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HEAT

4.1 UNDERSTANDING THERMAL EQUILIBRIUM1. Define:

(a) Temperature The measure of the degree of hotness of an object.Measured in SI unit Kelvin, KA hot object is at a higher temperature than a coldobject.

Form of energy, measured in Joules, JHeat is transferred from hotter object (highertemperature) to colder object (lower temperature)

(b) Heat

When an object is heated, it will absorb heat energy anthe temperature will increase.When an object is cooled, it will release heat energy an

the temperature will decrease.(c) Thermal

contactTwo objects are in thermal contact when heat energycan be transferred between them.

(d) Heat transfer When two objects with different degrees of hotnesscome into thermal contact, heat energy is transferredbetween the two objects.

(e) Mechanism of Thermal Equilibrium

Energy is transferred at a faster ratefrom the hotter object to the colderobject. Energy is also transferredfrom the colder object to the hotter

one, but at a slower rate.There is a net flow of energy from thehotter object to the colder object.

The hotter object cools down whilethe colder object warms up .After some time, energy istransferred at the same rate betweenthe two objects. There is no net heatransfer between the objects.The two objects are said to be inthermal equilibrium.

(f) ThermalEquilibrium

When two objects are in thermal equilibrium, there no net flow of heat between them.Two objects in thermal equilibrium have the sametemperature

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Example of thermal equilibrium

A wet towel is placed on the forehead of a person who has high fever.Initially the temperature of the cloth is lower than the body temperatureof the person. Heat energy is transferred from the forehead to thetowel until thermal equilibrium is reached. The towel is rinsed in tapwater and the procedure is repeated. In this way heat energy isremoved from the person.

Cooling drinksA hot drink can be cooled by adding a few ice cubes to the drink. Heatfrom the hot drink is transferred to the colder ice until thermalequilibrium between the ice and water is reached. The finaltemperature of the drink equal the final temperature of ices.

Liquid-in-glass ThermometerThe characteristic ofthe liquid used inliquid-in-glassthermometer

1. be easily seen2. expand and contract rapidly over a wide range

of temperature/ expand uniformly when heated3. not stick to the glass wall of the capillary tube.

How a liquid-in-glass thermometerworks?

1. The bulb of the thermometer contains a fixedmass of mercury. The volume of the mercuryincreases when it absorbs heat.

2. The mercury expands and rises in the capillary

tube. The length of the mercury column in thecapillary tube indicates the magnitude of thetemperature.

How can athermometer becalibrated?

1. A temperature scale is obtained by choosingtwo temperatures, called the fixed point.

2. Definition of ice point and steam point

Fixed point Definition Value

Lowerpoint: IcePoint

The temperature ofpure melting ice

0C

Upperpoint:steam point

The temperature ofsteam from water thatis boiling understandard atmosphericpressure.

100C

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When the two fixed points have been marked onthe stem of the thermometer, the range betweenthem is divided equally into 100 divisions ordegrees. The thermometer now has a scale.

Explain the workingprinciple of athermometer

When the thermometer is placed in contact withhot water, heat is transferred from hot water to thethermometer.Thermal equilibrium between the thermometer andhot water is reached when the net rate of heat

transfer is zero.The thermometer and the water are at the sametemperature. At this point, the thermometerreading shows the temperature of the water.

What arecharacteristics ofmercury that makesit suitable as a

liquid-in-glassthermometer?

1. It is a good conductor of heat2. it has a high boiling point, 357C3. it expands uniformly when heated4. it is opaque (does not allow light to pass

through) and it can be seen easily.Mercury freezes at a temperature of - 39C and it istherefore not suitable for measuring temperaturesbelow this temperature, such at the north pole.

How to increase thesensitivity of amercurythermometer?

1. Thin capillary tube2. A glass bulb with thinner wall3. Large bulb

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4.2 UNDERSTANDING SPECIFIC HEAT CAPACITY

1. Heatcapacity, C

The amount of heat required to change itstemperature by one degree.

2. SpecificHeat capacity,c

The amount of heat that must be supplied to increasethe temperature by 1 C for a mass of 1 kg of thesubstance

Specific heat capacity, c = Q SI unit: = J kg-1C-1

m

Q = heat absorbed / released, unit Jm = mass of the substance, unit kg = temperature difference , unit C

3. Quantity of heatabsorbed or lost bya substance

Q = mc

4. What does specific heatof aluminium 900 J kg-1C-1 mean?

900 J of heat needs to be supplied to 1 kgof aluminium to produce a 1 Ctemperature increase.

5. What does specific heatof water 4 200 J kg-1C-1

mean?

4 200 J of heat needs to be supplied to 1kg of water to produce a 1 C temperature

increase.6. The physicalmeaning ofspecific heatcapacity, c

When two objects of equal mass are heated atequal rates, the object with the smaller specificheat capacity will have a faster temperature.When two objects of equal mass are left to cooldown, the temperature of the object with smallerheat capacity will drop faster.

7. A substancewith a small

value ofspecific heatcapacity

1. heats up and cools at a faster rate.For example, metal like iron, steel, copper and

aluminium is used as pots and pans becausethey can be quickly heated up when there isonly small heat absorption.

2. sensitive to temperature changesA thermometer has low specific heat capacitiesso it enables heat to be easily absorbed andreleased even when small quantities of heat areinvolved.

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8. A substancewith a highvalue ofspecific heatcapacity

1. heats up and cools at slower rate. Require moreheat to raise its temperature by a specificamount.Poor conductor of heat handle of pot

2. can absorb a great amount of heat without ahigh increase in temperature.

For example, water acts a heat reservoir as it canabsorb a great amount of heat before it boils.Water is used as a cooling agent in a car radiator.

9. Applications of Specific Heat Capacity

Cooking pot

(a) Copper base

(b) WoodenHandle

(c) Alumni body

Low specific heat capacity. The pot becomeshot very quickly. This enables quick cookingof the food in the pot.

High density. The heavier base ensures thatthe pot is stable and will not topple overeasily.

Large specific heat capacity. The handle willnot become too hot when heat is absorbed.

Poor conductor of heat. Relatively low specific heat capacity. The pot

becomes hot quickly.

Low density so it will be lighter Does not react with the food in the pot

Sea Breeze Land has a smaller specific heatcapacity than sea. Faster increasein temperature, ie land is warmer

than the sea Air above the land is heated up and

rises.

Cooler air from the sea moves fromtowards the land as sea breeze.

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Land Breeze At night, heat is lost from the landand sea.

Sea has a larger specific heatcapacity so sea is warmer than

land. Warmer air above the sea rises Cooler air from the land moves

towards the sea as land breeze.

The cooling system of a car engine

Water has a high specific heat capacity and lower cost. So water canbe a useful a cooling agent. A water pump circulates the water.Heat produced by the engine is absorbed by the water that flows alongthe space in engine walls. The hot water flows to the radiator whereheat is lost to the cooler air that flows through the cooling fans.

10. A boy drinking hot soupwith a spoon. If he accidentallyspills a spoonful of soup ontohis hand, he would experienceonly a slight pain. However, if

he spills the whole contents ofthe bowl of soup onto himself,he would suffer seriousinjuries.

The mass of the spoonful soupis smaller than the mass of thewhole bowl of soup althoughboth are at the sametemperature and have same

specific heat capacity. Q = mc The mass is directly

proportional to the quantity ofheat.

The soup in the bowl containsmore heat

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Example 1Calculate the total heat that isobserved by a copper block ofmass 500 g and which has beenheated from 31 C to 80C.(specific heat capacity of copper= 390 JKg-1C-1)

Example 2When an electric heater is suppliedwith an electric power of 2 kW toheat 4 kg of water for 1 minute,calculate the increase intemperature of the water. [specificheat capacity of water = 4 200 JKg-1C-1) Assume there is no heat lossto the surroundings.

Example 3A lead bullet moves horizontallywith a velocity of 130 ms-1 andembedded into a cement wallafter collision. If the specific heatcapacity of lead = 130JKg-1C-1 and all heat produces is

absorbed by the bullet, what isthe increase in temperature of thebullet?

Example 4An aluminium block of mass 1 kgis heated by an electric heater for 3minutes and a temperature rise of15 C is recorded. If the electricheater is connected to a voltmeterwhich gives a reading of 30 V and

an ammeter which gives a readingof 2.5 A, calculate the specific heatcapacity of the aluminium.

Example 5300 g of water at temperature 40C is mixed with 900 g of water attemperature 80 C. If there is noheat loss to the surroundings,what is the final temperaturewhen thermal equilibrium isachieved by the mixture of water?

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4.3 UNDERSTANDING SPECIFIC LATENT HEAT

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1. Latent heat

The heat absorbed or the heat released at constanttemperature during change of phase.

2. 4 main Changes of phase When a solid melts, latent heat offusion is absorbed but thetemperature remains constant atits melting point

For a liquid to solidify at itsfreezing point, latent heat offusion has to be removed.

When a liquid is boiling, latentheat of vaporization is absorbed

but the temperature remainsconstant at its boiling point.

When vapour condenses backinto the liquid phase, latent heatof vaporization is released.

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3. The common

characteristics of the four

processes inthe change ofphase

A Substance undergoes a change of phase at aparticular temperature.

Heat energy is transferred during change of phase During change of phase, the temperature remains

constant even though there is transfer of heat.

Notes: The temperature of a substance is proportional to the averagekinetic energy of its particles.

Temperature increases when the average kinetic energy of theparticles increase

Temperature decreases when the average kinetic energy of theparticles decreases.

Temperature remains constant when the average kinetic energy doesnot change.4. Why does the

temperatureremainsconstantduring changeof phase?

During change of phase, the transfer of heat doesnot cause a change in the kinetic energy of themolecules.

During melting, the heat absorbed is used tobreak up the bonds between the particles. Theparticles are freed from their fixed positions and

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are able to vibrate and move among each other.

When a liquids boils, the heat absorbed is usedto completely break the bonds between theparticles and also to do work against

atmospheric pressure when the gaseous vapourexpands into the atmosphere.

5. Specific LatentHeat, l

The amount of heat required to change the phase of1 kg of the substance at a constant temperature.

m

Ql =

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unit : J kg-1

Q = latent heat absorbed or released by thesubstancem = mass of the substance

6. Specific latent

heat of fusion

The amount of heat required to change 1 kg of the

substance from solid to liquid phase without achange in temperature.

7. Specific latentheat ofvaporization

The amount of heat required to change 1 kg of thesubstance from the liquid to gaseous phase withouta change in temperature.

8. Specific latent heatof fusion of ice is336 000 Jkg-1

336 000 J of latent heat is needed for 1 kg iceto melt to become water at 0 C.

9. Specific latent heatof vaporization ofwater is 2.26 x 106 Jkg-1

2.26 x 106

J of latent heat is needed for 1 kgwater to boil to become vapour at 100C.

When the heat added or removedchanges the temperature of anobject, the heat is calculatedusing

Q = mc

When the heat added or removedchanges the phase of an object atconstant temperature, the heat iscalculated using

Q = mlIf heat is supplied electrically to change the phase of a substance, theequation Q = ml can be written as

Q = Pt = ml P = power of the heater, unit in W,t = time , unit is seconds

Example 1The specific latent heat of fusionof ice is 336 000 Jkg-1. What isthe quantity of heat required tomelt 2.5 kg of ice at 0 C?

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Example 2An electric kettle contains 3 kg ofwater. Calculate the amount ofheat required to boil away all thewater after the boiling point hasbeen reached.

Example 3What is the quantity of heat thatis required to convert 4 g of iceinto steam at 100 C.specific latent heat of fusion of ice is336 000 Jkg-1

Specific latent heat of vaporization ofwater is 2.26 x 106 Jkg-1

Specific heat capacity of water = 4.2 x103 J kg-1C-1

9. Experiment to determine the specific latent heat of fusion

Figure (a) shows the apparatus for determining the latent heat of fusionof ice. The control experiment in (b) is for the purpose of determine themass of ice melted by the surrounding heat.

The power supplied to the heater is 36 W. After 5 minutes, the powersupply in Figure (a) is cut off and both beakers are removed.

Mass of beaker (a) = 50 g

Mass of beaker (b) = 53 gMass of beaker (a) with water = 108 gMass of beaker (b) with water = 78 gMass of ice melted in beaker (a) =Mass of ice melted in beaker (b) =

Mass of ice melted by the heater

only = .Energy supplied by the heater,

Q = Pt = .Specific latent heat of fusion,

m

Ql = =

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10. Experiment to determine the specificlatent heat of vaporization for water.

Electric power supply = 2 kW.Time taken = 5 minutes.Initial reading of the electronic balance =685 gFinal reading of the electronic balance =565 gMass of water vaporized, m = Energy supplied by the heater, Q = Pt =..Specific latent heat of vaporization:

11. Applications of Specific Latent Heat Drinks can be cooled by adding in several cubes of ice. When ice

melts a large amount of heat is absorbed and this lowers thetemperature of the drink.

The freshness of fish and meat can bemaintained by placing them in contactwith ice. With its larger latent heat, iceis able to absorb a large quantity of heat

from the fish as it melts. Thus, food canbe kept at a low temperature for anextended period of time.

Water has a large specific latent heat ofvaporization. This property enablessteam to be used for cooking by themethod of steaming. When steamcondenses on the food, the latent heatis released directly onto the foodenables the food to be cooked at afaster rate.

Our bodies feel cool after sweating. This is because latent heat ofvaporization is absorbed from the body when sweat evaporates. Asa result, the body is cooled by the removal of heat.

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Always be very careful when openingthe lid of a pot when the water in it isboiling. Water has a large specificlatent heat of vaporization. When steam

condenses on the skin of your arm, thevery large amount of latent heatreleased can cause a serious burn.

4.4 UNDERSTANDING THE GAS LAWS

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Boyless Law statesthat for a fixed mass ofgas, the pressure of thegas, P is inverselyproportional to itsvolume, V when thetemperature, T is keptconstant

Charles law statesthat for a fixed massof gas, the volume ofthe gas, V is directlyproportional to itsabsolute temperature,T when its pressure, Pis kept constant.

Pressures Law statesthat for a fixed massof gas, the pressure ofthe gas, P is directlyproportional to itsabsolute temperature,T when its volume, Vis kept constant.

2211

tan

1

VPVP

tconsPV

VP

=

=

2

2

1

1

tan

T

V

T

V

tconsT

VTV

=

=

2

2

1

1

tan

T

P

T

P

tconsT

PTP

=

=

Boyles Law Charles Law Pressures Law

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When the volume ofa gas is decreased,the number ofmolecules per unitvolume increases.

The same number ofmolecules moves ina smaller space.

The moleculescollide morefrequently with thewalls of thecontainer.

This increase in therate of collisionresults in an increasein the pressure

exerted by the gas.

When a gas isheated, the averagekinetic energy ofthe moleculesincreases. Thetemperature of thegas increases.

The rate of collisionbetween themolecules and thewalls will increaseif the volume isconstant.

It the gas is allowedto expand, thefaster moleculesnow move in a

bigger space. Therefore, the rate

of collisionbetween themolecules and thewalls remainconstant and thusthe pressure isconstant.

When a gas isheated, the averagekinetic energyincreases. Thetemperature of thegas increases.

The faster movingmolecules strikethe walls of thecontainer morefrequently.

Thus, the pressureof the gasincreases.

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Experiments

Manipulated: Volume of air in asyringeResponding: Pressure of trappedairFixed : mass and

temperature of airinside a syringe

Manipulated: Temperature of trapairResponding: Length of air columnFixed : atmosphericpressure,

Mass of trapped air

The length of the air column, xrepresents the volume of airtrapped inside the capillary tube.The pressure of the trapped air =atmospheric pressure + pressuredue to the concentrated acid

Manipulated: Temperature of trapairResponding: Pressure of thetrapped airFixed : Volume of air

Mass of trapped air

The reading on the Bourdongauge is the pressure of the air inthe round flask and the

thermometer reading representsthe air temperature in the flask

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TemscaCoCo

Absolute temperature peratures measured in the Kelvin, Kle.

nvert C to Kelvin: + 273nvert Kelvin to C : T 273

Absolute zero The0 K0 KAt

V l K G

lowest possible temperature which is -273C or

= -273 Cthis point:

o ume and pressure of gas is zero

inetic energy of the gas molecules is zero

as molecules are stationary.

Example 1The air in a foot pump has an initial

volume of 2800 cm3 and pressure 100kPa. The outlet of the pump is closedand the piston pushed inwards untilthe volume of the air becomes 700cm3. What is the pressure of thecompressed air in the pump?

Example 2The pressure of a bubble under thesea is 120 cm Hg. When the bubble

rises to the surface of the sea, itsvolume becomes 25.0 cm3. Assumingthat the atmospheric pressure is 76cm Hg, what is the original volume ofthe bubble?

Example 3A cylinder contains 200 cm of gas ata temperature of 27 C. The gas isheated until its temperature increasesby 30 C. If the piston of the cylinderexpands under constant p

3

ressure,final volume of the gas?what is the

Example 4A fixed mass of gas in an enclosedmetal container has a pressure of 2.5x 10 Pa. It the gas is heated from 27C to 87

5

C, calculate the finalressure of the gas.p

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