Transfer of Thermal Energy

A bird can reduce the heat loss from its body during cold weather by fluffing up its feathers.

What are the processes of heat transfer?

A bird can reduce the heat loss from its body during cold weather by fluffing up its feathers.

What are the processes of heat transfer?

Click to proceed

~microLESSON Information~~microLESSON Information~• Level: Secondary Three (Special)• Time: 60 mins• Objectives:

Show understanding that thermal energy is transferred from a region of higher temperature to a region of lower temperature

Show understanding that regions of equal temperature will be in thermal equilibrium

Describe in molecular terms, how energy transfer occurs in solids

~microLESSON Information~~microLESSON Information~• Objectives: ( continue…)

Describe, in terms of density changes, convection in fluids.

Show understanding that energy transfer of a body by radiation does not require a material medium and the 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

IntroductionIntroduction

This dessert tastes hot on the outside and on biting into the bread; you will taste the cold, unmelted ice cream.

Have you ever heard or tried this dessert before?

Can you think of two reasons why the ice cream did not melt while it was fried with the bread in the hot oil?

Watch the following demo ….

Do you know why the part where the paper is wrapped around the plastic container burnt faster than the paper wrapped around the metal can?

Click to see explanation

Plastics vs metal can

ExplanationMetal can is a good conductor of heat, it conducts away the heat faster hence, taking the paper a longer time to catch fire as opposed to the part just wrapped around the plastic container that catches fire faster.

CONCEPT MAP

by processby process

Low temperatureLow temperature

Transfer of Transfer of thermal energy thermal energy

to achieve to achieve thermal thermal

equilibriumequilibrium

ConductionConduction RadiationRadiation

1. Vibration of atoms1. Vibration of atoms or moleculesor molecules2. Movement of free2. Movement of free electronselectrons

ConvectionConvection

byby

High temperatureHigh temperature

Density changesDensity changes

byby

Everyday applicationsEveryday applications

Emission ofEmission ofInfra-red radiationInfra-red radiation

• colour and texture colour and texture of the surface of the surface

• surface surface temperaturetemperature

• surface areasurface area

byby

rate affected byrate affected by

havehave

ContentsContentsSectSect

AA

SectSectBB ConductionConduction

Thermal EquilibriumThermal Equilibrium

SectSectCC ConvectionConvection

SectSectDD

QuitQuitNowNow QuitQuit

SectSectEE

ExplorationExploration

SectSectFF

Worksheet / Worksheet / Class noteClass note

SectSectGG

QuizQuiz

RadiationRadiation

What happens if a hot object is placed next to a cold object?

Well, we expect the two objects to finally come to the same final temperature after sometime.

We say that the objects now are in thermal equilibrium with each other.

How does this happen? Click here.

thermal equilibriumthermal equilibrium

Heat travels from a place of higher temperature to a place of lower temperature.Heat travels from a place of higher temperature to a place of lower temperature.

molecules in hot object lose kinetic energy by slowing down

molecules in cold object gain energy by speeding up

molecules have higher kinetic energy

molecules have higher kinetic energy

molecules have lower kinetic energy

molecules have lower kinetic energy

final temp

Molecules have same

kinetic energy

Transfer of energy stops when molecules of both objects have the same average kinetic energy. The objects are said to be in thermal equilibrium and they are at the same temperature.

Temperature is a measure of average kinetic energy of molecules in an object.

Two objects in thermal contact have the same temperature are in thermal equilibrium because the molecules have the same average kinetic energy.

transfer of thermal energytransfer of thermal energy

Heat travels from a place of higher temperature to a place of lower temperature.

Three processes by which heat may be transmitted

Heat travels from a place of higher temperature to a place of lower temperature.

Three processes by which heat may be transmitted

Does the hand receives heat from the electric iron via the three processes

of heat transfer?

1. Conduction2. Convection 3. Radiation

conductionconductionhow conduction workshow conduction works

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

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

Conduction takes place by molecular vibration or free electron diffusion.

Click to view

conductionconductionMolecular VibrationMolecular Vibration

In molecular vibration, the molecules of the substance being heated vibrate faster.

They jostle their neighbouring molecules, making them vibrate as well. In this way, heat flows one point to another.

direction of heat direction of heat transfer along the rodtransfer along the rod

Click on the picture to animate

conductionconductionFree electron diffusionFree electron diffusion

In free electron diffusion, free electrons at the heated end move faster as a result of having more kinetic energy.

These fast moving, energy carrying electrons spread to the cooler parts of the metal and then transfer their kinetic energy to other electrons by colliding with them.

This usually occurs in metals and is faster than the vibration process.

Click on the picture to animate

conductionconductionDemonstrationDemonstration

heat travels from the hot end of the metal pipe to the cooler end

This demonstration shows how heat travels along a metal pipe by conduction.Match sticks are attached to

the pipe with wax. The heat melts the wax and this causes the match sticks to fall.

Loading video clip………..

conductionconductionhow conduction workshow conduction works

this transfer of energy from particle to particle continues until thermal equilibrium is reached

no net movement of particles during conduction, the particles merely vibrate about its rest position

this transfer of energy from particle to particle continues until thermal equilibrium is reached

no net movement of particles during conduction, the particles merely vibrate about its rest position

Click on the following link:Look under conduction & click on Start heating

conductionconductionrate of conductionrate of conduction

Solids are better conductors of heat than liquids and gases

in solids, the particles are closer together than in liquids and gases

kinetic energy is transferred more quickly

Solids are better conductors of heat than liquids and gases

in solids, the particles are closer together than in liquids and gases

kinetic energy is transferred more quickly

Conduction in solids

Conduction in liquid

Conduction in gas

Comparison of conduction in all three states of matter

Click on the following link:

conductionconductionrate of conductionrate of conduction

Metals conduct heat much faster than non-metals

heat is conducted in metals by ‘free’ electrons and by the vibration of molecules whereas heat is conducted in non-metals only by the vibration of molecules

Metals conduct heat much faster than non-metals

heat is conducted in metals by ‘free’ electrons and by the vibration of molecules whereas heat is conducted in non-metals only by the vibration of molecules

Heat conduction in a metalHeat conduction in a non metal

Attention: in the video clip, the rod is a non metal and not a metal as indicated in the clip.

heat transfer by electrons is faster than by vibration

collision between moving electrons transfer some energy

metals are good thermal conductors of heat but are poor insulators

Gases are very poor conductors of heat as their molecules are too far apart to affect each other much. This means that air is a terrible conductor of heat energy.

rate of conductionrate of conduction

Thermal Conductivity

The ability to transfer heat within an object.

It varies for different materials.

Good conducting materials are those materials where there is little space between the particles - like most metals. These are called heat conductors.

Poor conductors, like glass and wood are called heat insulators.

conductionconductionconductors and insulatorsconductors and insulators

Good Good ConductorsConductors Poor ConductorsPoor Conductors

goldsilver

copperiron

brassaluminium

glassstonewater

plasticswood

Materials containing trapped air:

polystyrene foamwool

fibreglass

uses of good conductorsuses of good conductorsApplicationApplication

Good conductors are used in situations where heat has to be transmitted quickly.Good conductors are used in situations where heat has to be transmitted quickly.

A stone floor feels cool to a bare feet, but a floor mat in the same

room feels warm. Why?

good conductors

insulators

a saucepan makes use of good conductors and

insulators

ApplicationApplicationuses of poor conductors (insulators)uses of poor conductors (insulators)

Poor conductors (insulators) are used in situations where unwanted heat has to be kept away or to prevent heat loss.Poor conductors (insulators) are used in situations where unwanted heat has to be kept away or to prevent heat loss.

How does the fur of a polar bear help to minimise heat loss from the body?

How can we save energy?ApplicationApplication

Insulators can reduce transfer of energy, therefore using insulation results in the saving of energy.

1. lagging hot water tanks and pipes

2. cavity wall insulation

3. Floor insulation

4. Vacuum flasks

5. Double glazing

6. Thick curtain

7. Use of fiberglass, felt and expanded polystrene foam as insulators in the wall of houses

Following are examples of insulation to save energy cost.

convectionconvectionconvection in liquids and gasesconvection in liquids and gases

occurs in fluids (liquids and gases), but not in solids

density changes cause convection currents; hot fluids rise and cold fluids sink

occurs in fluids (liquids and gases), but not in solids

density changes cause convection currents; hot fluids rise and cold fluids sink

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

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

transfer of heat by convection

hot(lower density)

liquid or gas

cool (higher density)

Look under convection

convectionconvection

Click on image to animate

When a portion of water is heated by direct contact, it expands and decreases in density. Therefore, warm water rises.

convectionconvection

The surrounding cold water (being relatively denser) moves in to take its place. This process is repeated and the water gradually becomes hot.

When a portion of water is heated by direct contact, it expands and decreases in density. Therefore, warm water rises.

Click on image to animate

convection in liquidsconvection in liquidsconvectionconvection

The circulation of a liquid in this manner is called a convection current.The circulation of a liquid in this manner is called a convection current.

small flame

waterpotassium

permanganate crystals

purple streak

flask

convectionconvection

Convection occurs more readily in gases than in liquids because they expand much more than liquids when their temperature rises.

Convection occurs more readily in gases than in liquids because they expand much more than liquids when their temperature rises.

convection in gasesconvection in gases

smouldering paper smoke

glass window

glass cylinders

box

lighted candle

convection currents in air

Click here for explanation for above experiment

Explanation for experiment

A candle is lit below one chimney. The heat from this initiates convection current. The hot air flows out of this chimney. Cold air flows in through the other chimney. This is tested by holding a piece of smouldering paper over the top of the chimney through which the cold air enters the box and observing the path of the smoke. This simple method of air circulation was once used to ventilate underground mines.

hot water systemhot water systemApplicationApplication

A hot water system makes use of the principle of convection.A hot water system makes use of the principle of convection.

hot water rising

boiler

hot water storage tank

expansion pipe

ball valve

cold tank

cold water drawn down

Click http://www.howstuffworks.com/water-heater.htm to understand how the water heater separates hot water from cold water in the tank.

cooling system in a car – the radiatorcooling system in a car – the radiatorApplicationApplication

When a car engine is running for a long time, a lot of heat energy is produced. It is necessary to cool the engine so that it does not overheat.

When a car engine is running for a long time, a lot of heat energy is produced. It is necessary to cool the engine so that it does not overheat.

air into radiator

copper tubes with cooling fins

hot water

cool water

pump engine

When water in the water jacket gets heated, it flows into copper tubes and cool the water in them. The cooled water flows down and back into the engine through a hose at the bottom.

When water in the water jacket gets heated, it flows into copper tubes and cool the water in them. The cooled water flows down and back into the engine through a hose at the bottom.

ApplicationApplication

Air-conditioners are best positioned high, near the ceiling of a room.Air-conditioners are best positioned high, near the ceiling of a room.

air-conditionersair-conditioners

cold air

warm air

air-conditioner

cold air which is denser sinks

warm air which is less dense rises

cycle repeats until room air temperature is the same as the temperature set on the thermostat of the air-conditioner

cold air which is denser sinks

warm air which is less dense rises

cycle repeats until room air temperature is the same as the temperature set on the thermostat of the air-conditioner

ApplicationApplication

Click below for an animation on land and sea breeze.

Click for an explanation of land breeze and sea breeze.

You may also visit the following website:http://earthsci.terc.edu/content/visualizations/es1903/es1903page01.cfm?chapter_no=visualization

land breezeland breeze ApplicationApplication

sea cools slower than the land

warm air above sea rises

cool air above land moves to the sea

sea cools slower than the land

warm air above sea rises

cool air above land moves to the sea

Do you know why the sea cools slower than the land?

sea breezesea breezeApplicationApplication

land heats up faster than the sea

air above land heats up and rises

cool air above sea rushes in

land heats up faster than the sea

air above land heats up and rises

cool air above sea rushes in

Do you know why the land heats up faster than the land?

does not require any medium can take place in a vacuum

does not require any medium can take place in a vacuum

radiationradiation

A method of heat transfer by which a heat source transmits infra-red (IR) electromagnetic waves.A method of heat transfer by which a heat source transmits infra-red (IR) electromagnetic waves.

Heat from the sun reaches the Earth by the process of radiation

• Infra-red radiation is absorbed by all objects and surfaces.

• This absorption causes heat gain and a temperature rise.

• Dull,black surfaces absorb infra-red rays very quickly.

Radiation

absorbed by the box

radiationradiation

• Infra-red radiation is emitted by all objects and surfaces.

• This emission causes the temperature to fall as heat is lost.

• Dull, black surfaces are the best radiators.

Radiation is quickly

emitted by a black box

Radiation is slowly

emitted by a shiny box

radiationradiation

emission of radiationemission of radiationradiationradiation

the hotter an object is, the more energy it radiates dull black surfaces are good emitters or radiators than

shiny ones the greater the surface area and temperature of the

object, the faster is the rate of heat transfer from it

the hotter an object is, the more energy it radiates dull black surfaces are good emitters or radiators than

shiny ones the greater the surface area and temperature of the

object, the faster is the rate of heat transfer from it

boiling water

dull black

shiny

which side feels hotter?

emission of radiationemission of radiationradiationradiation

the greater the surface area and temperature of the object, the faster is the rate of heat transfer from it

the greater the surface area and temperature of the object, the faster is the rate of heat transfer from it

the big ears of the African elephant provide a large surface area for the giant mammal to cool off quickly

in hot weather

absorption of radiationabsorption of radiationradiationradiation

dull blackpolished

cork held by wax cork

falls off

wax melts first

dull black surfaces are good absorbers of radiation than shiny ones

in general, good emitters or also good absorbers

dull black surfaces are good absorbers of radiation than shiny ones

in general, good emitters or also good absorbers

uses of good and poor emittersuses of good and poor emittersApplicationApplication

Good emitters are used in situations where heat has to be quickly emitted.Good emitters are used in situations where heat has to be quickly emitted.

Good EmittersGood Emitters Poor EmittersPoor Emitters

Cooling fins at the back of a refrigerator

painted dull black

A shiny metal teapot

refrigerator

cooling fins at the back of a

refrigerator painted dull black

uses of good and poor absorbersuses of good and poor absorbersApplicationApplication

Good AbsorbersGood Absorbers Poor AbsorbersPoor Absorbers

Solar heating panels are painted in dull black

paint

Houses in hot countries and factory roofs are

painted in white, light-coloured paint, aluminium paint

Light colours are chosen for clothes and cars in

hot weather

Good absorbers are used in situations where heat has to be quickly absorbed.Good absorbers are used in situations where heat has to be quickly absorbed.

solar panels

keeps hot liquids hot and keeps cold liquids cold keeps hot liquids hot and keeps cold liquids cold

vacuum flaskvacuum flask

plastic cap

vacuum

hot liquid

glass silvered surfaces

foam plastic support

outer case

Application Application

minimize heat losses by conduction, convection, radiation and evaporation

minimize heat losses by conduction, convection, radiation and evaporation

Click here to view design features of a vacuum flaskClick here to return to main menu

Design features of a vacuum flask

Design features of a vacuum flask

Prevent heat gain/lost due to

Conduction

Convection

Radiation

Evaporation

Vacuum between the walls

Both walls are silvered on the vacuum side

Supported on foam plastic Plastic cap

QuizQuiz

 2) Metals are better conductors of heat than non-metals because in metals there are

A. many free electrons that are in motion

B. many free electrons that are in vibration

C. less free electrons that are in motion

D. less electrons that are in vibration

1) Heat is conducted through a plastic rod by

A. collisions of molecules in random motion

B. collisions of vibrating molecules

C. convection currents

D. radiation

There are three questions in this section. Try them and check the answer provided on the next slide.

3) Water in the electric heater gets heated up by

A. conduction

B. convection

C. conduction and convection

D. none of the above

Click here for answer

The answers are as follows:

1)B2)A3)C

Good! Did you get all the answers correct? Now you may click here to proceed to section G to further explore on this topic.

Answer for QuizAnswer for Quiz

Worksheet / Lecture noteWorksheet / Lecture note

worksheetClass note

You should print out a copy of the above worksheet and class note to accompany you through this e-lesson

ExplorationExploration

How does insulation work for us?

http://howthingswork.virginia.edu/book.html

Clothing and insulationhttp://plastics.about.com/library/weekly/aa980630.htm Fiberglass insulationhttp://www.edquest.ca/Notes/7unitc.html

Heat and temperature

http://www.ornl.gov/roofs+walls/insulation/ins_01.html

How does insulation work for you?

You may visit the following websites to discover more about the application of heat transfer in our daily life.

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