AQA GCSE 1a-1

Heat Transfer

AQA GCSE Physics pages 24 to 37AQA GCSE Science pages 228 to 241

August 21st 2010

AQA GCSE Specification

HEAT ENERGY TRANSFER11.1 How is heat (thermal energy) transferred and what factors affect the rate at which heat is transferred

Using skills, knowledge and understanding of how science works:• to evaluate ways in which heat is transferred in and out of bodies and ways in which the rates of these transfers can be reduced.

Skills, knowledge and understanding of how science works set in the context of:• Thermal (infra red) radiation is the transfer of energy by electromagnetic waves.• All bodies emit and absorb thermal radiation.• The hotter a body is the more energy it radiates.• Dark, matt surfaces are good absorbers and good emitters of radiation.• Light, shiny surfaces are poor absorbers and poor emitters of radiation.• The transfer of energy by conduction and convection involves particles and how this transfer takes place.• Under similar conditions different materials transfer heat at different rates.• The shape and dimensions of a body affect the rate at which it transfers heat.• The bigger the temperature difference between a body and its surroundings, the faster the rate at which heat is transferred.

Heat Energy• Heat is energy that flows from a

hot region to a cold region because of the temperature difference between the two regions.

• Heat energy flows by one or more of the processes of:1. CONDUCTION2. CONVECTION3. RADIATION

Conduction

This is how heat energy flows from a hot to cold region of a substance without the molecules of the substance moving from the hot to cold region.

Conduction by molecules• Molecules 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.• Solids are the best conductors and gases the worst.• A poor conductor is called an insulator.• A vacuum does not allow conduction.

Conduction through metals• Metals have an additional method of conduction.• Metals have electrons that can move from one atom to

another.• These free electrons can pass energy through the metal

very quickly.• Most of the best conductors are metals.• However, diamond, a non-metal, is the best conductor

despite only using the molecule to molecule method.

Rate of heat transferHeat transfer by conduction can increased by:

1. using a better conducting substance

2. decreasing the thickness of the substance

3. increasing the area of the substance

4. increasing the temperature difference across the substance

glasscopper60°C 20°Ccoppercopper100°C

Finding the best conductor• All 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 & wood

• The gummed paper singes and burns first on the wood 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 conductor• A 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 insulator

Air 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.

heatclose movemoleculesgood electronstransfercollide

heat

close

movemolecules

good

electronstransfer

collide

WORD SELECTION:

Question 2Write down three insulating materials and three conducting materials as well as their uses in the table below:

substance conductor or insulator

use

feathers insulator keeping birds warm

fibre glass insulator roof insulation

air insulator clothing

copper conductor cooking pan bases

water conductor cooling hot substances

steel conductor radiators

Question 3Explain why

a) newspaper wrapping keeps hot things hot, e.g. fish and chips, and cold things cold, e.g. ice cream,

b) fur coats would keep their owners warmer if they were worn inside out,

c) a string vest keeps a person warm even though it is a collection of holes bounded by string,

d) a concrete floor feels colder than a carpeted one even though they are at the same temperature.

ConductionNotes questions from pages 28/232 & 29/233

1. Describe, with the aid of a diagram, an experiment to compare how well materials conduct heat.

2. Explain the process of how conduction occurs in (a) metals and (b) non-metals.

3. Why are metals generally better conductors than non-metals?

4. Copy and answer questions (a), (b), (c) and (d) on pages 28/232 and 29/233.

5. Copy the Key Points on page 29/233.6. Answer the summary questions on page 29/233.

Conduction ANSWERS

In text questions:(a) Plastic and wood are poor

thermal conductors, so the handle doesn’t get hot.

(b) The material the rods are made from is the independent variable. The dependent variable is the time taken for the wax to melt.

(c) Felt.(d) The starting temperature.

Summary questions:1. (a) Fibreglass (b) Wood, plastic

(c) Steel2. (a) Felt or synthetic fur could be

used, because they are good insulators.

(b) Is your plan a fair test?3. The free electrons that gain

kinetic energy diffuse through the metal quickly, passing on energy to the other electrons and ions in the metal.

Convection

This is how heat energy flows from a hot to cold region of a substance by the molecules of the substance moving from the hot to cold region.

Convection in fluids• A fluid is a liquid or a gas.• When 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 does not occur in solids.

Convection in water• Potassium 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.

House water heating system• The 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 system

• Water 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.

Hot air rises• Heated 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 breezes• During a hot day heated air

rises up from land that is warmer than the sea.

• Cooler air moves in from the sea to replace the heated air. This is the sea breeze.

• At night the sea is often warmer than the land.

• Air now flows to the sea. This is called a land breeze.

Simple mine ventilation

The fire causes hot air to rise up the shaft above it.

Cooler, fresher air is drawn down the other shaft..

hot air rising

fresh air

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.

gases fluidsmoleculesconvection quicklyrisesdense

WORD SELECTION:

gasesfluids

molecules

convection

quickly

rises

dense

Question 2Explain why

a) a hot drink will cool more slowly if a lid is put on the top of the drink,

b) clouds are often formed at the top of thermals,

c) top loading freezers are more efficient than those with side doors,

d) hot water radiators are best placed\under a window.

ConvectionNotes questions from pages 30/234 & 31/235

1. Describe, with the aid of a diagram, an experiment to demonstrate convection in air.

2. Copy Figure 3 on page 30/234 and explain how hot water travels from the boiler to the hot water tap.

3. Copy and answer question (a) on page 30/234.4. Draw a diagram and explain the cause of a sea breeze.5. Explain how convection occurs and why it only occurs in

fluids.6. Copy the Key Points on page 31/235.7. Answer the summary questions on page 31/235.

Convection ANSWERS

In text question:(a) The water

from the taps would be cold.

Summary questions:1. Rises, mixes, cools, falls.2. (a) It heats it and makes it rise. (b) The hot air passes through the grille into the

room. (c) Cold air flows in at the bottom.3. Drop the crystal into the beaker of water through

a tube. Heat gently under one corner. the colour rises above the point of heating, travels across the top and falls at the opposite side of the beaker (where density of cooler water is greater). The colour then travels across the bottom of the beaker to replace lower density warmer water that rises above the Bunsen flame.

Thermal RadiationThis is how heat energy moves from

a hot to cold region by means of electromagnetic waves.

Thermal radiation is also called: infra-red

or IR radiation.

Properties of thermal radiation• Radiation travels equally in all directions.• Radiation travels far more quickly than the other forms of heat

transfer.• Radiation is the only type of heat transfer that can travel

through a vacuum.• Radiation travels best through a vacuum where its speed is that

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 radiation• We can feel thermal

radiation with our skin.

• A thermopile is a device that converts thermal radiation into electricity.

• Special cameras can be used to take infra-red pictures (Thermal Camera Pictures)

Thermal radiation & temperature• All objects above

absolute zero (-273 oC) give off thermal radiation.

• The hotter an object the more radiation it gives off.

Surface and emissionDarker surfaces emit radiation better than bright surfaces.

Rough surfaces emit radiation better than polished surfaces.

Absorption & Reflection

Dark surfaces absorb radiation better than bright surfaces.

Polished surfaces reflect radiation better than rough surfaces.

Silvered surfaces

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

The Greenhouse

The ‘Greenhouse Effect’ is where gases such as carbon dioxide in the Earth’s atmosphere have the same effect as the glass of a greenhouse.

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.

heat darkdirectionselectromagnetic 300 000- 273vacuum

WORD SELECTION:

smooth

heat

dark

directions

electromagnetic

300 000

- 273

vacuum

smooth

Question 2Explain why

a) heat from the Sun can only reach us by thermal radiation,

b) in hot countries houses are often painted white,

c) car radiators are black,

d) solar cells are black.

Thermal radiationNotes questions from pages 24/228 & 25/229

1. Draw figure 1 on page 24/228 and describe the experiment.

2. Why is thermal radiation sometimes called infra-red radiation?

3. Copy the ‘Key points’ on page 25/229.4. In what form does energy travel from the Sun?5. Explain what is meant by the ‘Greenhouse Effect’6. Copy and answer questions (a) and (b) on page 24/229.7. Copy the Key Points on page 25/229.8. Answer the summary questions on page 25/229.

Thermal radiation ANSWERS

In text questions:(a) The water is cooler than

the rhino, so there is less radiation from it.

(b) The radiation from the rhino reflects at the water surface so the water surface acts like a mirror.

Summary questions:1. Hot iron: infra-red only;

light bulb: both; TV screen: light only; Sun: both

2. Put your hand near it and see if it gets warm due to radiation from the iron.

3. (a) They lose less heat through radiation when they huddle together because they radiate heat to each other.(b) Is your design a fair test?

Surfaces and radiationNotes questions from pages 26/230 & 27/231

1. Which surfaces are: (a) the best emitters; (b) the best absorbers; (c) the worst emitters; (d) the best reflectors of radiation.

2. Draw diagrams to explain why rough surfaces absorb radiation better than smooth ones.

3. Copy and answer questions (a) and (b) on page 26/230.

4. Describe an experiment to compare how different surfaces absorb radiation.

5. Copy the Key Points on page 27/231.6. Answer the summary questions on page 27/231.

Surfaces and radiation ANSWERS

In text questions:(a) A sandy surface is

rough, not smooth, so it absorbs solar radiation more than a smooth icy surface. The sand grains become warm and melt the ice.

(b) They absorb solar radiation better than any other type of surface.

Summary questions:1. (a) White buildings stay cooler than dark

buildings, are better reflectors than dark buildings and they absorb less solar radiation.

(b) A black surface absorbs solar radiation better than a light surface.

2. (a) To ensure a fair test. (b) (i) D (ii) B (c) (i) B (ii) D (d) Greater accuracy, collects multiple

sets of data at whatever time intervals you choose.

3. In direct sunlight black will absorb heat readily, but in shade black will emit heat more readily.

A model of heat transfer processes

1. CONDUCTION

– book passed from student to student.

2. CONVECTION

– book taken a student to the back of the room.

3. RADIATION

– book thrown to the back of the room.

Heating a room using a water radiator

• Hot 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 flask

CONDUCTION – 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.

thermal temperature heattravels

WORD SELECTION:

convection

thermal

temperature

heat travels

convection

Question 2

part of flask processes reduced (eg ‘conduction’)

Write down the ways in which a vacuum flask reduces heat transfer in the table below:

outer cap / cup conduction & convection

plastic cap convection & conduction

shiny mirror surfaces radiation

vacuum conduction & convection

sponge conduction

air conduction

plastic case conduction

Question 3

device or part of a house processes reduced

Write down six different ways in which heat flow from a house can be reduced in the table below:

draught excluders convection

double glazing conduction & convection

reflecting strips behind radiators radiation

loft insulation conduction & convection

carpets conduction

cavity wall insulation conduction & convection

Question 4

Heat transfer by designNotes questions from pages 32/236 & 33/237

1. Explain how a car engine is kept cold.2. Copy and answer questions (a) and (b) on page

32/236.3. Copy Figure 2 on page 32/236 and explain how the

different parts of the vacuum flask reduce heat transfer.4. Explain four ways of reducing heat loss from a house.5. Copy and answer question (c) on page 33/237.6. Copy the Key Points on page 33/237.7. Answer the summary questions on page 33/237.

Heat transfer by design ANSWERS

In text questions:(a) To increase heat

loss by radiation.(b) It increases.(d) Cavity wall

insulation does not transfer heat by convection, because it is a solid but air would.

Summary questions:1. (a) Conduction (b) Convection

(c) Radiation2. (a) A metal frame conducts heat to the

outside; plastic is a poor thermal conductor so it does not.

(b) A vacuum would not transfer heat by convection, but air would.

(c) Is your design a fair test?3. Plastic cap – reduces convection and

conduction; double walled plastic container – reduces conduction; silvered surfaces – reduce radiation; vacuum – stops conduction and convection.

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

Hot issues Notes questions from pages 34/238 & 35/239

1. Answer part (a) from the activity on page 34/238.

2. Answer questions 1, 2, 3, 5, 6, and 7 on pages 34/238 and 35/239.

Hot issues ANSWERS

Activity part (a): (i) Add cavity wall insulation to reduce heat loss through the walls and loft insulation to reduce heat loss through the roof. (ii) 20 years.

1. Thermal energy from a deep tumour would be conducted to the surrounding parts and would not reach the surface of the body.

2. Appreciation that devices can assist with a range of treatments but they come at a cost. Is the money better spent on other things?

3. The hospitals may be part of a research programme, they could be sponsored by manufacturer to test the technology or they could be given extra money by a private benefactor or they could even be charging for its use.

5. (a) white (b) black6. (a) non-reversible

(b) reversible7. (a) The heat from the flame

could escape, raising the temperature of the methane so that it ignites.(b) To make sure that the gauze was not damaged.(c) A modern flame safety-lamp and electronic methanometers.

How Science Works ANSWERS

a) Minimum should be that pins on the different rods are all the same distance along the rod. (Better: placed at intervals along the rods).

b) For example: use the same amount of Vaseline each time.

c) The metal rods should be heated in the same way each time by the Bunsen (e.g. in the same part of the flame). The rods should also be protected from direct radiant energy from the Bunsen’s flame which would make the results invalid.

d) Repeat procedure.

e) Are there accurate labels with a simple layout?

f) Bar chart – categoric independent variable with a continuous dependent variable.

g) A second person achieving the same results increases reliability.

h) The results support the conclusion. However, the conclusion is invalid due to different initial temperatures.