Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9...

Transfer of Thermal EnergyTransfer of Thermal Energy

T H E M E T H R E E : T H E R M A L P H Y S I C S

C h a p t e r 9Learning outcomes• Appreciate how thermal energy is

transferred form a higher temperature region to one of lower temperature

• Describe how energy transfer occurs through molecules in solids

• Describe density changes in fluids leading to convection

• Explain that radiation does not require a material medium and rate of energy transfer is affected by: (i) colour and texture of the surface;

(ii) surface temperature; (iii) surface area• Apply the concept of thermal energy

transfer to everyday applications



C h a p t e r 99.1 Conduction

• Conduction is the process by which heat is transmitted through a medium from one particle to another



C h a p t e r 99.1 ConductionHow Conduction Works• Solids: conduction occurs when

thermal energy transferred from one particle to another.

• When one end of a rod is heated, molecules there gain energy and vibrate faster.

• These molecules collide with less energetic neighbors.

• Some of their energy is transferred to neighboring molecules which thus gains kinetic energy.



C h a p t e r 99.1 Conduction• Thermal energy is passed along the rod by the

vibrating molecules.• Vibration transfers energy from molecule to

molecule until cold end reaches same temperature as hot end

• No net movement of molecule during conduction.• Solids are better conductors of heat than liquids

and gases as molecules are closer together.• Kinetic energy transferred more quickly.




• Conduction takes place through vibrating molecules as well as free electrons in metals.

• When heated, free electrons gain energy and move faster.

• They move in the spaces between atoms before colliding with some atoms and transferring energy to them.

• Much faster than conduction by vibration of molecules in non-metals.

• Metals conduct heat much faster then non-metals which have no free electrons.




• Examples of good and poor heat conductors (insulators)

Good conductors

Poor conductors (insulators)

GoldSilver

CopperIron

BrassAluminium

GlassStoneWater

PlasticsWood

Polystyrene foam*Wool*

fibreglass**Materials containing trapped air



C h a p t e r 99.1 ConductionApplications of Conduction• Uses of Good Conductors• Used where thermal energy has to be quickly

transmitted• E.g. Pans, kettles, cooking utensils are made of

metals like aluminium, copper, steel• Copper pipes for heat exchangers• Heat sinks in electronic equipment



C h a p t e r 99.1 ConductionUses of Poor Conductors

(Insulators)• E.g. Cloth, plastics, wood• Help keep unwanted heat away• Help prevent heat loss



C h a p t e r 9

• Air is good insulator, reduces conduction of heat

• Felt material used as insulators to trap air pockets

9.1 Conduction

Prevent?



C h a p t e r 9

1. Thermal energy of a body is the total kinetic energy of the atoms or molecules in the body.

2. Heat is energy in transit from a high temperature object to a lower temperature object. It is recognised only as it crosses the boundary of a system.

3. Conduction is the process by which thermal energy is transmitted through a medium from one particle to another.

4. Conduction in metals is far better than in other solids because other than the vibrating atoms, the free electrons in a metal are also responsible for the conduction of thermal energy.

5. Liquids and gases are poor thermal conductors.



C h a p t e r 9



C h a p t e r 99.2 Convection

• Convection is the process by which heat is transmitted from one place to another by the net movement of heated particles of a gas or liquid.

• Convection takes place in liquids and gases where atoms or molecules move from one place to another

• Transfer of thermal energy depends on transfer of heated matter.



C h a p t e r 99.2 ConvectionConvection in Liquids• Add potassium

permanganate crystals into a flask of water

• Crystals sink to bottom of the flask

• Heat flask gently just below the crystals

• Purple-streaked water rise upwards and fan outwards.

• Water becomes uniformly purplish after a while.




• Circulation of a liquid called a convection current

• Due to heating at the bottom of liquid• When a liquid is heated, it expands,

becomes less dense and floats upwards.• Cold and denser liquid moves down to

replace the rising liquid masses.• This cold liquid, in turn, gets heated up.



C h a p t e r 9

A Hot Water System• based on convection principle• Consists of a boiler, water

storage tank and cold tank interconnected by pipes

• Convection currents drive hot water from the top of boiler into hot water storage tank.

• Cold water from storage tank is drawn down to the boiler where it in turn becomes heated.

9.2 Convection



C h a p t e r 99.2 ConvectionCar Radiator • Car engines are kept cool by a water jacket • When water in the water jacket gets heated, it

flows into copper tubes which include many cooling fans.

• A fan causes air to flow past the tubes and cool water in them.

• Cooled water flows down and back into engine through a hose at the bottom.



C h a p t e r 99.2 ConvectionConvection in Gases• To show convection currents in air• Fit two glass chimneys to the top of a

wooden box with plane glass window • Light a candle below one chimney• Heat from this initiates a convection

current• Hot air flows out of this chimney • Cold air flows in through other

chimney• Used to ventilate underground mines



C h a p t e r 99.2 ConvectionAir-conditioners• Cold air blown in sinks

because it’s denser then warm air.

• Warm air displaced by cold air rises and is drawn into air-conditioner to be cooled.

Refrigerator• Cold air below freezer sinks• Set up convection currents

Today’s fridge, freezer below??



C h a p t e r 99.2 ConvectionVentilation in a Kitchen• Hot air rises through chimney • Sets up a convection current• Cool air drawn into kitchen



C h a p t e r 99.2 ConvectionSea and Land Breezes• In the day, land heats up faster than the sea• Air above land expands and rises• Cool air above sea moves inland • Result in sea breeze




• At night, the land cools faster than the sea• Warm air above sea rises • Cool air above land moves out to take its

place• Result in land breeze



C h a p t e r 9

1. The process of thermal energy transfer most significant in fluids (liquids and gases) is convection.

2. Convection is the process by which heat is transmitted from one place to another by the movement of heated particles of a gas or liquid.

3. Convection currents are caused by density changes.

4. Hot fluids rise and cold fluids sink.



C h a p t e r 9



C h a p t e r 99.3 Radiation

• Radiation is the transfer of energy by electromagnetic waves

• Emission of Radiation• The rate of energy transfer by radiation is

affected by: (a) surface temperature

(b) colour and texture of the surface(c) surface area



C h a p t e r 99.3 RadiationUses of Poor Emitters of RadiationShiny surfaces are poor radiators E.g. Teapots and aluminum foil

Uses of Good Emitters of RadiationDull surfaces radiate quickly

E.g. Cooling fins of refrigerators and heat sinks



C h a p t e r 99.3 RadiationAbsorption of Radiation• Dull black surfaces better absorbers than

shiny surfaces• Good emitter is also good absorber• Shiny surfaces good reflectors of heat



C h a p t e r 99.3 RadiationUses of Good and Poor Absorbers of

Radiation• Houses in hot countries painted white to

keep them cooler.• Factory roofs coated with aluminium to

reduce radiation absorption.• Solar control films allow light to pass

through but keep out infrared radiation.• Dark clothes absorb radiation readily.• Solar panels painted black to absorb as

much radiation from sun as possible.



C h a p t e r 99.3 RadiationVacuum Flask• Reduces transfer of

thermal energy by conduction, convection and radiation

• Consists of a double-walled glass container with a vacuum in between

• Silvered walls on vacuum side

• Container supported by foam plastic, a poor conductor



C h a p t e r 99.3 Radiation

• No heat can enter or leave flask by conduction or convection across the vacuum

• Inner silvered surface reflects radiation from hot fluids back into flask

• Outer silvered surface reflects radiation in external surroundings away from flask

• Foam plastic support and plastic cup minimises heat transmitted by conduction through thin glass walls

• Plastic cup stops convection and evaporation



C h a p t e r 9

1. Radiation is the transfer of energy by infra-red electromagnetic waves.

2. Dull black surfaces are good emitters and absorbers of radiation.

3. The greater the surface area of the object, the faster is the rate of thermal transfer from it.

4. The greater the temperature of an object, the faster is the rate of thermal energy transfer from it.



C h a p t e r 9

Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9...

Documents

Transcript of Transfer of Thermal Energy T H E M E T H R E E : T H E R M A L P H Y S I C S C h a p t e r 9...