EDEXCEL IGCSE / CERTIFICATE IN PHYSICS 4-2Thermal EnergyEdexcel IGCSE Physics pages 133 to 141June 17th 2012 All content applies for Triple & Double Science

Edexcel SpecificationSection 4: Energy resources and energy transfer b) Energy transferdescribe how energy transfer may take place by conduction, convection and radiationexplain the role of convection in everyday phenomenaexplain how insulation is used to reduce energy transfers from buildings and the human body.

Thermal energyThermal or heat energy is the energy that flows from a hot region to a cold region by one or more of the processes of:

CONDUCTIONCONVECTIONand RADIATION

Thermal ConductionThermal conduction is how thermal energy flows through a substance without the substance itself moving.

Solids and metals are the best conductors, gases are the worst.

A vacuum does not allow conduction.

A poor conductor is called an insulator.

Thermal Conduction by moleculesMolecules vibrate more when they are heated.Intermolecular forces allow the molecules to pass their vibrations from one to another.The stronger the forces the faster the vibrations are passed.

Conduction through metalsMetals have an additional method of conduction. They have electrons that can move from one atom to another. These electrons can pass energy through the metal very quickly.Most of the best conductors are metals. However, diamond, a non-metal, is an excellent conductor because it has very strong intermolecular bonds.

Keeping warm with clothingAir is a good insulator.Clothes keep us warm by trapping air around our bodies.The more air layers are trapped the greater is the insulating effect of the clothing.

Animals keep warm in a similar way by using fur or feathers to trap insulating layers of air around their bodies.

Rate of thermal transferHeat transfer by conduction can increased by:using a better conducting substancedecreasing the thickness of the substanceincreasing the area of the substanceincreasing the temperature difference across the substance60C20C100C

Finding the best conductorAll the rods have the same length and cross-sectional area.They are all heated equally at one end with the bunsen burner.When the other end of a rod reaches a certain temperature the paraffin wax melts and the match stick falls off.A match stick will fall off the best conductor first.This should be the rod made of copper.

Comparing brass & woodThe gummed paper singes and burns first on the wooden side of the rod.This is because the brass removes the heat away from the paper more quickly than the wood.Brass is therefore the better conductor.

Water - a poor conductorA boiling tube of water is heated near the top of the water.Water boils at the top.The bottom of the tube remains cool enough to hold.This shows that water (and glass) only conduct heat relatively slowly.

Air - a good insulatorAir trapped in feathers, cavity wall insulation, loft insulation, duvets and lagging greatly reduces heat loss by conduction.

Question 1Choose appropriate words to fill the gaps below:Conduction is the main form of ______ transfer in solids. This is because the molecules are relatively _______ together. Extra heat energy makes the molecules __________ more. They pass on their extra vibrational energy to neighbouring ______________. Metals are __________ conductors of heat energy because they contain many free __________ which can move through the solid and ____________ energy. The electrons give up their energy when they ___________ with other molecules.heatclosemovemoleculesgoodelectronstransfercollideheatclosemovemoleculesgoodelectronstransfercollideWORD SELECTION:

Question 2Complete the table below:feathersinsulatorkeeping birds warmfibre glassinsulatorroof insulationairinsulatorclothingcopperconductorcooking pan baseswaterconductorcooling hot substancessteelconductorradiators

substanceconductor or insulatoruse

Question 3Explain whynewspaper wrapping keeps hot things hot, e.g. fish and chips, and cold things cold, e.g. ice cream,fur coats would keep their owners warmer if they were worn inside out,a string vest keeps a person warm even though it is a collection of holes bounded by string,a concrete floor feels colder than a carpeted one even though they are at the same temperature.

ConvectionConvection is the transfer of heat through fluids (liquids and gases) by the upward movement of warmer, less dense regions of fluid.

Convection does not occur in solids.

A vacuum does not allow convection.

Convection currentsWhen molecules are heated they move more quickly and occupy more space.Hotter fluids are therefore less dense than colder fluids.Hotter fluids rise up to float on top of colder fluids.A convection current is the path taken by rising hot fluids and sinking cold fluids.

Convection in waterPotassium permanganate crystals are used to dye water purple.When the bottom of the flask of water is heated warm less dense water rises.The potassium permanganate dye rises with the warmed water.When the water cools it becomes denser and sinks down to the bottom of the flask.

Heating a room by convection

Simple house water heating systemThe boiler heats the water.Hot water rises to the top of the boiler and up to the top of the storage tank.Colder water in the tank falls to the bottom of the boiler to be heater.A hot water tap draws water from the top of the storage tank.The cold water supply replenishes the hot water drawn off.

Engine water cooling systemWater heated by the engine rises to the top of the engine.This water is pumped into the top of the radiator.The fan cools the water in the radiator.Cooled water falls from the radiator into the engine.

ThermalsHeated air provides lift for a hot air balloon.Heated land causes rising air currents called thermals.Thermals can be used by gliders to provide lift.Many birds also use thermals for lift.

Sea and land breezesDuring a hot day heated air rises up from land that is warmer than the sea.A sea breeze consists of cooler air that moves in from the sea to replace the heated air.

At night the sea is often warmer than the land.Air now flows to the sea. This is called a land breeze.

Simple mine ventilationThe fire causes hot air to rise up the shaft above it. Cooler, fresher air is drawn down the other shaft..

Question 1Choose appropriate words to fill in the gaps below:Convection is a method of heat transfer that only occurs in ________ (liquids and ________). When part of a fluid is heated the ___________ in that region move more ______ and take up more space. The heated fluid expands and becomes less _______ than the surrounding cooler fluid. The heated fluid _______ on top of the cooler fluid. The upward path of the heated fluid is called a ____________current.gasesfluidsmoleculesconvectionquicklyrisesdenseWORD SELECTION:gasesfluidsmoleculesconvectionquicklyrisesdense

Question 2Explain whya hot drink will cool more slowly if a lid is put on the top of the drink,clouds are often formed at the top of thermals,top loading freezers are more efficient than those with side doors,hot water radiators are best placed\under a window.

Thermal RadiationThermal radiation is the transfer of energy by infra-red (IR) waves.

Radiation is the only type of heat transfer that can travel through a vacuum through which it travels at the speed of light (300 000 km/s or 186 000 miles per second).

Gases tend to allow radiation through better than liquids, liquids better than solids.

Detecting thermal radiationWe can feel thermal radiation with our skin.Special cameras can be used to take infra-red pictures.

Thermal radiation & temperatureAll objects above absolute zero (-273 C) give off thermal radiation. The hotter an object the more radiation it gives off.

Surface and emissionDark surfaces emit radiation better than bright surfaces.

Rough surfaces emit radiation better than polished surfaces.

Absorption & Reflection Dark surfaces absorb radiation best. Bright surfaces reflect radiation best.

Rough surfaces absorb radiation best.Polished surfaces reflect radiation best.

Silvered surfacesA metal kettle, a firefighter and a marathon runner make use of silvered surfaces.

Question 1Choose appropriate words to fill in the gaps below:Thermal radiation, also known as __________ radiation, is how heat travels by _____________ waves. Radiation travels equally in all ________and most quickly through a __________ where its speed is ___________ km/s. All objects above absolute zero ( ________ oC ) give off thermal radiation. Hot, _______ and rough surfaces emit radiation best. Bright and ___________surfaces reflect radiation best.heatdarkdirectionselectromagnetic300 000- 273vacuumWORD SELECTION:smoothheatdarkdirectionselectromagnetic300 000- 273vacuumsmooth

Question 2Explain whyheat from the Sun can only reach us by thermal radiation,in hot countries houses are often painted white,car radiators are black,solar cells are black.

Heat losses from a house

How heat losses are reduced

Heating a room using a water radiatorHot water heats the metal radiator by conduction.The outer metal surface heats air in contact by conduction and radiation.Hot air circulates a room by convection.

Reducing heat transfer using a vacuum flaskCONDUCTION reduced by the vacuum, stopper, glass, cork and air spaces.CONVECTION reduced by the vacuum, stopper and the trapped air spaces.RADIATION reduced by the silvered glass walls.

Question 1Choose appropriate words to fill in the gaps in below:_________ is the form of energy that ________ from a hot place to a cold one because of the __________ difference between these two places. Heat moves by conduction, ____________ and radiation._________ energy is an alternative name for heat energy.thermaltemperatureheattravelsWORD SELECTION:convectionthermaltemperatureheattravelsconvection

Question 2Write down the ways in which a vacuum flask reduces heat transferouter cap / cupconduction & convectionplastic capconvection & conductionshiny mirror surfacesradiationvacuumconduction & convectionspongeconductionairconductionplastic caseconduction

part of flaskprocesses reduced (eg conduction)

Question 3Write down six different ways in which heat flow from a house can be reduced in the table below:draught excludersconvectiondouble glazingconduction & convectionreflecting strips behind radiatorsradiationloft insulationconduction & convectioncarpetsconductioncavity wall insulationconduction & convection

device or part of a houseprocesses reduced

Online SimulationsConduction of heat in solids - eChalk Relative conductivities of metal rods - eChalk Convection currents in a room & fridge - eChalk Sequential Puzzle on Heat Conductor order- by KT - Microsoft WORD Energy House Simulation. For instructions click here. Worksheet for Energy House Simulation (KT) Gas molecule simulation of convection - falstad Thermal Camera Pictures - falstad Black Body Spectrum - PhET - How does the blackbody spectrum of the sun compare to visible light? Learn about the blackbody spectrum of the sun, a light bulb, an oven, and the earth. Adjust the temperature to see the wavelength and intensity of the spectrum change. View the color of the peak of the spectral curve

The Greenhouse Effect - PhET - Just how do greenhouse gases change the climate? Select the level of atmospheric greenhouse gases during an ice age, in the year 1750, today, or some time in the future and see how the Earth's temperature changes. Add clouds or panes of glass. BBC KS3 Bitesize Revision: Temperature and heat Transfer of thermal energy - includes applet showing convection currents in a room BBC Bitesize Revision: Introduction Page on AQA Heat Transfer & Efficiency Heat transfer by conduction & convection - includes applet showing convection currents in a room Heat transfer by radiation Reducing heat loss Test bite on Heat transfer, Energy transfers & Efficiency

Thermal EnergyNotes questions from pages 133 to 141Define what is meant by (a) thermal conduction, (b) convection and (c) thermal radiation.Describe how the process of convection is involved in (a) heating a room using a convector heater, (b) a lava lamp.Explain in terms of the methods of heat transfer how clothing keeps us warm.Copy the pie chart on page 139 and explain how the heat loss from a house can be reduced from each of the five sections.

Answer the questions on page 141.Verify that you can do all of the items listed in the end of chapter checklist on page 141.

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