human contributions to greenhouse gases can also have great consequences For more information httphyperphysicsphy-astrgsueduhbasehframehtml
10 httphyperphysicsphy-astrgsueduhbasehframehtml
Heat Energy in transit EFEU May 2009
Renilde Nihoul 13 HUBrussel
72 Involving all three basic heat transfer mechanisms cooling of the human body
Figure 10 Cooling of the human body11
This is a simplified model of the process by which the human body gives off heat Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism One implication of the model is that radiation is the most important heat transfer mechanism at ordinary room temperatures This model indicates that an unclothed person at rest in a room temperature of 23 Celsius would be uncomfortably cool The skin temperature of 34 degC is a typical skin temperature taken from physiology texts compared to the normal core body temperature of 37 degC What will happen if the ambient temperature is above the skin temperature Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism This becomes a problem when the ambient temperature is above body temperature because all three standard heat transfer mechanisms work against this heat loss by transferring heat into the body
Figure 11 Cooling of the human body when the ambient temperature is above the skin temperature
11 httphyperphysicsphy-astrgsueduhbasethermocoobodhtmlc1
Heat Energy in transit EFEU May 2009
Renilde Nihoul 14 HUBrussel
Our ability to exist in such conditions comes from the efficiency of cooling by the evaporation of perspiration At a temperature of 45 Celsius the evaporation process must overcome the transfer of heat into the body and give off enough heat to accomplish a 90 watt net outward flow rate of energy Because of the bodys temperature regulation mechanisms the skin temperature would be expected to rise to 37degC at which point perspiration is initiated and increases until the evaporation cooling is sufficient to hold the skin at 37degC if possible With those assumptions about the temperatures one can calculate that there will be a net input power of 109 watts to the body (by radiation) The perspiration cooling must overcome that and produce the net outflow of 90 watts for equilibrium
8 Dealing with misconceptions by training the scientific method
As mentioned before some misconceptions about heat can stubbornly persist in pupils mind In order to explain how to deal with this misconceptions by training the scientific method we illustrate this concept with an example
81 Step 1 Formulation of a problem
A problem (of a daily life situation) can be introduced by the teacher or by a pupil For instance
I want to keep the tea as hot as possible as long as possible in a teapot Therefore I better choose a thick teapot A metal teapot hellip
82 Step 2 Formulation of workhypotheses
The pupils discuss the problem in small groups (3 ndash 4 pupils) and try to formulate workhypotheses The emphasis has to lay on the formulation of the hypotheses and not on the ldquocorrectrdquo formulation As a teacher you try to stimulate the discussion by asking questions as ldquoWhat exactly do you mean Why do you think that is How could you write this downhelliprdquo Don‟t give any information with respect to content Discuss in class all the preconceptions made by the pupils It is important they realise not everyone has the same preconceptions It is also important that they can practice to express their visions They have to become aware of their own visions
Summarize all the visions of the pupils
A metal teapot is better than a porcelain one to keep the tea hot
In a thicker teapot the tea will remain longer hot It doesn‟t matter which teapot you use (the tea will chill
anyway) hellip
Heat Energy in transit EFEU May 2009
Renilde Nihoul 15 HUBrussel
83 Step 3 Experimental verification
Normally the teacher thinks out the experiment However it is important that the pupils plan their own experiment so they see the experiment as a mean to answer questions and not only as a recipe they have to follow
At first they will use their sensory perceptions to plan the experiment Let them realise that these sensory perceptions are valuable but they give only vague and superficial information Then they ought to be able to translate the problem in measurable quantities All quantities that are important for the experiment need to be listed Some of these quantities need to be kept at a constant level during the experiment Other quantities need to be measured The pupils should estimate the magnitude of the results in advance so they can select the appropriate measuring instruments They can use the following scheme to think out their experiment
Workhypothesis What is the question
Description and outline of the experiment Which experiment can answer that question
Material list Which equipment is necessary
Procedure How exactly is it done
For the experiment with the teapot this could be resolved as follows
Tea can be replaced by boiling water In stead of a real teapot we can use cups of different materials
(with the same volume) To discover the effect of the thickness we use cups of the
same material but with a different thickness We measure the temperature with a thermometer the time with
a chronometer and the volume with a pipette
Procedure Fill the cups with equal volumes of boiling water (use the
different kind of cups) Measure the temperature of the water at different points of time
(every 15 seconds) Put the results in a convenient table and try to figure out which
cup is most suitable to keep the tea hot
Heat Energy in transit EFEU May 2009
Renilde Nihoul 16 HUBrussel
84 Step 4 Reflection
When the experiment is finished they have to select the work hypothesis confirmed by the experiment and give one‟s motives for this choice All the difficulties have to be put into words If their own vision is in conflict with the outcome of the experiment the pupils will be intellectually challenged
Selection of the hypothesis confirmed by the experiment a porcelain teapot will keep the tea longer hot than a metal teapot The thickness of the teapot is important The tea will chill in every teapot but the time it takes to do so will be different
Motivation of the selection
85 Step 5 Scientific explanation
The scientific explanation should start from the concrete problem from a conceptual approach and lead then to a mathematical formulation The scientific explanation should be told in a fascinating way so they will remember it longer
The better you prevent an energy exchange between the hot tea and the surrounding air the longer the tea remains hot
A metal pot is a very bad insulator (A complete explanation is not given here)
86 Conclusion
To deal with the existing misconceptions let the pupils make their own experiments By practicing a scientific method they will be able to discover the bdquotruth‟ The link between the physical principles and situations of daily life will become clearer They might be more interested in physics lessons as they understand they can use these theoretical principles to explain daily life problems Evaluate the concept afterwards (Evalution of your practice is a powerful tool for enhancing your teaching as well as their students learning) Did the strategie capture and retain the pupil‟s attention How effective was this in encouraging their scientific skills Hopefully the pupils will perceive themselves as participants in the journey of discovering and learning and as co-operating detectives looking for and evaluating evidence Let them share your feelings of curiosity enthusiasm and critical reflection
Heat Energy in transit EFEU May 2009
Renilde Nihoul 17 HUBrussel
9 References
Balck C Cocquyt B Van Peteghem R (2005) Misvattingen fysica te lijf Syllabus Universiteit Antwerpen ndash Centrum Nascholing Onderwijs
Giancoli DC (1999) Natuurkunde voor Wetenschap en Techniek deel II Golven en Geluid Kinetische Theorie en Thermodynamica Elektriciteit en Magnetisme Licht Academic Service Schoonhoven
Hathaway D (2008) Solar Physics NASAs Marshall Space Flight Center httpsolarsciencemsfcnasagov
NaveCR (2005) HyperPhysics Departement of Physics and Astronomy Georgia State University httphyperphysicsphy-astrgsueduhbasehframehtml
Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 5 HUBrussel
ldquoheatrdquo as a verb since this feeds the misconceptions Introduce and use the concept of internal energy as quickly as possible
4 The mechanical equivalent of heat
In a classic experiment in 1850 James Joule showed the energy equivalence of heating and doing work by using the change in potential energy of falling masses to stir an insulated container of water with paddles
Figure 3 Joules arrangement for measuring the mechanical equivalent of heat The falling weights turn paddles
that stir the water in the container thus raising the temperature3
The mechanical work done by the falling weights produces a measurable temperature rise of the water At that time calories were the accepted unit of heat It was originally defined as the quantity of heat necessary to raise the temperature of 1 g of water from 145 to 155 degC From the measured temperature increase of the water Joule was able to deduce the amount of calories of heat Q that would have produced the same temperature increase The work done on the water by the falling weights (in joules) therefore produced a temperature rise equivalent to the absorption by the water of a certain heat (in calories) and from this equivalence it is possible to determine the relationship between the calorie and the joule This experiment is noteworthy not only for the skill ingenuity and its precision but also for the direction it provided in showing that heat like work could properly be regarded as a means of transferring energy In the 18th century it was believed that a material fluid called caloric was exchanged between bodies at different temperatures
5 Internal energy and the first law of thermodynamics
3 httpenwikipediaorgwikiImageJoule27s_heat_apparatusJPG
Heat Energy in transit EFEU May 2009
Renilde Nihoul 6 HUBrussel
Internal energy is defined as the energy associated with the random disordered motion of molecules It is separated in scale from the macroscopic ordered energy associated with moving objects it refers to the invisible microscopic energy on the atomic and molecular scale For example a glass of water at room temperature standing on a table has no apparent energy either potential or kinetic But on the microscopic scale it is a seething mass of high speed molecules travelling at hundreds of meters per second
Figure 4 Internal energy of a glass of water at room temperature4
U is the most common symbol used for internal energy The change in internal energy between equilibrium states 1 and 2 is ΔU = U2 ndash U1
The value of U1 depends only on the coordinates of the state 1(only on temperature for an ideal gas) Similarly U2 depends only on the coordinates of point 2 Such a function is called a state function it depends only on the state of a system and not at all on how the system arrived at that state This brings us to the first law of thermodynamics which can be stated as follows In any thermodynamic process between equilibrium states 1 and 2 the quantity Q ndash W has the same value for any path between state 1 and 2 This quantity is equal to the change in the value of a state function called the internal energy Mathematically the first law is ΔU = Q ndash W The change in internal energy of a system is equal to the heat added to the system minus the work done by the system In other textbooks you may find the first law written as ΔU = Q + W It is the same law namely the thermodynamic expression of the conservation of energy principle In this case W is defined as the work done on the system instead of work done by the system For many processes (eg an internal combustion engine) the common
4 httphyperphysicsphy-astrgsueduhbasethermointenghtmlc2
Heat Energy in transit EFEU May 2009
Renilde Nihoul 7 HUBrussel
scenario is one of adding heat to a volume of gas and using the expansion of that gas to do work Therefore we prefer to use the first notation
6 The transfer of heat
The transfer of heat between a system and its environment can take place by three mechanisms conduction convection and radiation
61 Conduction
If you leave a metal poker in a fire for any length of time its handle will become hot Energy is transferred from the fire to the handle by conduction along the length of the metal shaft The atoms at the hot end by virtue of the high temperature at that end are vibrating with large amplitude These large vibrational amplitudes are passed along the shaft from atom to atom by interactions between adjacent atoms In this way a region of rising temperature travels along the shaft to your hand The energy will be transferred because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones The rate of conduction heat transfer between two plane surfaces can be calculated Consider a thin slab of homogeneous material of thickness d and cross-sectional area A The temperature is T + ΔT (Thot) on one face and T (Tcold) on the other The rate of heat flow through the slab is directly proportional to A (the more area available the more heat can flow per unit time) inversely proportional to d (the thicker the slab the less heat can flow per unit time) directly proportional to ΔT (the larger the temperature difference the more heat can flow per unit time) [To minimize the loss of heat from your house in winter make the surface area smaller (a two-story house is more efficient than a one-story house of the same total floor area) use thick walls filled with insulation and perhaps most important move to a warmer climate] Mathematically we can summarize this as
dTA
tQ=
With Q = heat transferred in time = Δt κ = thermal conductivity of the barrier A = area ΔT = temperature difference d = thickness of the barrier A substance with a large value of κ is a good heat conductor one with a small value of κ is a poor conductor or a good insulator In the case of solids the properties of materials that make them good electrical conductors (namely the ability of electrons to move
Heat Energy in transit EFEU May 2009
Renilde Nihoul 8 HUBrussel
relatively easily throughout the bulk of the material) also make them good thermal conductors Table 1 shows some representative values of κ
Material Conductivity κ (Wm middot K)
Metals
Stainless steel 14
Lead 35
Aluminium 235
Copper 401
Silver 428
Gases
Air 0026
Helium 015
Hydrogen 018
Building materials
Polyurethane foam 0024
Rock wool 0043
Fiberglas 0048
White pine 011
Window glass 10
Table 1 Some thermal conductivities5
Over the range of temperatures we normally encounter we can regard κ as a constant but over wide temperature ranges it does show a slight variation with T Gases transfer heat by direct collisions between molecules and as would be expected their thermal conductivity is low compared to most solids since they are dilute media
62 Convection
If you look at the flame of a candle or a match you are watching heat energy being transported upward by convection Heat transfer by convection occurs when a fluid such as air of water is in contact with an object whose temperature is higher than that of its surroundings The temperature of the fluid that is in contact with the hot object increases and (in most cases) the fluid expands Being less dense than the surrounding cooler fluid it rises because of buoyant forces
5 Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Heat Energy in transit EFEU May 2009
Renilde Nihoul 9 HUBrussel
Figure 5 Heat convection6
Convection can also lead to circulation in a liquid as in heating a pot of water over a flame Heated water expands and becomes more buoyant Cooler denser water near the surface descends and patterns of circulation can be formed
Figure 6 Convection currents7
Atmospheric convection plays a fundamental role in determining the global climate patterns and in our daily weather variations Glider pilots and condors alike seek the convective thermals that rising from the warmer Earth beneath keep them aloft Huge energy transfers take place within the oceans by the same process The outer region of the sun called the photosphere contains a vast array of convection cells that transport energy from the interior to the solar surface and give the surface a granulated appearance with a typical dimension of a granule being 1000 kilometres
6 httphyperphysicsphy-astrgsueduhbasethermoheatrahtmlc2 7 httpwwwphysicsarizonaedu
Heat Energy in transit EFEU May 2009
Renilde Nihoul 10 HUBrussel
Figure 7 The photosphere of the sun8
Heat-induced fluid motion in initially static fluids is known as free convection For cases where the fluid is already in motion heat conducted into the fluid will be transported away chiefly by fluid convection These cases known as forced convection require a pressure gradient to drive the fluid motion as opposed to a gravity gradient to induce motion through buoyancy It is difficult to quantify the effects of convection since it inherently depends upon small no uniformities in an otherwise fairly homogeneous medium
63 Radiation
Energy is carried from the sun to us by electromagnetic waves that travel freely through the near vacuum of the intervening space If you stand near a bonfire or an open fireplace you are warmed by the same process
8 httpsolarsciencemsfcnasagovimagesgranulesjpg
Heat Energy in transit EFEU May 2009
Renilde Nihoul 11 HUBrussel
Figure 8 The sun at 5800K and a campfire at 800K9
The sun at 5800 K and a hot campfire at perhaps 800 K give off radiation at a rate proportional to the 4th power of the temperature All objects emit such electromagnetic radiation because of their temperature and also absorb some radiation that falls on them from the objects The higher the temperature of an object the more it radiates The energy is carried by photons of light in the infrared and visible portions of the electromagnetic spectrum When temperatures are uniform the radiative flux between objects is in equilibrium and no net thermal energy is exchanged The balance is upset when temperatures are not uniform and thermal energy is transported from surfaces of higher to surfaces of lower temperatures
7 Heat Transfer Examples
71 With focus on radiation the Greenhouse effect
The greenhouse effect refers to circumstances where the short wavelengths of visible light from the sun pass through a transparent medium and are absorbed but the longer wavelengths of the infrared re-radiation from the heated objects are unable to pass through that medium The trapping of the long wavelength radiation leads to more heating and a higher resultant temperature Besides the heating of an automobile by sunlight through the windshield and the namesake example of heating the greenhouse by sunlight passing through sealed transparent windows the greenhouse effect has been widely used to describe the trapping of excess heat by the rising concentration of carbon dioxide
9 httphyperphysicsphy-astrgsueduhbasethermostefanhtmlc2
Heat Energy in transit EFEU May 2009
Renilde Nihoul 12 HUBrussel
in the atmosphere The carbon dioxide strongly absorbs infrared and does not allow as much of it to escape into space
Figure 9 The greenhouse effect10
A major part of the efficiency of the heating of an actual greenhouse is the trapping of the air so that the energy is not lost by convection Keeping the hot air from escaping out the top is part of the practical greenhouse effect but it is common usage to refer to the infrared trapping as the greenhouse effect in atmospheric applications where the air trapping is not applicable
The action of carbon dioxide and other greenhouse gases in trapping outgoing infrared energy from the Earth thereby warming the planet is called the greenhouse effect Those gas molecules in the Earths atmosphere with three or more atoms are called greenhouse gases The greenhouse gases include water vapour (H2O) ozone (O3) carbon dioxide (CO2) and methane (CH4) Also trace quantities of chlorofluorocarbons (CFCs) can have a disproportionately large effect Current analysis suggests that the combustion of fossil fuels is a major contributor to the increase in the carbon dioxide concentration It may measurably increase the overall average temperature of the Earth which could have disastrous consequences Because the potential consequences of global warming in terms of loss of snow cover sea level rise change in weather patterns etc are so great it is a major societal concern On the other hand proposed measures to reduce human contributions to greenhouse gases can also have great consequences For more information httphyperphysicsphy-astrgsueduhbasehframehtml
10 httphyperphysicsphy-astrgsueduhbasehframehtml
Heat Energy in transit EFEU May 2009
Renilde Nihoul 13 HUBrussel
72 Involving all three basic heat transfer mechanisms cooling of the human body
Figure 10 Cooling of the human body11
This is a simplified model of the process by which the human body gives off heat Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism One implication of the model is that radiation is the most important heat transfer mechanism at ordinary room temperatures This model indicates that an unclothed person at rest in a room temperature of 23 Celsius would be uncomfortably cool The skin temperature of 34 degC is a typical skin temperature taken from physiology texts compared to the normal core body temperature of 37 degC What will happen if the ambient temperature is above the skin temperature Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism This becomes a problem when the ambient temperature is above body temperature because all three standard heat transfer mechanisms work against this heat loss by transferring heat into the body
Figure 11 Cooling of the human body when the ambient temperature is above the skin temperature
11 httphyperphysicsphy-astrgsueduhbasethermocoobodhtmlc1
Heat Energy in transit EFEU May 2009
Renilde Nihoul 14 HUBrussel
Our ability to exist in such conditions comes from the efficiency of cooling by the evaporation of perspiration At a temperature of 45 Celsius the evaporation process must overcome the transfer of heat into the body and give off enough heat to accomplish a 90 watt net outward flow rate of energy Because of the bodys temperature regulation mechanisms the skin temperature would be expected to rise to 37degC at which point perspiration is initiated and increases until the evaporation cooling is sufficient to hold the skin at 37degC if possible With those assumptions about the temperatures one can calculate that there will be a net input power of 109 watts to the body (by radiation) The perspiration cooling must overcome that and produce the net outflow of 90 watts for equilibrium
8 Dealing with misconceptions by training the scientific method
As mentioned before some misconceptions about heat can stubbornly persist in pupils mind In order to explain how to deal with this misconceptions by training the scientific method we illustrate this concept with an example
81 Step 1 Formulation of a problem
A problem (of a daily life situation) can be introduced by the teacher or by a pupil For instance
I want to keep the tea as hot as possible as long as possible in a teapot Therefore I better choose a thick teapot A metal teapot hellip
82 Step 2 Formulation of workhypotheses
The pupils discuss the problem in small groups (3 ndash 4 pupils) and try to formulate workhypotheses The emphasis has to lay on the formulation of the hypotheses and not on the ldquocorrectrdquo formulation As a teacher you try to stimulate the discussion by asking questions as ldquoWhat exactly do you mean Why do you think that is How could you write this downhelliprdquo Don‟t give any information with respect to content Discuss in class all the preconceptions made by the pupils It is important they realise not everyone has the same preconceptions It is also important that they can practice to express their visions They have to become aware of their own visions
Summarize all the visions of the pupils
A metal teapot is better than a porcelain one to keep the tea hot
In a thicker teapot the tea will remain longer hot It doesn‟t matter which teapot you use (the tea will chill
anyway) hellip
Heat Energy in transit EFEU May 2009
Renilde Nihoul 15 HUBrussel
83 Step 3 Experimental verification
Normally the teacher thinks out the experiment However it is important that the pupils plan their own experiment so they see the experiment as a mean to answer questions and not only as a recipe they have to follow
At first they will use their sensory perceptions to plan the experiment Let them realise that these sensory perceptions are valuable but they give only vague and superficial information Then they ought to be able to translate the problem in measurable quantities All quantities that are important for the experiment need to be listed Some of these quantities need to be kept at a constant level during the experiment Other quantities need to be measured The pupils should estimate the magnitude of the results in advance so they can select the appropriate measuring instruments They can use the following scheme to think out their experiment
Workhypothesis What is the question
Description and outline of the experiment Which experiment can answer that question
Material list Which equipment is necessary
Procedure How exactly is it done
For the experiment with the teapot this could be resolved as follows
Tea can be replaced by boiling water In stead of a real teapot we can use cups of different materials
(with the same volume) To discover the effect of the thickness we use cups of the
same material but with a different thickness We measure the temperature with a thermometer the time with
a chronometer and the volume with a pipette
Procedure Fill the cups with equal volumes of boiling water (use the
different kind of cups) Measure the temperature of the water at different points of time
(every 15 seconds) Put the results in a convenient table and try to figure out which
cup is most suitable to keep the tea hot
Heat Energy in transit EFEU May 2009
Renilde Nihoul 16 HUBrussel
84 Step 4 Reflection
When the experiment is finished they have to select the work hypothesis confirmed by the experiment and give one‟s motives for this choice All the difficulties have to be put into words If their own vision is in conflict with the outcome of the experiment the pupils will be intellectually challenged
Selection of the hypothesis confirmed by the experiment a porcelain teapot will keep the tea longer hot than a metal teapot The thickness of the teapot is important The tea will chill in every teapot but the time it takes to do so will be different
Motivation of the selection
85 Step 5 Scientific explanation
The scientific explanation should start from the concrete problem from a conceptual approach and lead then to a mathematical formulation The scientific explanation should be told in a fascinating way so they will remember it longer
The better you prevent an energy exchange between the hot tea and the surrounding air the longer the tea remains hot
A metal pot is a very bad insulator (A complete explanation is not given here)
86 Conclusion
To deal with the existing misconceptions let the pupils make their own experiments By practicing a scientific method they will be able to discover the bdquotruth‟ The link between the physical principles and situations of daily life will become clearer They might be more interested in physics lessons as they understand they can use these theoretical principles to explain daily life problems Evaluate the concept afterwards (Evalution of your practice is a powerful tool for enhancing your teaching as well as their students learning) Did the strategie capture and retain the pupil‟s attention How effective was this in encouraging their scientific skills Hopefully the pupils will perceive themselves as participants in the journey of discovering and learning and as co-operating detectives looking for and evaluating evidence Let them share your feelings of curiosity enthusiasm and critical reflection
Heat Energy in transit EFEU May 2009
Renilde Nihoul 17 HUBrussel
9 References
Balck C Cocquyt B Van Peteghem R (2005) Misvattingen fysica te lijf Syllabus Universiteit Antwerpen ndash Centrum Nascholing Onderwijs
Giancoli DC (1999) Natuurkunde voor Wetenschap en Techniek deel II Golven en Geluid Kinetische Theorie en Thermodynamica Elektriciteit en Magnetisme Licht Academic Service Schoonhoven
Hathaway D (2008) Solar Physics NASAs Marshall Space Flight Center httpsolarsciencemsfcnasagov
NaveCR (2005) HyperPhysics Departement of Physics and Astronomy Georgia State University httphyperphysicsphy-astrgsueduhbasehframehtml
Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 6 HUBrussel
Internal energy is defined as the energy associated with the random disordered motion of molecules It is separated in scale from the macroscopic ordered energy associated with moving objects it refers to the invisible microscopic energy on the atomic and molecular scale For example a glass of water at room temperature standing on a table has no apparent energy either potential or kinetic But on the microscopic scale it is a seething mass of high speed molecules travelling at hundreds of meters per second
Figure 4 Internal energy of a glass of water at room temperature4
U is the most common symbol used for internal energy The change in internal energy between equilibrium states 1 and 2 is ΔU = U2 ndash U1
The value of U1 depends only on the coordinates of the state 1(only on temperature for an ideal gas) Similarly U2 depends only on the coordinates of point 2 Such a function is called a state function it depends only on the state of a system and not at all on how the system arrived at that state This brings us to the first law of thermodynamics which can be stated as follows In any thermodynamic process between equilibrium states 1 and 2 the quantity Q ndash W has the same value for any path between state 1 and 2 This quantity is equal to the change in the value of a state function called the internal energy Mathematically the first law is ΔU = Q ndash W The change in internal energy of a system is equal to the heat added to the system minus the work done by the system In other textbooks you may find the first law written as ΔU = Q + W It is the same law namely the thermodynamic expression of the conservation of energy principle In this case W is defined as the work done on the system instead of work done by the system For many processes (eg an internal combustion engine) the common
4 httphyperphysicsphy-astrgsueduhbasethermointenghtmlc2
Heat Energy in transit EFEU May 2009
Renilde Nihoul 7 HUBrussel
scenario is one of adding heat to a volume of gas and using the expansion of that gas to do work Therefore we prefer to use the first notation
6 The transfer of heat
The transfer of heat between a system and its environment can take place by three mechanisms conduction convection and radiation
61 Conduction
If you leave a metal poker in a fire for any length of time its handle will become hot Energy is transferred from the fire to the handle by conduction along the length of the metal shaft The atoms at the hot end by virtue of the high temperature at that end are vibrating with large amplitude These large vibrational amplitudes are passed along the shaft from atom to atom by interactions between adjacent atoms In this way a region of rising temperature travels along the shaft to your hand The energy will be transferred because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones The rate of conduction heat transfer between two plane surfaces can be calculated Consider a thin slab of homogeneous material of thickness d and cross-sectional area A The temperature is T + ΔT (Thot) on one face and T (Tcold) on the other The rate of heat flow through the slab is directly proportional to A (the more area available the more heat can flow per unit time) inversely proportional to d (the thicker the slab the less heat can flow per unit time) directly proportional to ΔT (the larger the temperature difference the more heat can flow per unit time) [To minimize the loss of heat from your house in winter make the surface area smaller (a two-story house is more efficient than a one-story house of the same total floor area) use thick walls filled with insulation and perhaps most important move to a warmer climate] Mathematically we can summarize this as
dTA
tQ=
With Q = heat transferred in time = Δt κ = thermal conductivity of the barrier A = area ΔT = temperature difference d = thickness of the barrier A substance with a large value of κ is a good heat conductor one with a small value of κ is a poor conductor or a good insulator In the case of solids the properties of materials that make them good electrical conductors (namely the ability of electrons to move
Heat Energy in transit EFEU May 2009
Renilde Nihoul 8 HUBrussel
relatively easily throughout the bulk of the material) also make them good thermal conductors Table 1 shows some representative values of κ
Material Conductivity κ (Wm middot K)
Metals
Stainless steel 14
Lead 35
Aluminium 235
Copper 401
Silver 428
Gases
Air 0026
Helium 015
Hydrogen 018
Building materials
Polyurethane foam 0024
Rock wool 0043
Fiberglas 0048
White pine 011
Window glass 10
Table 1 Some thermal conductivities5
Over the range of temperatures we normally encounter we can regard κ as a constant but over wide temperature ranges it does show a slight variation with T Gases transfer heat by direct collisions between molecules and as would be expected their thermal conductivity is low compared to most solids since they are dilute media
62 Convection
If you look at the flame of a candle or a match you are watching heat energy being transported upward by convection Heat transfer by convection occurs when a fluid such as air of water is in contact with an object whose temperature is higher than that of its surroundings The temperature of the fluid that is in contact with the hot object increases and (in most cases) the fluid expands Being less dense than the surrounding cooler fluid it rises because of buoyant forces
5 Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Heat Energy in transit EFEU May 2009
Renilde Nihoul 9 HUBrussel
Figure 5 Heat convection6
Convection can also lead to circulation in a liquid as in heating a pot of water over a flame Heated water expands and becomes more buoyant Cooler denser water near the surface descends and patterns of circulation can be formed
Figure 6 Convection currents7
Atmospheric convection plays a fundamental role in determining the global climate patterns and in our daily weather variations Glider pilots and condors alike seek the convective thermals that rising from the warmer Earth beneath keep them aloft Huge energy transfers take place within the oceans by the same process The outer region of the sun called the photosphere contains a vast array of convection cells that transport energy from the interior to the solar surface and give the surface a granulated appearance with a typical dimension of a granule being 1000 kilometres
6 httphyperphysicsphy-astrgsueduhbasethermoheatrahtmlc2 7 httpwwwphysicsarizonaedu
Heat Energy in transit EFEU May 2009
Renilde Nihoul 10 HUBrussel
Figure 7 The photosphere of the sun8
Heat-induced fluid motion in initially static fluids is known as free convection For cases where the fluid is already in motion heat conducted into the fluid will be transported away chiefly by fluid convection These cases known as forced convection require a pressure gradient to drive the fluid motion as opposed to a gravity gradient to induce motion through buoyancy It is difficult to quantify the effects of convection since it inherently depends upon small no uniformities in an otherwise fairly homogeneous medium
63 Radiation
Energy is carried from the sun to us by electromagnetic waves that travel freely through the near vacuum of the intervening space If you stand near a bonfire or an open fireplace you are warmed by the same process
8 httpsolarsciencemsfcnasagovimagesgranulesjpg
Heat Energy in transit EFEU May 2009
Renilde Nihoul 11 HUBrussel
Figure 8 The sun at 5800K and a campfire at 800K9
The sun at 5800 K and a hot campfire at perhaps 800 K give off radiation at a rate proportional to the 4th power of the temperature All objects emit such electromagnetic radiation because of their temperature and also absorb some radiation that falls on them from the objects The higher the temperature of an object the more it radiates The energy is carried by photons of light in the infrared and visible portions of the electromagnetic spectrum When temperatures are uniform the radiative flux between objects is in equilibrium and no net thermal energy is exchanged The balance is upset when temperatures are not uniform and thermal energy is transported from surfaces of higher to surfaces of lower temperatures
7 Heat Transfer Examples
71 With focus on radiation the Greenhouse effect
The greenhouse effect refers to circumstances where the short wavelengths of visible light from the sun pass through a transparent medium and are absorbed but the longer wavelengths of the infrared re-radiation from the heated objects are unable to pass through that medium The trapping of the long wavelength radiation leads to more heating and a higher resultant temperature Besides the heating of an automobile by sunlight through the windshield and the namesake example of heating the greenhouse by sunlight passing through sealed transparent windows the greenhouse effect has been widely used to describe the trapping of excess heat by the rising concentration of carbon dioxide
9 httphyperphysicsphy-astrgsueduhbasethermostefanhtmlc2
Heat Energy in transit EFEU May 2009
Renilde Nihoul 12 HUBrussel
in the atmosphere The carbon dioxide strongly absorbs infrared and does not allow as much of it to escape into space
Figure 9 The greenhouse effect10
A major part of the efficiency of the heating of an actual greenhouse is the trapping of the air so that the energy is not lost by convection Keeping the hot air from escaping out the top is part of the practical greenhouse effect but it is common usage to refer to the infrared trapping as the greenhouse effect in atmospheric applications where the air trapping is not applicable
The action of carbon dioxide and other greenhouse gases in trapping outgoing infrared energy from the Earth thereby warming the planet is called the greenhouse effect Those gas molecules in the Earths atmosphere with three or more atoms are called greenhouse gases The greenhouse gases include water vapour (H2O) ozone (O3) carbon dioxide (CO2) and methane (CH4) Also trace quantities of chlorofluorocarbons (CFCs) can have a disproportionately large effect Current analysis suggests that the combustion of fossil fuels is a major contributor to the increase in the carbon dioxide concentration It may measurably increase the overall average temperature of the Earth which could have disastrous consequences Because the potential consequences of global warming in terms of loss of snow cover sea level rise change in weather patterns etc are so great it is a major societal concern On the other hand proposed measures to reduce human contributions to greenhouse gases can also have great consequences For more information httphyperphysicsphy-astrgsueduhbasehframehtml
10 httphyperphysicsphy-astrgsueduhbasehframehtml
Heat Energy in transit EFEU May 2009
Renilde Nihoul 13 HUBrussel
72 Involving all three basic heat transfer mechanisms cooling of the human body
Figure 10 Cooling of the human body11
This is a simplified model of the process by which the human body gives off heat Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism One implication of the model is that radiation is the most important heat transfer mechanism at ordinary room temperatures This model indicates that an unclothed person at rest in a room temperature of 23 Celsius would be uncomfortably cool The skin temperature of 34 degC is a typical skin temperature taken from physiology texts compared to the normal core body temperature of 37 degC What will happen if the ambient temperature is above the skin temperature Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism This becomes a problem when the ambient temperature is above body temperature because all three standard heat transfer mechanisms work against this heat loss by transferring heat into the body
Figure 11 Cooling of the human body when the ambient temperature is above the skin temperature
11 httphyperphysicsphy-astrgsueduhbasethermocoobodhtmlc1
Heat Energy in transit EFEU May 2009
Renilde Nihoul 14 HUBrussel
Our ability to exist in such conditions comes from the efficiency of cooling by the evaporation of perspiration At a temperature of 45 Celsius the evaporation process must overcome the transfer of heat into the body and give off enough heat to accomplish a 90 watt net outward flow rate of energy Because of the bodys temperature regulation mechanisms the skin temperature would be expected to rise to 37degC at which point perspiration is initiated and increases until the evaporation cooling is sufficient to hold the skin at 37degC if possible With those assumptions about the temperatures one can calculate that there will be a net input power of 109 watts to the body (by radiation) The perspiration cooling must overcome that and produce the net outflow of 90 watts for equilibrium
8 Dealing with misconceptions by training the scientific method
As mentioned before some misconceptions about heat can stubbornly persist in pupils mind In order to explain how to deal with this misconceptions by training the scientific method we illustrate this concept with an example
81 Step 1 Formulation of a problem
A problem (of a daily life situation) can be introduced by the teacher or by a pupil For instance
I want to keep the tea as hot as possible as long as possible in a teapot Therefore I better choose a thick teapot A metal teapot hellip
82 Step 2 Formulation of workhypotheses
The pupils discuss the problem in small groups (3 ndash 4 pupils) and try to formulate workhypotheses The emphasis has to lay on the formulation of the hypotheses and not on the ldquocorrectrdquo formulation As a teacher you try to stimulate the discussion by asking questions as ldquoWhat exactly do you mean Why do you think that is How could you write this downhelliprdquo Don‟t give any information with respect to content Discuss in class all the preconceptions made by the pupils It is important they realise not everyone has the same preconceptions It is also important that they can practice to express their visions They have to become aware of their own visions
Summarize all the visions of the pupils
A metal teapot is better than a porcelain one to keep the tea hot
In a thicker teapot the tea will remain longer hot It doesn‟t matter which teapot you use (the tea will chill
anyway) hellip
Heat Energy in transit EFEU May 2009
Renilde Nihoul 15 HUBrussel
83 Step 3 Experimental verification
Normally the teacher thinks out the experiment However it is important that the pupils plan their own experiment so they see the experiment as a mean to answer questions and not only as a recipe they have to follow
At first they will use their sensory perceptions to plan the experiment Let them realise that these sensory perceptions are valuable but they give only vague and superficial information Then they ought to be able to translate the problem in measurable quantities All quantities that are important for the experiment need to be listed Some of these quantities need to be kept at a constant level during the experiment Other quantities need to be measured The pupils should estimate the magnitude of the results in advance so they can select the appropriate measuring instruments They can use the following scheme to think out their experiment
Workhypothesis What is the question
Description and outline of the experiment Which experiment can answer that question
Material list Which equipment is necessary
Procedure How exactly is it done
For the experiment with the teapot this could be resolved as follows
Tea can be replaced by boiling water In stead of a real teapot we can use cups of different materials
(with the same volume) To discover the effect of the thickness we use cups of the
same material but with a different thickness We measure the temperature with a thermometer the time with
a chronometer and the volume with a pipette
Procedure Fill the cups with equal volumes of boiling water (use the
different kind of cups) Measure the temperature of the water at different points of time
(every 15 seconds) Put the results in a convenient table and try to figure out which
cup is most suitable to keep the tea hot
Heat Energy in transit EFEU May 2009
Renilde Nihoul 16 HUBrussel
84 Step 4 Reflection
When the experiment is finished they have to select the work hypothesis confirmed by the experiment and give one‟s motives for this choice All the difficulties have to be put into words If their own vision is in conflict with the outcome of the experiment the pupils will be intellectually challenged
Selection of the hypothesis confirmed by the experiment a porcelain teapot will keep the tea longer hot than a metal teapot The thickness of the teapot is important The tea will chill in every teapot but the time it takes to do so will be different
Motivation of the selection
85 Step 5 Scientific explanation
The scientific explanation should start from the concrete problem from a conceptual approach and lead then to a mathematical formulation The scientific explanation should be told in a fascinating way so they will remember it longer
The better you prevent an energy exchange between the hot tea and the surrounding air the longer the tea remains hot
A metal pot is a very bad insulator (A complete explanation is not given here)
86 Conclusion
To deal with the existing misconceptions let the pupils make their own experiments By practicing a scientific method they will be able to discover the bdquotruth‟ The link between the physical principles and situations of daily life will become clearer They might be more interested in physics lessons as they understand they can use these theoretical principles to explain daily life problems Evaluate the concept afterwards (Evalution of your practice is a powerful tool for enhancing your teaching as well as their students learning) Did the strategie capture and retain the pupil‟s attention How effective was this in encouraging their scientific skills Hopefully the pupils will perceive themselves as participants in the journey of discovering and learning and as co-operating detectives looking for and evaluating evidence Let them share your feelings of curiosity enthusiasm and critical reflection
Heat Energy in transit EFEU May 2009
Renilde Nihoul 17 HUBrussel
9 References
Balck C Cocquyt B Van Peteghem R (2005) Misvattingen fysica te lijf Syllabus Universiteit Antwerpen ndash Centrum Nascholing Onderwijs
Giancoli DC (1999) Natuurkunde voor Wetenschap en Techniek deel II Golven en Geluid Kinetische Theorie en Thermodynamica Elektriciteit en Magnetisme Licht Academic Service Schoonhoven
Hathaway D (2008) Solar Physics NASAs Marshall Space Flight Center httpsolarsciencemsfcnasagov
NaveCR (2005) HyperPhysics Departement of Physics and Astronomy Georgia State University httphyperphysicsphy-astrgsueduhbasehframehtml
Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 7 HUBrussel
scenario is one of adding heat to a volume of gas and using the expansion of that gas to do work Therefore we prefer to use the first notation
6 The transfer of heat
The transfer of heat between a system and its environment can take place by three mechanisms conduction convection and radiation
61 Conduction
If you leave a metal poker in a fire for any length of time its handle will become hot Energy is transferred from the fire to the handle by conduction along the length of the metal shaft The atoms at the hot end by virtue of the high temperature at that end are vibrating with large amplitude These large vibrational amplitudes are passed along the shaft from atom to atom by interactions between adjacent atoms In this way a region of rising temperature travels along the shaft to your hand The energy will be transferred because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones The rate of conduction heat transfer between two plane surfaces can be calculated Consider a thin slab of homogeneous material of thickness d and cross-sectional area A The temperature is T + ΔT (Thot) on one face and T (Tcold) on the other The rate of heat flow through the slab is directly proportional to A (the more area available the more heat can flow per unit time) inversely proportional to d (the thicker the slab the less heat can flow per unit time) directly proportional to ΔT (the larger the temperature difference the more heat can flow per unit time) [To minimize the loss of heat from your house in winter make the surface area smaller (a two-story house is more efficient than a one-story house of the same total floor area) use thick walls filled with insulation and perhaps most important move to a warmer climate] Mathematically we can summarize this as
dTA
tQ=
With Q = heat transferred in time = Δt κ = thermal conductivity of the barrier A = area ΔT = temperature difference d = thickness of the barrier A substance with a large value of κ is a good heat conductor one with a small value of κ is a poor conductor or a good insulator In the case of solids the properties of materials that make them good electrical conductors (namely the ability of electrons to move
Heat Energy in transit EFEU May 2009
Renilde Nihoul 8 HUBrussel
relatively easily throughout the bulk of the material) also make them good thermal conductors Table 1 shows some representative values of κ
Material Conductivity κ (Wm middot K)
Metals
Stainless steel 14
Lead 35
Aluminium 235
Copper 401
Silver 428
Gases
Air 0026
Helium 015
Hydrogen 018
Building materials
Polyurethane foam 0024
Rock wool 0043
Fiberglas 0048
White pine 011
Window glass 10
Table 1 Some thermal conductivities5
Over the range of temperatures we normally encounter we can regard κ as a constant but over wide temperature ranges it does show a slight variation with T Gases transfer heat by direct collisions between molecules and as would be expected their thermal conductivity is low compared to most solids since they are dilute media
62 Convection
If you look at the flame of a candle or a match you are watching heat energy being transported upward by convection Heat transfer by convection occurs when a fluid such as air of water is in contact with an object whose temperature is higher than that of its surroundings The temperature of the fluid that is in contact with the hot object increases and (in most cases) the fluid expands Being less dense than the surrounding cooler fluid it rises because of buoyant forces
5 Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Heat Energy in transit EFEU May 2009
Renilde Nihoul 9 HUBrussel
Figure 5 Heat convection6
Convection can also lead to circulation in a liquid as in heating a pot of water over a flame Heated water expands and becomes more buoyant Cooler denser water near the surface descends and patterns of circulation can be formed
Figure 6 Convection currents7
Atmospheric convection plays a fundamental role in determining the global climate patterns and in our daily weather variations Glider pilots and condors alike seek the convective thermals that rising from the warmer Earth beneath keep them aloft Huge energy transfers take place within the oceans by the same process The outer region of the sun called the photosphere contains a vast array of convection cells that transport energy from the interior to the solar surface and give the surface a granulated appearance with a typical dimension of a granule being 1000 kilometres
6 httphyperphysicsphy-astrgsueduhbasethermoheatrahtmlc2 7 httpwwwphysicsarizonaedu
Heat Energy in transit EFEU May 2009
Renilde Nihoul 10 HUBrussel
Figure 7 The photosphere of the sun8
Heat-induced fluid motion in initially static fluids is known as free convection For cases where the fluid is already in motion heat conducted into the fluid will be transported away chiefly by fluid convection These cases known as forced convection require a pressure gradient to drive the fluid motion as opposed to a gravity gradient to induce motion through buoyancy It is difficult to quantify the effects of convection since it inherently depends upon small no uniformities in an otherwise fairly homogeneous medium
63 Radiation
Energy is carried from the sun to us by electromagnetic waves that travel freely through the near vacuum of the intervening space If you stand near a bonfire or an open fireplace you are warmed by the same process
8 httpsolarsciencemsfcnasagovimagesgranulesjpg
Heat Energy in transit EFEU May 2009
Renilde Nihoul 11 HUBrussel
Figure 8 The sun at 5800K and a campfire at 800K9
The sun at 5800 K and a hot campfire at perhaps 800 K give off radiation at a rate proportional to the 4th power of the temperature All objects emit such electromagnetic radiation because of their temperature and also absorb some radiation that falls on them from the objects The higher the temperature of an object the more it radiates The energy is carried by photons of light in the infrared and visible portions of the electromagnetic spectrum When temperatures are uniform the radiative flux between objects is in equilibrium and no net thermal energy is exchanged The balance is upset when temperatures are not uniform and thermal energy is transported from surfaces of higher to surfaces of lower temperatures
7 Heat Transfer Examples
71 With focus on radiation the Greenhouse effect
The greenhouse effect refers to circumstances where the short wavelengths of visible light from the sun pass through a transparent medium and are absorbed but the longer wavelengths of the infrared re-radiation from the heated objects are unable to pass through that medium The trapping of the long wavelength radiation leads to more heating and a higher resultant temperature Besides the heating of an automobile by sunlight through the windshield and the namesake example of heating the greenhouse by sunlight passing through sealed transparent windows the greenhouse effect has been widely used to describe the trapping of excess heat by the rising concentration of carbon dioxide
9 httphyperphysicsphy-astrgsueduhbasethermostefanhtmlc2
Heat Energy in transit EFEU May 2009
Renilde Nihoul 12 HUBrussel
in the atmosphere The carbon dioxide strongly absorbs infrared and does not allow as much of it to escape into space
Figure 9 The greenhouse effect10
A major part of the efficiency of the heating of an actual greenhouse is the trapping of the air so that the energy is not lost by convection Keeping the hot air from escaping out the top is part of the practical greenhouse effect but it is common usage to refer to the infrared trapping as the greenhouse effect in atmospheric applications where the air trapping is not applicable
The action of carbon dioxide and other greenhouse gases in trapping outgoing infrared energy from the Earth thereby warming the planet is called the greenhouse effect Those gas molecules in the Earths atmosphere with three or more atoms are called greenhouse gases The greenhouse gases include water vapour (H2O) ozone (O3) carbon dioxide (CO2) and methane (CH4) Also trace quantities of chlorofluorocarbons (CFCs) can have a disproportionately large effect Current analysis suggests that the combustion of fossil fuels is a major contributor to the increase in the carbon dioxide concentration It may measurably increase the overall average temperature of the Earth which could have disastrous consequences Because the potential consequences of global warming in terms of loss of snow cover sea level rise change in weather patterns etc are so great it is a major societal concern On the other hand proposed measures to reduce human contributions to greenhouse gases can also have great consequences For more information httphyperphysicsphy-astrgsueduhbasehframehtml
10 httphyperphysicsphy-astrgsueduhbasehframehtml
Heat Energy in transit EFEU May 2009
Renilde Nihoul 13 HUBrussel
72 Involving all three basic heat transfer mechanisms cooling of the human body
Figure 10 Cooling of the human body11
This is a simplified model of the process by which the human body gives off heat Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism One implication of the model is that radiation is the most important heat transfer mechanism at ordinary room temperatures This model indicates that an unclothed person at rest in a room temperature of 23 Celsius would be uncomfortably cool The skin temperature of 34 degC is a typical skin temperature taken from physiology texts compared to the normal core body temperature of 37 degC What will happen if the ambient temperature is above the skin temperature Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism This becomes a problem when the ambient temperature is above body temperature because all three standard heat transfer mechanisms work against this heat loss by transferring heat into the body
Figure 11 Cooling of the human body when the ambient temperature is above the skin temperature
11 httphyperphysicsphy-astrgsueduhbasethermocoobodhtmlc1
Heat Energy in transit EFEU May 2009
Renilde Nihoul 14 HUBrussel
Our ability to exist in such conditions comes from the efficiency of cooling by the evaporation of perspiration At a temperature of 45 Celsius the evaporation process must overcome the transfer of heat into the body and give off enough heat to accomplish a 90 watt net outward flow rate of energy Because of the bodys temperature regulation mechanisms the skin temperature would be expected to rise to 37degC at which point perspiration is initiated and increases until the evaporation cooling is sufficient to hold the skin at 37degC if possible With those assumptions about the temperatures one can calculate that there will be a net input power of 109 watts to the body (by radiation) The perspiration cooling must overcome that and produce the net outflow of 90 watts for equilibrium
8 Dealing with misconceptions by training the scientific method
As mentioned before some misconceptions about heat can stubbornly persist in pupils mind In order to explain how to deal with this misconceptions by training the scientific method we illustrate this concept with an example
81 Step 1 Formulation of a problem
A problem (of a daily life situation) can be introduced by the teacher or by a pupil For instance
I want to keep the tea as hot as possible as long as possible in a teapot Therefore I better choose a thick teapot A metal teapot hellip
82 Step 2 Formulation of workhypotheses
The pupils discuss the problem in small groups (3 ndash 4 pupils) and try to formulate workhypotheses The emphasis has to lay on the formulation of the hypotheses and not on the ldquocorrectrdquo formulation As a teacher you try to stimulate the discussion by asking questions as ldquoWhat exactly do you mean Why do you think that is How could you write this downhelliprdquo Don‟t give any information with respect to content Discuss in class all the preconceptions made by the pupils It is important they realise not everyone has the same preconceptions It is also important that they can practice to express their visions They have to become aware of their own visions
Summarize all the visions of the pupils
A metal teapot is better than a porcelain one to keep the tea hot
In a thicker teapot the tea will remain longer hot It doesn‟t matter which teapot you use (the tea will chill
anyway) hellip
Heat Energy in transit EFEU May 2009
Renilde Nihoul 15 HUBrussel
83 Step 3 Experimental verification
Normally the teacher thinks out the experiment However it is important that the pupils plan their own experiment so they see the experiment as a mean to answer questions and not only as a recipe they have to follow
At first they will use their sensory perceptions to plan the experiment Let them realise that these sensory perceptions are valuable but they give only vague and superficial information Then they ought to be able to translate the problem in measurable quantities All quantities that are important for the experiment need to be listed Some of these quantities need to be kept at a constant level during the experiment Other quantities need to be measured The pupils should estimate the magnitude of the results in advance so they can select the appropriate measuring instruments They can use the following scheme to think out their experiment
Workhypothesis What is the question
Description and outline of the experiment Which experiment can answer that question
Material list Which equipment is necessary
Procedure How exactly is it done
For the experiment with the teapot this could be resolved as follows
Tea can be replaced by boiling water In stead of a real teapot we can use cups of different materials
(with the same volume) To discover the effect of the thickness we use cups of the
same material but with a different thickness We measure the temperature with a thermometer the time with
a chronometer and the volume with a pipette
Procedure Fill the cups with equal volumes of boiling water (use the
different kind of cups) Measure the temperature of the water at different points of time
(every 15 seconds) Put the results in a convenient table and try to figure out which
cup is most suitable to keep the tea hot
Heat Energy in transit EFEU May 2009
Renilde Nihoul 16 HUBrussel
84 Step 4 Reflection
When the experiment is finished they have to select the work hypothesis confirmed by the experiment and give one‟s motives for this choice All the difficulties have to be put into words If their own vision is in conflict with the outcome of the experiment the pupils will be intellectually challenged
Selection of the hypothesis confirmed by the experiment a porcelain teapot will keep the tea longer hot than a metal teapot The thickness of the teapot is important The tea will chill in every teapot but the time it takes to do so will be different
Motivation of the selection
85 Step 5 Scientific explanation
The scientific explanation should start from the concrete problem from a conceptual approach and lead then to a mathematical formulation The scientific explanation should be told in a fascinating way so they will remember it longer
The better you prevent an energy exchange between the hot tea and the surrounding air the longer the tea remains hot
A metal pot is a very bad insulator (A complete explanation is not given here)
86 Conclusion
To deal with the existing misconceptions let the pupils make their own experiments By practicing a scientific method they will be able to discover the bdquotruth‟ The link between the physical principles and situations of daily life will become clearer They might be more interested in physics lessons as they understand they can use these theoretical principles to explain daily life problems Evaluate the concept afterwards (Evalution of your practice is a powerful tool for enhancing your teaching as well as their students learning) Did the strategie capture and retain the pupil‟s attention How effective was this in encouraging their scientific skills Hopefully the pupils will perceive themselves as participants in the journey of discovering and learning and as co-operating detectives looking for and evaluating evidence Let them share your feelings of curiosity enthusiasm and critical reflection
Heat Energy in transit EFEU May 2009
Renilde Nihoul 17 HUBrussel
9 References
Balck C Cocquyt B Van Peteghem R (2005) Misvattingen fysica te lijf Syllabus Universiteit Antwerpen ndash Centrum Nascholing Onderwijs
Giancoli DC (1999) Natuurkunde voor Wetenschap en Techniek deel II Golven en Geluid Kinetische Theorie en Thermodynamica Elektriciteit en Magnetisme Licht Academic Service Schoonhoven
Hathaway D (2008) Solar Physics NASAs Marshall Space Flight Center httpsolarsciencemsfcnasagov
NaveCR (2005) HyperPhysics Departement of Physics and Astronomy Georgia State University httphyperphysicsphy-astrgsueduhbasehframehtml
Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 8 HUBrussel
relatively easily throughout the bulk of the material) also make them good thermal conductors Table 1 shows some representative values of κ
Material Conductivity κ (Wm middot K)
Metals
Stainless steel 14
Lead 35
Aluminium 235
Copper 401
Silver 428
Gases
Air 0026
Helium 015
Hydrogen 018
Building materials
Polyurethane foam 0024
Rock wool 0043
Fiberglas 0048
White pine 011
Window glass 10
Table 1 Some thermal conductivities5
Over the range of temperatures we normally encounter we can regard κ as a constant but over wide temperature ranges it does show a slight variation with T Gases transfer heat by direct collisions between molecules and as would be expected their thermal conductivity is low compared to most solids since they are dilute media
62 Convection
If you look at the flame of a candle or a match you are watching heat energy being transported upward by convection Heat transfer by convection occurs when a fluid such as air of water is in contact with an object whose temperature is higher than that of its surroundings The temperature of the fluid that is in contact with the hot object increases and (in most cases) the fluid expands Being less dense than the surrounding cooler fluid it rises because of buoyant forces
5 Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Heat Energy in transit EFEU May 2009
Renilde Nihoul 9 HUBrussel
Figure 5 Heat convection6
Convection can also lead to circulation in a liquid as in heating a pot of water over a flame Heated water expands and becomes more buoyant Cooler denser water near the surface descends and patterns of circulation can be formed
Figure 6 Convection currents7
Atmospheric convection plays a fundamental role in determining the global climate patterns and in our daily weather variations Glider pilots and condors alike seek the convective thermals that rising from the warmer Earth beneath keep them aloft Huge energy transfers take place within the oceans by the same process The outer region of the sun called the photosphere contains a vast array of convection cells that transport energy from the interior to the solar surface and give the surface a granulated appearance with a typical dimension of a granule being 1000 kilometres
6 httphyperphysicsphy-astrgsueduhbasethermoheatrahtmlc2 7 httpwwwphysicsarizonaedu
Heat Energy in transit EFEU May 2009
Renilde Nihoul 10 HUBrussel
Figure 7 The photosphere of the sun8
Heat-induced fluid motion in initially static fluids is known as free convection For cases where the fluid is already in motion heat conducted into the fluid will be transported away chiefly by fluid convection These cases known as forced convection require a pressure gradient to drive the fluid motion as opposed to a gravity gradient to induce motion through buoyancy It is difficult to quantify the effects of convection since it inherently depends upon small no uniformities in an otherwise fairly homogeneous medium
63 Radiation
Energy is carried from the sun to us by electromagnetic waves that travel freely through the near vacuum of the intervening space If you stand near a bonfire or an open fireplace you are warmed by the same process
8 httpsolarsciencemsfcnasagovimagesgranulesjpg
Heat Energy in transit EFEU May 2009
Renilde Nihoul 11 HUBrussel
Figure 8 The sun at 5800K and a campfire at 800K9
The sun at 5800 K and a hot campfire at perhaps 800 K give off radiation at a rate proportional to the 4th power of the temperature All objects emit such electromagnetic radiation because of their temperature and also absorb some radiation that falls on them from the objects The higher the temperature of an object the more it radiates The energy is carried by photons of light in the infrared and visible portions of the electromagnetic spectrum When temperatures are uniform the radiative flux between objects is in equilibrium and no net thermal energy is exchanged The balance is upset when temperatures are not uniform and thermal energy is transported from surfaces of higher to surfaces of lower temperatures
7 Heat Transfer Examples
71 With focus on radiation the Greenhouse effect
The greenhouse effect refers to circumstances where the short wavelengths of visible light from the sun pass through a transparent medium and are absorbed but the longer wavelengths of the infrared re-radiation from the heated objects are unable to pass through that medium The trapping of the long wavelength radiation leads to more heating and a higher resultant temperature Besides the heating of an automobile by sunlight through the windshield and the namesake example of heating the greenhouse by sunlight passing through sealed transparent windows the greenhouse effect has been widely used to describe the trapping of excess heat by the rising concentration of carbon dioxide
9 httphyperphysicsphy-astrgsueduhbasethermostefanhtmlc2
Heat Energy in transit EFEU May 2009
Renilde Nihoul 12 HUBrussel
in the atmosphere The carbon dioxide strongly absorbs infrared and does not allow as much of it to escape into space
Figure 9 The greenhouse effect10
A major part of the efficiency of the heating of an actual greenhouse is the trapping of the air so that the energy is not lost by convection Keeping the hot air from escaping out the top is part of the practical greenhouse effect but it is common usage to refer to the infrared trapping as the greenhouse effect in atmospheric applications where the air trapping is not applicable
The action of carbon dioxide and other greenhouse gases in trapping outgoing infrared energy from the Earth thereby warming the planet is called the greenhouse effect Those gas molecules in the Earths atmosphere with three or more atoms are called greenhouse gases The greenhouse gases include water vapour (H2O) ozone (O3) carbon dioxide (CO2) and methane (CH4) Also trace quantities of chlorofluorocarbons (CFCs) can have a disproportionately large effect Current analysis suggests that the combustion of fossil fuels is a major contributor to the increase in the carbon dioxide concentration It may measurably increase the overall average temperature of the Earth which could have disastrous consequences Because the potential consequences of global warming in terms of loss of snow cover sea level rise change in weather patterns etc are so great it is a major societal concern On the other hand proposed measures to reduce human contributions to greenhouse gases can also have great consequences For more information httphyperphysicsphy-astrgsueduhbasehframehtml
10 httphyperphysicsphy-astrgsueduhbasehframehtml
Heat Energy in transit EFEU May 2009
Renilde Nihoul 13 HUBrussel
72 Involving all three basic heat transfer mechanisms cooling of the human body
Figure 10 Cooling of the human body11
This is a simplified model of the process by which the human body gives off heat Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism One implication of the model is that radiation is the most important heat transfer mechanism at ordinary room temperatures This model indicates that an unclothed person at rest in a room temperature of 23 Celsius would be uncomfortably cool The skin temperature of 34 degC is a typical skin temperature taken from physiology texts compared to the normal core body temperature of 37 degC What will happen if the ambient temperature is above the skin temperature Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism This becomes a problem when the ambient temperature is above body temperature because all three standard heat transfer mechanisms work against this heat loss by transferring heat into the body
Figure 11 Cooling of the human body when the ambient temperature is above the skin temperature
11 httphyperphysicsphy-astrgsueduhbasethermocoobodhtmlc1
Heat Energy in transit EFEU May 2009
Renilde Nihoul 14 HUBrussel
Our ability to exist in such conditions comes from the efficiency of cooling by the evaporation of perspiration At a temperature of 45 Celsius the evaporation process must overcome the transfer of heat into the body and give off enough heat to accomplish a 90 watt net outward flow rate of energy Because of the bodys temperature regulation mechanisms the skin temperature would be expected to rise to 37degC at which point perspiration is initiated and increases until the evaporation cooling is sufficient to hold the skin at 37degC if possible With those assumptions about the temperatures one can calculate that there will be a net input power of 109 watts to the body (by radiation) The perspiration cooling must overcome that and produce the net outflow of 90 watts for equilibrium
8 Dealing with misconceptions by training the scientific method
As mentioned before some misconceptions about heat can stubbornly persist in pupils mind In order to explain how to deal with this misconceptions by training the scientific method we illustrate this concept with an example
81 Step 1 Formulation of a problem
A problem (of a daily life situation) can be introduced by the teacher or by a pupil For instance
I want to keep the tea as hot as possible as long as possible in a teapot Therefore I better choose a thick teapot A metal teapot hellip
82 Step 2 Formulation of workhypotheses
The pupils discuss the problem in small groups (3 ndash 4 pupils) and try to formulate workhypotheses The emphasis has to lay on the formulation of the hypotheses and not on the ldquocorrectrdquo formulation As a teacher you try to stimulate the discussion by asking questions as ldquoWhat exactly do you mean Why do you think that is How could you write this downhelliprdquo Don‟t give any information with respect to content Discuss in class all the preconceptions made by the pupils It is important they realise not everyone has the same preconceptions It is also important that they can practice to express their visions They have to become aware of their own visions
Summarize all the visions of the pupils
A metal teapot is better than a porcelain one to keep the tea hot
In a thicker teapot the tea will remain longer hot It doesn‟t matter which teapot you use (the tea will chill
anyway) hellip
Heat Energy in transit EFEU May 2009
Renilde Nihoul 15 HUBrussel
83 Step 3 Experimental verification
Normally the teacher thinks out the experiment However it is important that the pupils plan their own experiment so they see the experiment as a mean to answer questions and not only as a recipe they have to follow
At first they will use their sensory perceptions to plan the experiment Let them realise that these sensory perceptions are valuable but they give only vague and superficial information Then they ought to be able to translate the problem in measurable quantities All quantities that are important for the experiment need to be listed Some of these quantities need to be kept at a constant level during the experiment Other quantities need to be measured The pupils should estimate the magnitude of the results in advance so they can select the appropriate measuring instruments They can use the following scheme to think out their experiment
Workhypothesis What is the question
Description and outline of the experiment Which experiment can answer that question
Material list Which equipment is necessary
Procedure How exactly is it done
For the experiment with the teapot this could be resolved as follows
Tea can be replaced by boiling water In stead of a real teapot we can use cups of different materials
(with the same volume) To discover the effect of the thickness we use cups of the
same material but with a different thickness We measure the temperature with a thermometer the time with
a chronometer and the volume with a pipette
Procedure Fill the cups with equal volumes of boiling water (use the
different kind of cups) Measure the temperature of the water at different points of time
(every 15 seconds) Put the results in a convenient table and try to figure out which
cup is most suitable to keep the tea hot
Heat Energy in transit EFEU May 2009
Renilde Nihoul 16 HUBrussel
84 Step 4 Reflection
When the experiment is finished they have to select the work hypothesis confirmed by the experiment and give one‟s motives for this choice All the difficulties have to be put into words If their own vision is in conflict with the outcome of the experiment the pupils will be intellectually challenged
Selection of the hypothesis confirmed by the experiment a porcelain teapot will keep the tea longer hot than a metal teapot The thickness of the teapot is important The tea will chill in every teapot but the time it takes to do so will be different
Motivation of the selection
85 Step 5 Scientific explanation
The scientific explanation should start from the concrete problem from a conceptual approach and lead then to a mathematical formulation The scientific explanation should be told in a fascinating way so they will remember it longer
The better you prevent an energy exchange between the hot tea and the surrounding air the longer the tea remains hot
A metal pot is a very bad insulator (A complete explanation is not given here)
86 Conclusion
To deal with the existing misconceptions let the pupils make their own experiments By practicing a scientific method they will be able to discover the bdquotruth‟ The link between the physical principles and situations of daily life will become clearer They might be more interested in physics lessons as they understand they can use these theoretical principles to explain daily life problems Evaluate the concept afterwards (Evalution of your practice is a powerful tool for enhancing your teaching as well as their students learning) Did the strategie capture and retain the pupil‟s attention How effective was this in encouraging their scientific skills Hopefully the pupils will perceive themselves as participants in the journey of discovering and learning and as co-operating detectives looking for and evaluating evidence Let them share your feelings of curiosity enthusiasm and critical reflection
Heat Energy in transit EFEU May 2009
Renilde Nihoul 17 HUBrussel
9 References
Balck C Cocquyt B Van Peteghem R (2005) Misvattingen fysica te lijf Syllabus Universiteit Antwerpen ndash Centrum Nascholing Onderwijs
Giancoli DC (1999) Natuurkunde voor Wetenschap en Techniek deel II Golven en Geluid Kinetische Theorie en Thermodynamica Elektriciteit en Magnetisme Licht Academic Service Schoonhoven
Hathaway D (2008) Solar Physics NASAs Marshall Space Flight Center httpsolarsciencemsfcnasagov
NaveCR (2005) HyperPhysics Departement of Physics and Astronomy Georgia State University httphyperphysicsphy-astrgsueduhbasehframehtml
Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 9 HUBrussel
Figure 5 Heat convection6
Convection can also lead to circulation in a liquid as in heating a pot of water over a flame Heated water expands and becomes more buoyant Cooler denser water near the surface descends and patterns of circulation can be formed
Figure 6 Convection currents7
Atmospheric convection plays a fundamental role in determining the global climate patterns and in our daily weather variations Glider pilots and condors alike seek the convective thermals that rising from the warmer Earth beneath keep them aloft Huge energy transfers take place within the oceans by the same process The outer region of the sun called the photosphere contains a vast array of convection cells that transport energy from the interior to the solar surface and give the surface a granulated appearance with a typical dimension of a granule being 1000 kilometres
6 httphyperphysicsphy-astrgsueduhbasethermoheatrahtmlc2 7 httpwwwphysicsarizonaedu
Heat Energy in transit EFEU May 2009
Renilde Nihoul 10 HUBrussel
Figure 7 The photosphere of the sun8
Heat-induced fluid motion in initially static fluids is known as free convection For cases where the fluid is already in motion heat conducted into the fluid will be transported away chiefly by fluid convection These cases known as forced convection require a pressure gradient to drive the fluid motion as opposed to a gravity gradient to induce motion through buoyancy It is difficult to quantify the effects of convection since it inherently depends upon small no uniformities in an otherwise fairly homogeneous medium
63 Radiation
Energy is carried from the sun to us by electromagnetic waves that travel freely through the near vacuum of the intervening space If you stand near a bonfire or an open fireplace you are warmed by the same process
8 httpsolarsciencemsfcnasagovimagesgranulesjpg
Heat Energy in transit EFEU May 2009
Renilde Nihoul 11 HUBrussel
Figure 8 The sun at 5800K and a campfire at 800K9
The sun at 5800 K and a hot campfire at perhaps 800 K give off radiation at a rate proportional to the 4th power of the temperature All objects emit such electromagnetic radiation because of their temperature and also absorb some radiation that falls on them from the objects The higher the temperature of an object the more it radiates The energy is carried by photons of light in the infrared and visible portions of the electromagnetic spectrum When temperatures are uniform the radiative flux between objects is in equilibrium and no net thermal energy is exchanged The balance is upset when temperatures are not uniform and thermal energy is transported from surfaces of higher to surfaces of lower temperatures
7 Heat Transfer Examples
71 With focus on radiation the Greenhouse effect
The greenhouse effect refers to circumstances where the short wavelengths of visible light from the sun pass through a transparent medium and are absorbed but the longer wavelengths of the infrared re-radiation from the heated objects are unable to pass through that medium The trapping of the long wavelength radiation leads to more heating and a higher resultant temperature Besides the heating of an automobile by sunlight through the windshield and the namesake example of heating the greenhouse by sunlight passing through sealed transparent windows the greenhouse effect has been widely used to describe the trapping of excess heat by the rising concentration of carbon dioxide
9 httphyperphysicsphy-astrgsueduhbasethermostefanhtmlc2
Heat Energy in transit EFEU May 2009
Renilde Nihoul 12 HUBrussel
in the atmosphere The carbon dioxide strongly absorbs infrared and does not allow as much of it to escape into space
Figure 9 The greenhouse effect10
A major part of the efficiency of the heating of an actual greenhouse is the trapping of the air so that the energy is not lost by convection Keeping the hot air from escaping out the top is part of the practical greenhouse effect but it is common usage to refer to the infrared trapping as the greenhouse effect in atmospheric applications where the air trapping is not applicable
The action of carbon dioxide and other greenhouse gases in trapping outgoing infrared energy from the Earth thereby warming the planet is called the greenhouse effect Those gas molecules in the Earths atmosphere with three or more atoms are called greenhouse gases The greenhouse gases include water vapour (H2O) ozone (O3) carbon dioxide (CO2) and methane (CH4) Also trace quantities of chlorofluorocarbons (CFCs) can have a disproportionately large effect Current analysis suggests that the combustion of fossil fuels is a major contributor to the increase in the carbon dioxide concentration It may measurably increase the overall average temperature of the Earth which could have disastrous consequences Because the potential consequences of global warming in terms of loss of snow cover sea level rise change in weather patterns etc are so great it is a major societal concern On the other hand proposed measures to reduce human contributions to greenhouse gases can also have great consequences For more information httphyperphysicsphy-astrgsueduhbasehframehtml
10 httphyperphysicsphy-astrgsueduhbasehframehtml
Heat Energy in transit EFEU May 2009
Renilde Nihoul 13 HUBrussel
72 Involving all three basic heat transfer mechanisms cooling of the human body
Figure 10 Cooling of the human body11
This is a simplified model of the process by which the human body gives off heat Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism One implication of the model is that radiation is the most important heat transfer mechanism at ordinary room temperatures This model indicates that an unclothed person at rest in a room temperature of 23 Celsius would be uncomfortably cool The skin temperature of 34 degC is a typical skin temperature taken from physiology texts compared to the normal core body temperature of 37 degC What will happen if the ambient temperature is above the skin temperature Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism This becomes a problem when the ambient temperature is above body temperature because all three standard heat transfer mechanisms work against this heat loss by transferring heat into the body
Figure 11 Cooling of the human body when the ambient temperature is above the skin temperature
11 httphyperphysicsphy-astrgsueduhbasethermocoobodhtmlc1
Heat Energy in transit EFEU May 2009
Renilde Nihoul 14 HUBrussel
Our ability to exist in such conditions comes from the efficiency of cooling by the evaporation of perspiration At a temperature of 45 Celsius the evaporation process must overcome the transfer of heat into the body and give off enough heat to accomplish a 90 watt net outward flow rate of energy Because of the bodys temperature regulation mechanisms the skin temperature would be expected to rise to 37degC at which point perspiration is initiated and increases until the evaporation cooling is sufficient to hold the skin at 37degC if possible With those assumptions about the temperatures one can calculate that there will be a net input power of 109 watts to the body (by radiation) The perspiration cooling must overcome that and produce the net outflow of 90 watts for equilibrium
8 Dealing with misconceptions by training the scientific method
As mentioned before some misconceptions about heat can stubbornly persist in pupils mind In order to explain how to deal with this misconceptions by training the scientific method we illustrate this concept with an example
81 Step 1 Formulation of a problem
A problem (of a daily life situation) can be introduced by the teacher or by a pupil For instance
I want to keep the tea as hot as possible as long as possible in a teapot Therefore I better choose a thick teapot A metal teapot hellip
82 Step 2 Formulation of workhypotheses
The pupils discuss the problem in small groups (3 ndash 4 pupils) and try to formulate workhypotheses The emphasis has to lay on the formulation of the hypotheses and not on the ldquocorrectrdquo formulation As a teacher you try to stimulate the discussion by asking questions as ldquoWhat exactly do you mean Why do you think that is How could you write this downhelliprdquo Don‟t give any information with respect to content Discuss in class all the preconceptions made by the pupils It is important they realise not everyone has the same preconceptions It is also important that they can practice to express their visions They have to become aware of their own visions
Summarize all the visions of the pupils
A metal teapot is better than a porcelain one to keep the tea hot
In a thicker teapot the tea will remain longer hot It doesn‟t matter which teapot you use (the tea will chill
anyway) hellip
Heat Energy in transit EFEU May 2009
Renilde Nihoul 15 HUBrussel
83 Step 3 Experimental verification
Normally the teacher thinks out the experiment However it is important that the pupils plan their own experiment so they see the experiment as a mean to answer questions and not only as a recipe they have to follow
At first they will use their sensory perceptions to plan the experiment Let them realise that these sensory perceptions are valuable but they give only vague and superficial information Then they ought to be able to translate the problem in measurable quantities All quantities that are important for the experiment need to be listed Some of these quantities need to be kept at a constant level during the experiment Other quantities need to be measured The pupils should estimate the magnitude of the results in advance so they can select the appropriate measuring instruments They can use the following scheme to think out their experiment
Workhypothesis What is the question
Description and outline of the experiment Which experiment can answer that question
Material list Which equipment is necessary
Procedure How exactly is it done
For the experiment with the teapot this could be resolved as follows
Tea can be replaced by boiling water In stead of a real teapot we can use cups of different materials
(with the same volume) To discover the effect of the thickness we use cups of the
same material but with a different thickness We measure the temperature with a thermometer the time with
a chronometer and the volume with a pipette
Procedure Fill the cups with equal volumes of boiling water (use the
different kind of cups) Measure the temperature of the water at different points of time
(every 15 seconds) Put the results in a convenient table and try to figure out which
cup is most suitable to keep the tea hot
Heat Energy in transit EFEU May 2009
Renilde Nihoul 16 HUBrussel
84 Step 4 Reflection
When the experiment is finished they have to select the work hypothesis confirmed by the experiment and give one‟s motives for this choice All the difficulties have to be put into words If their own vision is in conflict with the outcome of the experiment the pupils will be intellectually challenged
Selection of the hypothesis confirmed by the experiment a porcelain teapot will keep the tea longer hot than a metal teapot The thickness of the teapot is important The tea will chill in every teapot but the time it takes to do so will be different
Motivation of the selection
85 Step 5 Scientific explanation
The scientific explanation should start from the concrete problem from a conceptual approach and lead then to a mathematical formulation The scientific explanation should be told in a fascinating way so they will remember it longer
The better you prevent an energy exchange between the hot tea and the surrounding air the longer the tea remains hot
A metal pot is a very bad insulator (A complete explanation is not given here)
86 Conclusion
To deal with the existing misconceptions let the pupils make their own experiments By practicing a scientific method they will be able to discover the bdquotruth‟ The link between the physical principles and situations of daily life will become clearer They might be more interested in physics lessons as they understand they can use these theoretical principles to explain daily life problems Evaluate the concept afterwards (Evalution of your practice is a powerful tool for enhancing your teaching as well as their students learning) Did the strategie capture and retain the pupil‟s attention How effective was this in encouraging their scientific skills Hopefully the pupils will perceive themselves as participants in the journey of discovering and learning and as co-operating detectives looking for and evaluating evidence Let them share your feelings of curiosity enthusiasm and critical reflection
Heat Energy in transit EFEU May 2009
Renilde Nihoul 17 HUBrussel
9 References
Balck C Cocquyt B Van Peteghem R (2005) Misvattingen fysica te lijf Syllabus Universiteit Antwerpen ndash Centrum Nascholing Onderwijs
Giancoli DC (1999) Natuurkunde voor Wetenschap en Techniek deel II Golven en Geluid Kinetische Theorie en Thermodynamica Elektriciteit en Magnetisme Licht Academic Service Schoonhoven
Hathaway D (2008) Solar Physics NASAs Marshall Space Flight Center httpsolarsciencemsfcnasagov
NaveCR (2005) HyperPhysics Departement of Physics and Astronomy Georgia State University httphyperphysicsphy-astrgsueduhbasehframehtml
Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 10 HUBrussel
Figure 7 The photosphere of the sun8
Heat-induced fluid motion in initially static fluids is known as free convection For cases where the fluid is already in motion heat conducted into the fluid will be transported away chiefly by fluid convection These cases known as forced convection require a pressure gradient to drive the fluid motion as opposed to a gravity gradient to induce motion through buoyancy It is difficult to quantify the effects of convection since it inherently depends upon small no uniformities in an otherwise fairly homogeneous medium
63 Radiation
Energy is carried from the sun to us by electromagnetic waves that travel freely through the near vacuum of the intervening space If you stand near a bonfire or an open fireplace you are warmed by the same process
8 httpsolarsciencemsfcnasagovimagesgranulesjpg
Heat Energy in transit EFEU May 2009
Renilde Nihoul 11 HUBrussel
Figure 8 The sun at 5800K and a campfire at 800K9
The sun at 5800 K and a hot campfire at perhaps 800 K give off radiation at a rate proportional to the 4th power of the temperature All objects emit such electromagnetic radiation because of their temperature and also absorb some radiation that falls on them from the objects The higher the temperature of an object the more it radiates The energy is carried by photons of light in the infrared and visible portions of the electromagnetic spectrum When temperatures are uniform the radiative flux between objects is in equilibrium and no net thermal energy is exchanged The balance is upset when temperatures are not uniform and thermal energy is transported from surfaces of higher to surfaces of lower temperatures
7 Heat Transfer Examples
71 With focus on radiation the Greenhouse effect
The greenhouse effect refers to circumstances where the short wavelengths of visible light from the sun pass through a transparent medium and are absorbed but the longer wavelengths of the infrared re-radiation from the heated objects are unable to pass through that medium The trapping of the long wavelength radiation leads to more heating and a higher resultant temperature Besides the heating of an automobile by sunlight through the windshield and the namesake example of heating the greenhouse by sunlight passing through sealed transparent windows the greenhouse effect has been widely used to describe the trapping of excess heat by the rising concentration of carbon dioxide
9 httphyperphysicsphy-astrgsueduhbasethermostefanhtmlc2
Heat Energy in transit EFEU May 2009
Renilde Nihoul 12 HUBrussel
in the atmosphere The carbon dioxide strongly absorbs infrared and does not allow as much of it to escape into space
Figure 9 The greenhouse effect10
A major part of the efficiency of the heating of an actual greenhouse is the trapping of the air so that the energy is not lost by convection Keeping the hot air from escaping out the top is part of the practical greenhouse effect but it is common usage to refer to the infrared trapping as the greenhouse effect in atmospheric applications where the air trapping is not applicable
The action of carbon dioxide and other greenhouse gases in trapping outgoing infrared energy from the Earth thereby warming the planet is called the greenhouse effect Those gas molecules in the Earths atmosphere with three or more atoms are called greenhouse gases The greenhouse gases include water vapour (H2O) ozone (O3) carbon dioxide (CO2) and methane (CH4) Also trace quantities of chlorofluorocarbons (CFCs) can have a disproportionately large effect Current analysis suggests that the combustion of fossil fuels is a major contributor to the increase in the carbon dioxide concentration It may measurably increase the overall average temperature of the Earth which could have disastrous consequences Because the potential consequences of global warming in terms of loss of snow cover sea level rise change in weather patterns etc are so great it is a major societal concern On the other hand proposed measures to reduce human contributions to greenhouse gases can also have great consequences For more information httphyperphysicsphy-astrgsueduhbasehframehtml
10 httphyperphysicsphy-astrgsueduhbasehframehtml
Heat Energy in transit EFEU May 2009
Renilde Nihoul 13 HUBrussel
72 Involving all three basic heat transfer mechanisms cooling of the human body
Figure 10 Cooling of the human body11
This is a simplified model of the process by which the human body gives off heat Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism One implication of the model is that radiation is the most important heat transfer mechanism at ordinary room temperatures This model indicates that an unclothed person at rest in a room temperature of 23 Celsius would be uncomfortably cool The skin temperature of 34 degC is a typical skin temperature taken from physiology texts compared to the normal core body temperature of 37 degC What will happen if the ambient temperature is above the skin temperature Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism This becomes a problem when the ambient temperature is above body temperature because all three standard heat transfer mechanisms work against this heat loss by transferring heat into the body
Figure 11 Cooling of the human body when the ambient temperature is above the skin temperature
11 httphyperphysicsphy-astrgsueduhbasethermocoobodhtmlc1
Heat Energy in transit EFEU May 2009
Renilde Nihoul 14 HUBrussel
Our ability to exist in such conditions comes from the efficiency of cooling by the evaporation of perspiration At a temperature of 45 Celsius the evaporation process must overcome the transfer of heat into the body and give off enough heat to accomplish a 90 watt net outward flow rate of energy Because of the bodys temperature regulation mechanisms the skin temperature would be expected to rise to 37degC at which point perspiration is initiated and increases until the evaporation cooling is sufficient to hold the skin at 37degC if possible With those assumptions about the temperatures one can calculate that there will be a net input power of 109 watts to the body (by radiation) The perspiration cooling must overcome that and produce the net outflow of 90 watts for equilibrium
8 Dealing with misconceptions by training the scientific method
As mentioned before some misconceptions about heat can stubbornly persist in pupils mind In order to explain how to deal with this misconceptions by training the scientific method we illustrate this concept with an example
81 Step 1 Formulation of a problem
A problem (of a daily life situation) can be introduced by the teacher or by a pupil For instance
I want to keep the tea as hot as possible as long as possible in a teapot Therefore I better choose a thick teapot A metal teapot hellip
82 Step 2 Formulation of workhypotheses
The pupils discuss the problem in small groups (3 ndash 4 pupils) and try to formulate workhypotheses The emphasis has to lay on the formulation of the hypotheses and not on the ldquocorrectrdquo formulation As a teacher you try to stimulate the discussion by asking questions as ldquoWhat exactly do you mean Why do you think that is How could you write this downhelliprdquo Don‟t give any information with respect to content Discuss in class all the preconceptions made by the pupils It is important they realise not everyone has the same preconceptions It is also important that they can practice to express their visions They have to become aware of their own visions
Summarize all the visions of the pupils
A metal teapot is better than a porcelain one to keep the tea hot
In a thicker teapot the tea will remain longer hot It doesn‟t matter which teapot you use (the tea will chill
anyway) hellip
Heat Energy in transit EFEU May 2009
Renilde Nihoul 15 HUBrussel
83 Step 3 Experimental verification
Normally the teacher thinks out the experiment However it is important that the pupils plan their own experiment so they see the experiment as a mean to answer questions and not only as a recipe they have to follow
At first they will use their sensory perceptions to plan the experiment Let them realise that these sensory perceptions are valuable but they give only vague and superficial information Then they ought to be able to translate the problem in measurable quantities All quantities that are important for the experiment need to be listed Some of these quantities need to be kept at a constant level during the experiment Other quantities need to be measured The pupils should estimate the magnitude of the results in advance so they can select the appropriate measuring instruments They can use the following scheme to think out their experiment
Workhypothesis What is the question
Description and outline of the experiment Which experiment can answer that question
Material list Which equipment is necessary
Procedure How exactly is it done
For the experiment with the teapot this could be resolved as follows
Tea can be replaced by boiling water In stead of a real teapot we can use cups of different materials
(with the same volume) To discover the effect of the thickness we use cups of the
same material but with a different thickness We measure the temperature with a thermometer the time with
a chronometer and the volume with a pipette
Procedure Fill the cups with equal volumes of boiling water (use the
different kind of cups) Measure the temperature of the water at different points of time
(every 15 seconds) Put the results in a convenient table and try to figure out which
cup is most suitable to keep the tea hot
Heat Energy in transit EFEU May 2009
Renilde Nihoul 16 HUBrussel
84 Step 4 Reflection
When the experiment is finished they have to select the work hypothesis confirmed by the experiment and give one‟s motives for this choice All the difficulties have to be put into words If their own vision is in conflict with the outcome of the experiment the pupils will be intellectually challenged
Selection of the hypothesis confirmed by the experiment a porcelain teapot will keep the tea longer hot than a metal teapot The thickness of the teapot is important The tea will chill in every teapot but the time it takes to do so will be different
Motivation of the selection
85 Step 5 Scientific explanation
The scientific explanation should start from the concrete problem from a conceptual approach and lead then to a mathematical formulation The scientific explanation should be told in a fascinating way so they will remember it longer
The better you prevent an energy exchange between the hot tea and the surrounding air the longer the tea remains hot
A metal pot is a very bad insulator (A complete explanation is not given here)
86 Conclusion
To deal with the existing misconceptions let the pupils make their own experiments By practicing a scientific method they will be able to discover the bdquotruth‟ The link between the physical principles and situations of daily life will become clearer They might be more interested in physics lessons as they understand they can use these theoretical principles to explain daily life problems Evaluate the concept afterwards (Evalution of your practice is a powerful tool for enhancing your teaching as well as their students learning) Did the strategie capture and retain the pupil‟s attention How effective was this in encouraging their scientific skills Hopefully the pupils will perceive themselves as participants in the journey of discovering and learning and as co-operating detectives looking for and evaluating evidence Let them share your feelings of curiosity enthusiasm and critical reflection
Heat Energy in transit EFEU May 2009
Renilde Nihoul 17 HUBrussel
9 References
Balck C Cocquyt B Van Peteghem R (2005) Misvattingen fysica te lijf Syllabus Universiteit Antwerpen ndash Centrum Nascholing Onderwijs
Giancoli DC (1999) Natuurkunde voor Wetenschap en Techniek deel II Golven en Geluid Kinetische Theorie en Thermodynamica Elektriciteit en Magnetisme Licht Academic Service Schoonhoven
Hathaway D (2008) Solar Physics NASAs Marshall Space Flight Center httpsolarsciencemsfcnasagov
NaveCR (2005) HyperPhysics Departement of Physics and Astronomy Georgia State University httphyperphysicsphy-astrgsueduhbasehframehtml
Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 11 HUBrussel
Figure 8 The sun at 5800K and a campfire at 800K9
The sun at 5800 K and a hot campfire at perhaps 800 K give off radiation at a rate proportional to the 4th power of the temperature All objects emit such electromagnetic radiation because of their temperature and also absorb some radiation that falls on them from the objects The higher the temperature of an object the more it radiates The energy is carried by photons of light in the infrared and visible portions of the electromagnetic spectrum When temperatures are uniform the radiative flux between objects is in equilibrium and no net thermal energy is exchanged The balance is upset when temperatures are not uniform and thermal energy is transported from surfaces of higher to surfaces of lower temperatures
7 Heat Transfer Examples
71 With focus on radiation the Greenhouse effect
The greenhouse effect refers to circumstances where the short wavelengths of visible light from the sun pass through a transparent medium and are absorbed but the longer wavelengths of the infrared re-radiation from the heated objects are unable to pass through that medium The trapping of the long wavelength radiation leads to more heating and a higher resultant temperature Besides the heating of an automobile by sunlight through the windshield and the namesake example of heating the greenhouse by sunlight passing through sealed transparent windows the greenhouse effect has been widely used to describe the trapping of excess heat by the rising concentration of carbon dioxide
9 httphyperphysicsphy-astrgsueduhbasethermostefanhtmlc2
Heat Energy in transit EFEU May 2009
Renilde Nihoul 12 HUBrussel
in the atmosphere The carbon dioxide strongly absorbs infrared and does not allow as much of it to escape into space
Figure 9 The greenhouse effect10
A major part of the efficiency of the heating of an actual greenhouse is the trapping of the air so that the energy is not lost by convection Keeping the hot air from escaping out the top is part of the practical greenhouse effect but it is common usage to refer to the infrared trapping as the greenhouse effect in atmospheric applications where the air trapping is not applicable
The action of carbon dioxide and other greenhouse gases in trapping outgoing infrared energy from the Earth thereby warming the planet is called the greenhouse effect Those gas molecules in the Earths atmosphere with three or more atoms are called greenhouse gases The greenhouse gases include water vapour (H2O) ozone (O3) carbon dioxide (CO2) and methane (CH4) Also trace quantities of chlorofluorocarbons (CFCs) can have a disproportionately large effect Current analysis suggests that the combustion of fossil fuels is a major contributor to the increase in the carbon dioxide concentration It may measurably increase the overall average temperature of the Earth which could have disastrous consequences Because the potential consequences of global warming in terms of loss of snow cover sea level rise change in weather patterns etc are so great it is a major societal concern On the other hand proposed measures to reduce human contributions to greenhouse gases can also have great consequences For more information httphyperphysicsphy-astrgsueduhbasehframehtml
10 httphyperphysicsphy-astrgsueduhbasehframehtml
Heat Energy in transit EFEU May 2009
Renilde Nihoul 13 HUBrussel
72 Involving all three basic heat transfer mechanisms cooling of the human body
Figure 10 Cooling of the human body11
This is a simplified model of the process by which the human body gives off heat Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism One implication of the model is that radiation is the most important heat transfer mechanism at ordinary room temperatures This model indicates that an unclothed person at rest in a room temperature of 23 Celsius would be uncomfortably cool The skin temperature of 34 degC is a typical skin temperature taken from physiology texts compared to the normal core body temperature of 37 degC What will happen if the ambient temperature is above the skin temperature Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism This becomes a problem when the ambient temperature is above body temperature because all three standard heat transfer mechanisms work against this heat loss by transferring heat into the body
Figure 11 Cooling of the human body when the ambient temperature is above the skin temperature
11 httphyperphysicsphy-astrgsueduhbasethermocoobodhtmlc1
Heat Energy in transit EFEU May 2009
Renilde Nihoul 14 HUBrussel
Our ability to exist in such conditions comes from the efficiency of cooling by the evaporation of perspiration At a temperature of 45 Celsius the evaporation process must overcome the transfer of heat into the body and give off enough heat to accomplish a 90 watt net outward flow rate of energy Because of the bodys temperature regulation mechanisms the skin temperature would be expected to rise to 37degC at which point perspiration is initiated and increases until the evaporation cooling is sufficient to hold the skin at 37degC if possible With those assumptions about the temperatures one can calculate that there will be a net input power of 109 watts to the body (by radiation) The perspiration cooling must overcome that and produce the net outflow of 90 watts for equilibrium
8 Dealing with misconceptions by training the scientific method
As mentioned before some misconceptions about heat can stubbornly persist in pupils mind In order to explain how to deal with this misconceptions by training the scientific method we illustrate this concept with an example
81 Step 1 Formulation of a problem
A problem (of a daily life situation) can be introduced by the teacher or by a pupil For instance
I want to keep the tea as hot as possible as long as possible in a teapot Therefore I better choose a thick teapot A metal teapot hellip
82 Step 2 Formulation of workhypotheses
The pupils discuss the problem in small groups (3 ndash 4 pupils) and try to formulate workhypotheses The emphasis has to lay on the formulation of the hypotheses and not on the ldquocorrectrdquo formulation As a teacher you try to stimulate the discussion by asking questions as ldquoWhat exactly do you mean Why do you think that is How could you write this downhelliprdquo Don‟t give any information with respect to content Discuss in class all the preconceptions made by the pupils It is important they realise not everyone has the same preconceptions It is also important that they can practice to express their visions They have to become aware of their own visions
Summarize all the visions of the pupils
A metal teapot is better than a porcelain one to keep the tea hot
In a thicker teapot the tea will remain longer hot It doesn‟t matter which teapot you use (the tea will chill
anyway) hellip
Heat Energy in transit EFEU May 2009
Renilde Nihoul 15 HUBrussel
83 Step 3 Experimental verification
Normally the teacher thinks out the experiment However it is important that the pupils plan their own experiment so they see the experiment as a mean to answer questions and not only as a recipe they have to follow
At first they will use their sensory perceptions to plan the experiment Let them realise that these sensory perceptions are valuable but they give only vague and superficial information Then they ought to be able to translate the problem in measurable quantities All quantities that are important for the experiment need to be listed Some of these quantities need to be kept at a constant level during the experiment Other quantities need to be measured The pupils should estimate the magnitude of the results in advance so they can select the appropriate measuring instruments They can use the following scheme to think out their experiment
Workhypothesis What is the question
Description and outline of the experiment Which experiment can answer that question
Material list Which equipment is necessary
Procedure How exactly is it done
For the experiment with the teapot this could be resolved as follows
Tea can be replaced by boiling water In stead of a real teapot we can use cups of different materials
(with the same volume) To discover the effect of the thickness we use cups of the
same material but with a different thickness We measure the temperature with a thermometer the time with
a chronometer and the volume with a pipette
Procedure Fill the cups with equal volumes of boiling water (use the
different kind of cups) Measure the temperature of the water at different points of time
(every 15 seconds) Put the results in a convenient table and try to figure out which
cup is most suitable to keep the tea hot
Heat Energy in transit EFEU May 2009
Renilde Nihoul 16 HUBrussel
84 Step 4 Reflection
When the experiment is finished they have to select the work hypothesis confirmed by the experiment and give one‟s motives for this choice All the difficulties have to be put into words If their own vision is in conflict with the outcome of the experiment the pupils will be intellectually challenged
Selection of the hypothesis confirmed by the experiment a porcelain teapot will keep the tea longer hot than a metal teapot The thickness of the teapot is important The tea will chill in every teapot but the time it takes to do so will be different
Motivation of the selection
85 Step 5 Scientific explanation
The scientific explanation should start from the concrete problem from a conceptual approach and lead then to a mathematical formulation The scientific explanation should be told in a fascinating way so they will remember it longer
The better you prevent an energy exchange between the hot tea and the surrounding air the longer the tea remains hot
A metal pot is a very bad insulator (A complete explanation is not given here)
86 Conclusion
To deal with the existing misconceptions let the pupils make their own experiments By practicing a scientific method they will be able to discover the bdquotruth‟ The link between the physical principles and situations of daily life will become clearer They might be more interested in physics lessons as they understand they can use these theoretical principles to explain daily life problems Evaluate the concept afterwards (Evalution of your practice is a powerful tool for enhancing your teaching as well as their students learning) Did the strategie capture and retain the pupil‟s attention How effective was this in encouraging their scientific skills Hopefully the pupils will perceive themselves as participants in the journey of discovering and learning and as co-operating detectives looking for and evaluating evidence Let them share your feelings of curiosity enthusiasm and critical reflection
Heat Energy in transit EFEU May 2009
Renilde Nihoul 17 HUBrussel
9 References
Balck C Cocquyt B Van Peteghem R (2005) Misvattingen fysica te lijf Syllabus Universiteit Antwerpen ndash Centrum Nascholing Onderwijs
Giancoli DC (1999) Natuurkunde voor Wetenschap en Techniek deel II Golven en Geluid Kinetische Theorie en Thermodynamica Elektriciteit en Magnetisme Licht Academic Service Schoonhoven
Hathaway D (2008) Solar Physics NASAs Marshall Space Flight Center httpsolarsciencemsfcnasagov
NaveCR (2005) HyperPhysics Departement of Physics and Astronomy Georgia State University httphyperphysicsphy-astrgsueduhbasehframehtml
Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 12 HUBrussel
in the atmosphere The carbon dioxide strongly absorbs infrared and does not allow as much of it to escape into space
Figure 9 The greenhouse effect10
A major part of the efficiency of the heating of an actual greenhouse is the trapping of the air so that the energy is not lost by convection Keeping the hot air from escaping out the top is part of the practical greenhouse effect but it is common usage to refer to the infrared trapping as the greenhouse effect in atmospheric applications where the air trapping is not applicable
The action of carbon dioxide and other greenhouse gases in trapping outgoing infrared energy from the Earth thereby warming the planet is called the greenhouse effect Those gas molecules in the Earths atmosphere with three or more atoms are called greenhouse gases The greenhouse gases include water vapour (H2O) ozone (O3) carbon dioxide (CO2) and methane (CH4) Also trace quantities of chlorofluorocarbons (CFCs) can have a disproportionately large effect Current analysis suggests that the combustion of fossil fuels is a major contributor to the increase in the carbon dioxide concentration It may measurably increase the overall average temperature of the Earth which could have disastrous consequences Because the potential consequences of global warming in terms of loss of snow cover sea level rise change in weather patterns etc are so great it is a major societal concern On the other hand proposed measures to reduce human contributions to greenhouse gases can also have great consequences For more information httphyperphysicsphy-astrgsueduhbasehframehtml
10 httphyperphysicsphy-astrgsueduhbasehframehtml
Heat Energy in transit EFEU May 2009
Renilde Nihoul 13 HUBrussel
72 Involving all three basic heat transfer mechanisms cooling of the human body
Figure 10 Cooling of the human body11
This is a simplified model of the process by which the human body gives off heat Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism One implication of the model is that radiation is the most important heat transfer mechanism at ordinary room temperatures This model indicates that an unclothed person at rest in a room temperature of 23 Celsius would be uncomfortably cool The skin temperature of 34 degC is a typical skin temperature taken from physiology texts compared to the normal core body temperature of 37 degC What will happen if the ambient temperature is above the skin temperature Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism This becomes a problem when the ambient temperature is above body temperature because all three standard heat transfer mechanisms work against this heat loss by transferring heat into the body
Figure 11 Cooling of the human body when the ambient temperature is above the skin temperature
11 httphyperphysicsphy-astrgsueduhbasethermocoobodhtmlc1
Heat Energy in transit EFEU May 2009
Renilde Nihoul 14 HUBrussel
Our ability to exist in such conditions comes from the efficiency of cooling by the evaporation of perspiration At a temperature of 45 Celsius the evaporation process must overcome the transfer of heat into the body and give off enough heat to accomplish a 90 watt net outward flow rate of energy Because of the bodys temperature regulation mechanisms the skin temperature would be expected to rise to 37degC at which point perspiration is initiated and increases until the evaporation cooling is sufficient to hold the skin at 37degC if possible With those assumptions about the temperatures one can calculate that there will be a net input power of 109 watts to the body (by radiation) The perspiration cooling must overcome that and produce the net outflow of 90 watts for equilibrium
8 Dealing with misconceptions by training the scientific method
As mentioned before some misconceptions about heat can stubbornly persist in pupils mind In order to explain how to deal with this misconceptions by training the scientific method we illustrate this concept with an example
81 Step 1 Formulation of a problem
A problem (of a daily life situation) can be introduced by the teacher or by a pupil For instance
I want to keep the tea as hot as possible as long as possible in a teapot Therefore I better choose a thick teapot A metal teapot hellip
82 Step 2 Formulation of workhypotheses
The pupils discuss the problem in small groups (3 ndash 4 pupils) and try to formulate workhypotheses The emphasis has to lay on the formulation of the hypotheses and not on the ldquocorrectrdquo formulation As a teacher you try to stimulate the discussion by asking questions as ldquoWhat exactly do you mean Why do you think that is How could you write this downhelliprdquo Don‟t give any information with respect to content Discuss in class all the preconceptions made by the pupils It is important they realise not everyone has the same preconceptions It is also important that they can practice to express their visions They have to become aware of their own visions
Summarize all the visions of the pupils
A metal teapot is better than a porcelain one to keep the tea hot
In a thicker teapot the tea will remain longer hot It doesn‟t matter which teapot you use (the tea will chill
anyway) hellip
Heat Energy in transit EFEU May 2009
Renilde Nihoul 15 HUBrussel
83 Step 3 Experimental verification
Normally the teacher thinks out the experiment However it is important that the pupils plan their own experiment so they see the experiment as a mean to answer questions and not only as a recipe they have to follow
At first they will use their sensory perceptions to plan the experiment Let them realise that these sensory perceptions are valuable but they give only vague and superficial information Then they ought to be able to translate the problem in measurable quantities All quantities that are important for the experiment need to be listed Some of these quantities need to be kept at a constant level during the experiment Other quantities need to be measured The pupils should estimate the magnitude of the results in advance so they can select the appropriate measuring instruments They can use the following scheme to think out their experiment
Workhypothesis What is the question
Description and outline of the experiment Which experiment can answer that question
Material list Which equipment is necessary
Procedure How exactly is it done
For the experiment with the teapot this could be resolved as follows
Tea can be replaced by boiling water In stead of a real teapot we can use cups of different materials
(with the same volume) To discover the effect of the thickness we use cups of the
same material but with a different thickness We measure the temperature with a thermometer the time with
a chronometer and the volume with a pipette
Procedure Fill the cups with equal volumes of boiling water (use the
different kind of cups) Measure the temperature of the water at different points of time
(every 15 seconds) Put the results in a convenient table and try to figure out which
cup is most suitable to keep the tea hot
Heat Energy in transit EFEU May 2009
Renilde Nihoul 16 HUBrussel
84 Step 4 Reflection
When the experiment is finished they have to select the work hypothesis confirmed by the experiment and give one‟s motives for this choice All the difficulties have to be put into words If their own vision is in conflict with the outcome of the experiment the pupils will be intellectually challenged
Selection of the hypothesis confirmed by the experiment a porcelain teapot will keep the tea longer hot than a metal teapot The thickness of the teapot is important The tea will chill in every teapot but the time it takes to do so will be different
Motivation of the selection
85 Step 5 Scientific explanation
The scientific explanation should start from the concrete problem from a conceptual approach and lead then to a mathematical formulation The scientific explanation should be told in a fascinating way so they will remember it longer
The better you prevent an energy exchange between the hot tea and the surrounding air the longer the tea remains hot
A metal pot is a very bad insulator (A complete explanation is not given here)
86 Conclusion
To deal with the existing misconceptions let the pupils make their own experiments By practicing a scientific method they will be able to discover the bdquotruth‟ The link between the physical principles and situations of daily life will become clearer They might be more interested in physics lessons as they understand they can use these theoretical principles to explain daily life problems Evaluate the concept afterwards (Evalution of your practice is a powerful tool for enhancing your teaching as well as their students learning) Did the strategie capture and retain the pupil‟s attention How effective was this in encouraging their scientific skills Hopefully the pupils will perceive themselves as participants in the journey of discovering and learning and as co-operating detectives looking for and evaluating evidence Let them share your feelings of curiosity enthusiasm and critical reflection
Heat Energy in transit EFEU May 2009
Renilde Nihoul 17 HUBrussel
9 References
Balck C Cocquyt B Van Peteghem R (2005) Misvattingen fysica te lijf Syllabus Universiteit Antwerpen ndash Centrum Nascholing Onderwijs
Giancoli DC (1999) Natuurkunde voor Wetenschap en Techniek deel II Golven en Geluid Kinetische Theorie en Thermodynamica Elektriciteit en Magnetisme Licht Academic Service Schoonhoven
Hathaway D (2008) Solar Physics NASAs Marshall Space Flight Center httpsolarsciencemsfcnasagov
NaveCR (2005) HyperPhysics Departement of Physics and Astronomy Georgia State University httphyperphysicsphy-astrgsueduhbasehframehtml
Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 13 HUBrussel
72 Involving all three basic heat transfer mechanisms cooling of the human body
Figure 10 Cooling of the human body11
This is a simplified model of the process by which the human body gives off heat Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism One implication of the model is that radiation is the most important heat transfer mechanism at ordinary room temperatures This model indicates that an unclothed person at rest in a room temperature of 23 Celsius would be uncomfortably cool The skin temperature of 34 degC is a typical skin temperature taken from physiology texts compared to the normal core body temperature of 37 degC What will happen if the ambient temperature is above the skin temperature Even when inactive an adult male must lose heat at a rate of about 90 watts as a result of his basal metabolism This becomes a problem when the ambient temperature is above body temperature because all three standard heat transfer mechanisms work against this heat loss by transferring heat into the body
Figure 11 Cooling of the human body when the ambient temperature is above the skin temperature
11 httphyperphysicsphy-astrgsueduhbasethermocoobodhtmlc1
Heat Energy in transit EFEU May 2009
Renilde Nihoul 14 HUBrussel
Our ability to exist in such conditions comes from the efficiency of cooling by the evaporation of perspiration At a temperature of 45 Celsius the evaporation process must overcome the transfer of heat into the body and give off enough heat to accomplish a 90 watt net outward flow rate of energy Because of the bodys temperature regulation mechanisms the skin temperature would be expected to rise to 37degC at which point perspiration is initiated and increases until the evaporation cooling is sufficient to hold the skin at 37degC if possible With those assumptions about the temperatures one can calculate that there will be a net input power of 109 watts to the body (by radiation) The perspiration cooling must overcome that and produce the net outflow of 90 watts for equilibrium
8 Dealing with misconceptions by training the scientific method
As mentioned before some misconceptions about heat can stubbornly persist in pupils mind In order to explain how to deal with this misconceptions by training the scientific method we illustrate this concept with an example
81 Step 1 Formulation of a problem
A problem (of a daily life situation) can be introduced by the teacher or by a pupil For instance
I want to keep the tea as hot as possible as long as possible in a teapot Therefore I better choose a thick teapot A metal teapot hellip
82 Step 2 Formulation of workhypotheses
The pupils discuss the problem in small groups (3 ndash 4 pupils) and try to formulate workhypotheses The emphasis has to lay on the formulation of the hypotheses and not on the ldquocorrectrdquo formulation As a teacher you try to stimulate the discussion by asking questions as ldquoWhat exactly do you mean Why do you think that is How could you write this downhelliprdquo Don‟t give any information with respect to content Discuss in class all the preconceptions made by the pupils It is important they realise not everyone has the same preconceptions It is also important that they can practice to express their visions They have to become aware of their own visions
Summarize all the visions of the pupils
A metal teapot is better than a porcelain one to keep the tea hot
In a thicker teapot the tea will remain longer hot It doesn‟t matter which teapot you use (the tea will chill
anyway) hellip
Heat Energy in transit EFEU May 2009
Renilde Nihoul 15 HUBrussel
83 Step 3 Experimental verification
Normally the teacher thinks out the experiment However it is important that the pupils plan their own experiment so they see the experiment as a mean to answer questions and not only as a recipe they have to follow
At first they will use their sensory perceptions to plan the experiment Let them realise that these sensory perceptions are valuable but they give only vague and superficial information Then they ought to be able to translate the problem in measurable quantities All quantities that are important for the experiment need to be listed Some of these quantities need to be kept at a constant level during the experiment Other quantities need to be measured The pupils should estimate the magnitude of the results in advance so they can select the appropriate measuring instruments They can use the following scheme to think out their experiment
Workhypothesis What is the question
Description and outline of the experiment Which experiment can answer that question
Material list Which equipment is necessary
Procedure How exactly is it done
For the experiment with the teapot this could be resolved as follows
Tea can be replaced by boiling water In stead of a real teapot we can use cups of different materials
(with the same volume) To discover the effect of the thickness we use cups of the
same material but with a different thickness We measure the temperature with a thermometer the time with
a chronometer and the volume with a pipette
Procedure Fill the cups with equal volumes of boiling water (use the
different kind of cups) Measure the temperature of the water at different points of time
(every 15 seconds) Put the results in a convenient table and try to figure out which
cup is most suitable to keep the tea hot
Heat Energy in transit EFEU May 2009
Renilde Nihoul 16 HUBrussel
84 Step 4 Reflection
When the experiment is finished they have to select the work hypothesis confirmed by the experiment and give one‟s motives for this choice All the difficulties have to be put into words If their own vision is in conflict with the outcome of the experiment the pupils will be intellectually challenged
Selection of the hypothesis confirmed by the experiment a porcelain teapot will keep the tea longer hot than a metal teapot The thickness of the teapot is important The tea will chill in every teapot but the time it takes to do so will be different
Motivation of the selection
85 Step 5 Scientific explanation
The scientific explanation should start from the concrete problem from a conceptual approach and lead then to a mathematical formulation The scientific explanation should be told in a fascinating way so they will remember it longer
The better you prevent an energy exchange between the hot tea and the surrounding air the longer the tea remains hot
A metal pot is a very bad insulator (A complete explanation is not given here)
86 Conclusion
To deal with the existing misconceptions let the pupils make their own experiments By practicing a scientific method they will be able to discover the bdquotruth‟ The link between the physical principles and situations of daily life will become clearer They might be more interested in physics lessons as they understand they can use these theoretical principles to explain daily life problems Evaluate the concept afterwards (Evalution of your practice is a powerful tool for enhancing your teaching as well as their students learning) Did the strategie capture and retain the pupil‟s attention How effective was this in encouraging their scientific skills Hopefully the pupils will perceive themselves as participants in the journey of discovering and learning and as co-operating detectives looking for and evaluating evidence Let them share your feelings of curiosity enthusiasm and critical reflection
Heat Energy in transit EFEU May 2009
Renilde Nihoul 17 HUBrussel
9 References
Balck C Cocquyt B Van Peteghem R (2005) Misvattingen fysica te lijf Syllabus Universiteit Antwerpen ndash Centrum Nascholing Onderwijs
Giancoli DC (1999) Natuurkunde voor Wetenschap en Techniek deel II Golven en Geluid Kinetische Theorie en Thermodynamica Elektriciteit en Magnetisme Licht Academic Service Schoonhoven
Hathaway D (2008) Solar Physics NASAs Marshall Space Flight Center httpsolarsciencemsfcnasagov
NaveCR (2005) HyperPhysics Departement of Physics and Astronomy Georgia State University httphyperphysicsphy-astrgsueduhbasehframehtml
Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 14 HUBrussel
Our ability to exist in such conditions comes from the efficiency of cooling by the evaporation of perspiration At a temperature of 45 Celsius the evaporation process must overcome the transfer of heat into the body and give off enough heat to accomplish a 90 watt net outward flow rate of energy Because of the bodys temperature regulation mechanisms the skin temperature would be expected to rise to 37degC at which point perspiration is initiated and increases until the evaporation cooling is sufficient to hold the skin at 37degC if possible With those assumptions about the temperatures one can calculate that there will be a net input power of 109 watts to the body (by radiation) The perspiration cooling must overcome that and produce the net outflow of 90 watts for equilibrium
8 Dealing with misconceptions by training the scientific method
As mentioned before some misconceptions about heat can stubbornly persist in pupils mind In order to explain how to deal with this misconceptions by training the scientific method we illustrate this concept with an example
81 Step 1 Formulation of a problem
A problem (of a daily life situation) can be introduced by the teacher or by a pupil For instance
I want to keep the tea as hot as possible as long as possible in a teapot Therefore I better choose a thick teapot A metal teapot hellip
82 Step 2 Formulation of workhypotheses
The pupils discuss the problem in small groups (3 ndash 4 pupils) and try to formulate workhypotheses The emphasis has to lay on the formulation of the hypotheses and not on the ldquocorrectrdquo formulation As a teacher you try to stimulate the discussion by asking questions as ldquoWhat exactly do you mean Why do you think that is How could you write this downhelliprdquo Don‟t give any information with respect to content Discuss in class all the preconceptions made by the pupils It is important they realise not everyone has the same preconceptions It is also important that they can practice to express their visions They have to become aware of their own visions
Summarize all the visions of the pupils
A metal teapot is better than a porcelain one to keep the tea hot
In a thicker teapot the tea will remain longer hot It doesn‟t matter which teapot you use (the tea will chill
anyway) hellip
Heat Energy in transit EFEU May 2009
Renilde Nihoul 15 HUBrussel
83 Step 3 Experimental verification
Normally the teacher thinks out the experiment However it is important that the pupils plan their own experiment so they see the experiment as a mean to answer questions and not only as a recipe they have to follow
At first they will use their sensory perceptions to plan the experiment Let them realise that these sensory perceptions are valuable but they give only vague and superficial information Then they ought to be able to translate the problem in measurable quantities All quantities that are important for the experiment need to be listed Some of these quantities need to be kept at a constant level during the experiment Other quantities need to be measured The pupils should estimate the magnitude of the results in advance so they can select the appropriate measuring instruments They can use the following scheme to think out their experiment
Workhypothesis What is the question
Description and outline of the experiment Which experiment can answer that question
Material list Which equipment is necessary
Procedure How exactly is it done
For the experiment with the teapot this could be resolved as follows
Tea can be replaced by boiling water In stead of a real teapot we can use cups of different materials
(with the same volume) To discover the effect of the thickness we use cups of the
same material but with a different thickness We measure the temperature with a thermometer the time with
a chronometer and the volume with a pipette
Procedure Fill the cups with equal volumes of boiling water (use the
different kind of cups) Measure the temperature of the water at different points of time
(every 15 seconds) Put the results in a convenient table and try to figure out which
cup is most suitable to keep the tea hot
Heat Energy in transit EFEU May 2009
Renilde Nihoul 16 HUBrussel
84 Step 4 Reflection
When the experiment is finished they have to select the work hypothesis confirmed by the experiment and give one‟s motives for this choice All the difficulties have to be put into words If their own vision is in conflict with the outcome of the experiment the pupils will be intellectually challenged
Selection of the hypothesis confirmed by the experiment a porcelain teapot will keep the tea longer hot than a metal teapot The thickness of the teapot is important The tea will chill in every teapot but the time it takes to do so will be different
Motivation of the selection
85 Step 5 Scientific explanation
The scientific explanation should start from the concrete problem from a conceptual approach and lead then to a mathematical formulation The scientific explanation should be told in a fascinating way so they will remember it longer
The better you prevent an energy exchange between the hot tea and the surrounding air the longer the tea remains hot
A metal pot is a very bad insulator (A complete explanation is not given here)
86 Conclusion
To deal with the existing misconceptions let the pupils make their own experiments By practicing a scientific method they will be able to discover the bdquotruth‟ The link between the physical principles and situations of daily life will become clearer They might be more interested in physics lessons as they understand they can use these theoretical principles to explain daily life problems Evaluate the concept afterwards (Evalution of your practice is a powerful tool for enhancing your teaching as well as their students learning) Did the strategie capture and retain the pupil‟s attention How effective was this in encouraging their scientific skills Hopefully the pupils will perceive themselves as participants in the journey of discovering and learning and as co-operating detectives looking for and evaluating evidence Let them share your feelings of curiosity enthusiasm and critical reflection
Heat Energy in transit EFEU May 2009
Renilde Nihoul 17 HUBrussel
9 References
Balck C Cocquyt B Van Peteghem R (2005) Misvattingen fysica te lijf Syllabus Universiteit Antwerpen ndash Centrum Nascholing Onderwijs
Giancoli DC (1999) Natuurkunde voor Wetenschap en Techniek deel II Golven en Geluid Kinetische Theorie en Thermodynamica Elektriciteit en Magnetisme Licht Academic Service Schoonhoven
Hathaway D (2008) Solar Physics NASAs Marshall Space Flight Center httpsolarsciencemsfcnasagov
NaveCR (2005) HyperPhysics Departement of Physics and Astronomy Georgia State University httphyperphysicsphy-astrgsueduhbasehframehtml
Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 15 HUBrussel
83 Step 3 Experimental verification
Normally the teacher thinks out the experiment However it is important that the pupils plan their own experiment so they see the experiment as a mean to answer questions and not only as a recipe they have to follow
At first they will use their sensory perceptions to plan the experiment Let them realise that these sensory perceptions are valuable but they give only vague and superficial information Then they ought to be able to translate the problem in measurable quantities All quantities that are important for the experiment need to be listed Some of these quantities need to be kept at a constant level during the experiment Other quantities need to be measured The pupils should estimate the magnitude of the results in advance so they can select the appropriate measuring instruments They can use the following scheme to think out their experiment
Workhypothesis What is the question
Description and outline of the experiment Which experiment can answer that question
Material list Which equipment is necessary
Procedure How exactly is it done
For the experiment with the teapot this could be resolved as follows
Tea can be replaced by boiling water In stead of a real teapot we can use cups of different materials
(with the same volume) To discover the effect of the thickness we use cups of the
same material but with a different thickness We measure the temperature with a thermometer the time with
a chronometer and the volume with a pipette
Procedure Fill the cups with equal volumes of boiling water (use the
different kind of cups) Measure the temperature of the water at different points of time
(every 15 seconds) Put the results in a convenient table and try to figure out which
cup is most suitable to keep the tea hot
Heat Energy in transit EFEU May 2009
Renilde Nihoul 16 HUBrussel
84 Step 4 Reflection
When the experiment is finished they have to select the work hypothesis confirmed by the experiment and give one‟s motives for this choice All the difficulties have to be put into words If their own vision is in conflict with the outcome of the experiment the pupils will be intellectually challenged
Selection of the hypothesis confirmed by the experiment a porcelain teapot will keep the tea longer hot than a metal teapot The thickness of the teapot is important The tea will chill in every teapot but the time it takes to do so will be different
Motivation of the selection
85 Step 5 Scientific explanation
The scientific explanation should start from the concrete problem from a conceptual approach and lead then to a mathematical formulation The scientific explanation should be told in a fascinating way so they will remember it longer
The better you prevent an energy exchange between the hot tea and the surrounding air the longer the tea remains hot
A metal pot is a very bad insulator (A complete explanation is not given here)
86 Conclusion
To deal with the existing misconceptions let the pupils make their own experiments By practicing a scientific method they will be able to discover the bdquotruth‟ The link between the physical principles and situations of daily life will become clearer They might be more interested in physics lessons as they understand they can use these theoretical principles to explain daily life problems Evaluate the concept afterwards (Evalution of your practice is a powerful tool for enhancing your teaching as well as their students learning) Did the strategie capture and retain the pupil‟s attention How effective was this in encouraging their scientific skills Hopefully the pupils will perceive themselves as participants in the journey of discovering and learning and as co-operating detectives looking for and evaluating evidence Let them share your feelings of curiosity enthusiasm and critical reflection
Heat Energy in transit EFEU May 2009
Renilde Nihoul 17 HUBrussel
9 References
Balck C Cocquyt B Van Peteghem R (2005) Misvattingen fysica te lijf Syllabus Universiteit Antwerpen ndash Centrum Nascholing Onderwijs
Giancoli DC (1999) Natuurkunde voor Wetenschap en Techniek deel II Golven en Geluid Kinetische Theorie en Thermodynamica Elektriciteit en Magnetisme Licht Academic Service Schoonhoven
Hathaway D (2008) Solar Physics NASAs Marshall Space Flight Center httpsolarsciencemsfcnasagov
NaveCR (2005) HyperPhysics Departement of Physics and Astronomy Georgia State University httphyperphysicsphy-astrgsueduhbasehframehtml
Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 16 HUBrussel
84 Step 4 Reflection
When the experiment is finished they have to select the work hypothesis confirmed by the experiment and give one‟s motives for this choice All the difficulties have to be put into words If their own vision is in conflict with the outcome of the experiment the pupils will be intellectually challenged
Selection of the hypothesis confirmed by the experiment a porcelain teapot will keep the tea longer hot than a metal teapot The thickness of the teapot is important The tea will chill in every teapot but the time it takes to do so will be different
Motivation of the selection
85 Step 5 Scientific explanation
The scientific explanation should start from the concrete problem from a conceptual approach and lead then to a mathematical formulation The scientific explanation should be told in a fascinating way so they will remember it longer
The better you prevent an energy exchange between the hot tea and the surrounding air the longer the tea remains hot
A metal pot is a very bad insulator (A complete explanation is not given here)
86 Conclusion
To deal with the existing misconceptions let the pupils make their own experiments By practicing a scientific method they will be able to discover the bdquotruth‟ The link between the physical principles and situations of daily life will become clearer They might be more interested in physics lessons as they understand they can use these theoretical principles to explain daily life problems Evaluate the concept afterwards (Evalution of your practice is a powerful tool for enhancing your teaching as well as their students learning) Did the strategie capture and retain the pupil‟s attention How effective was this in encouraging their scientific skills Hopefully the pupils will perceive themselves as participants in the journey of discovering and learning and as co-operating detectives looking for and evaluating evidence Let them share your feelings of curiosity enthusiasm and critical reflection
Heat Energy in transit EFEU May 2009
Renilde Nihoul 17 HUBrussel
9 References
Balck C Cocquyt B Van Peteghem R (2005) Misvattingen fysica te lijf Syllabus Universiteit Antwerpen ndash Centrum Nascholing Onderwijs
Giancoli DC (1999) Natuurkunde voor Wetenschap en Techniek deel II Golven en Geluid Kinetische Theorie en Thermodynamica Elektriciteit en Magnetisme Licht Academic Service Schoonhoven
Hathaway D (2008) Solar Physics NASAs Marshall Space Flight Center httpsolarsciencemsfcnasagov
NaveCR (2005) HyperPhysics Departement of Physics and Astronomy Georgia State University httphyperphysicsphy-astrgsueduhbasehframehtml
Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
Heat Energy in transit EFEU May 2009
Renilde Nihoul 17 HUBrussel
9 References
Balck C Cocquyt B Van Peteghem R (2005) Misvattingen fysica te lijf Syllabus Universiteit Antwerpen ndash Centrum Nascholing Onderwijs
Giancoli DC (1999) Natuurkunde voor Wetenschap en Techniek deel II Golven en Geluid Kinetische Theorie en Thermodynamica Elektriciteit en Magnetisme Licht Academic Service Schoonhoven
Hathaway D (2008) Solar Physics NASAs Marshall Space Flight Center httpsolarsciencemsfcnasagov
NaveCR (2005) HyperPhysics Departement of Physics and Astronomy Georgia State University httphyperphysicsphy-astrgsueduhbasehframehtml
Resnick R Halliday D Krane K (1992) PHYSICS Volume 1 4th Edition John Wiley amp Sons New York
Zemansky M (1970) The Use and Misuse of the Word bdquoHeat‟ The Physics Teacher 8 295
