Skip to content
Cambridge IGCSE0625

Transfer of thermal energy

Physics 0625 Chapter Notes

What this chapter covers

Transfer of thermal energy - ConductionTransfer of thermal energy - ConvectionTransfer of thermal energy - RadiationTransfer of thermal energy - Consequences of thermal energy transfer
ShareWhatsAppPost
Transfer of thermal energy notes

Read online free. Download a watermarked copy with a free account.

Read the notes

The full Transfer of thermal energy notes as text: skim, search, and jump between subtopics.

~25 min read

In this chapter

  1. 1Good and bad thermal conductors
  2. 2Conduction by lattice vibrations
  3. 3Free electrons in metals
  4. 4Why gases and most liquids conduct badly
  5. 5Conductors in between
  6. 6Where convection happens
  7. 7Convection currents and density
  8. 8Convection in experiments and equipment
  9. 9Thermal radiation
  10. 10Surface colour and texture
  11. 11Energy balance and temperature
  12. 12Rate of emission
  13. 13Experiments on emitters and absorbers
  14. 14Heating a pan: conduction
  15. 15Heating a room: convection
  16. 16Heating by radiation
  17. 17Black and shiny surfaces
  18. 18The vacuum flask
  19. 19More than one process at once

1. Good and bad thermal conductors

A good thermal conductor passes thermal energy through itself quickly. A thermal insulator is a bad conductor. A good conductor carries energy away from the point of heating.

To compare materials, heat each one in the same way and see how quickly the energy gets through. In one test wax is melted by each rod: the rod where the wax melts quickest is the best conductor.

In another test, containers of different metals heat water on a hotplate. Measure the time taken for the water to reach boiling point. Keep the volume of water, its starting temperature and the setting of the hotplate the same, then compare the times.

Examiner insight

“The majority of candidates failed to recognise that copper, as a good conductor, conducts the energy away from the point of heating, thereby keeping the paper cooler.” “Many candidates gained credit for using their knowledge and understanding of thermal conductivity experiments to describe methods that were practical and to state how the order of thermal conductivity would be established. However, some candidates tried to list the materials in order of thermal conductivity.”

Worked example 11 mark

On a cold day, a shiny metal rod feels colder to the touch than a black plastic rod.

Which statement explains this observation?

A The metal rod is a better absorber of infrared radiation than the plastic rod. · B The metal rod is a better thermal conductor than the plastic rod. · C The metal rod is a worse absorber of infrared radiation than the plastic rod. · D The metal rod is a worse thermal conductor than the plastic rod.

  1. 1

    B: the metal rod is a better thermal conductor than the plastic rod. [1]

2. Conduction by lattice vibrations

In a solid the particles are arranged in a lattice and vibrate. Heat one end and the particles there vibrate faster.

They pass the energy to their neighbouring particles, which pass it on again. This is how thermal energy is conducted through all solids.

Examiner insight

“Some candidates could have improved on their answers by clearly stating that the energy was passed to neighbouring particles.” “Many answers then referred to the molecules of the spoon beginning to vibrate, rather than having faster or increased vibration.”

3. Free electrons in metals

Metals also contain free (delocalised) electrons. The electrons gain energy from the atoms and move through the metal.

They collide with distant atoms and transfer energy to them. So a metal conducts in two ways.

Non-metals have no delocalised electrons, so they conduct less well than metals.

Examiner insight

“Weaker candidates usually mentioned delocalised or free electrons but often went on to explain the transfer in terms of vibrations rather than movement of electrons through the metal.” “There were some incorrect references to electrons vibrating.”

Worked example 11 mark

A metal pan contains cold water.

When the pan is placed on a hot stove, the water gets hotter.

By which processes does the heat pass through the metal pan?

A conduction and convection · B conduction only · C convection only · D radiation only

  1. 1

    B: through the metal the heat passes by conduction only. [1]

4. Why gases and most liquids conduct badly

In a gas the particles are far apart. Collisions that transfer energy are less frequent, and no lattice vibrations take place. So gases are poor thermal conductors. Most liquids are poor conductors too.

Examiner insight

“Stronger candidates completed their response by adding that the consequence of this greater particle separation is that collisions which transfer energy are less frequent or that no lattice vibrations take place in a gas.” “Weaker answers stated that gases were far apart, implying more than one gas, instead of that gas particles were far apart.”

5. Conductors in between

Many solids lie between good thermal conductors and thermal insulators. They conduct thermal energy better than thermal insulators but less well than good thermal conductors.

Worked example 11 mark

Four samples of materials with the same dimensions are tested.

Which material gives the highest rate of thermal conduction?

A rubber · B copper · C plastic · D steel

  1. 1

    B: copper conducts at the highest rate. [1]

Recap

  • A good thermal conductor passes energy quickly. A thermal insulator is a bad conductor.
  • Compare conductors by heating each in the same way. The wax melts quickest on the best conductor.
  • Extended: in all solids, vibrating particles in the lattice pass energy to neighbouring particles.
  • Extended: in metals, free (delocalised) electrons also carry energy through the metal.
  • Extended: gases and most liquids conduct badly. Gas particles are far apart, so energy-transferring collisions are less frequent.
  • Extended: some solids conduct better than insulators but less well than good conductors.

6. Where convection happens

Convection is an important method of thermal energy transfer in liquids and gases.

It occurs in gases and liquids only. It does not occur in solids. A vacuum has no particles, so there is no convection through it.

Heat passing through a metal wall is conduction, not convection. Read which part of the set-up the question asks about.

Examiner insight

“A considerable number of candidates incorrectly believed that convection currents occur in a vacuum.” “Many candidates did not read the question carefully enough and gave convection as the answer. The question was asking about heat transfer through the beaker to the water not through the water.”

Worked example 11 mark

Convection is an important method of thermal energy transfer.

In which states of matter does convection occur?

A gases, liquids and solids

B gases and liquids only

C gases and solids only

D liquids and solids only

  1. 1

    convection occurs in gases and liquids only: B [1]

7. Convection currents and density

When part of a liquid or gas is heated, the heated fluid expands. It becomes less dense than the fluid around it, so it rises.

The rising fluid displaces colder fluid. As more fluid is heated, a circulation of the fluid is set up. This is a convection current.

Write about the water or the air itself. Do not write about its particles expanding or becoming less dense.

Examiner insight

“Candidates should be careful not to describe molecules as being less dense or expanding.” “Weaker candidates gave statements such as colder, less dense air falls, or warm air rises because it is more dense.”

Worked example 12 marks

A flame under a hot-air balloon heats the air in the balloon.

Hot air inside the balloon rises to the top of the balloon.

Explain why the hot air rises.

  1. 1

    the heated air expands [1]

  2. 2

    so the hot air is less dense (than the cooler air around it) [1]

8. Convection in experiments and equipment

Heated air is less dense and rises to the top of a room, displacing colder air. In a tank of water, the hot water collects at the top.

Cooling sets up a convection current too. Cooled water contracts and becomes denser, so the cooler water falls and the warmer water rises.

Heating the bottom surface of a sample of air warms all the air by convection. This masks any conduction effects in the air. Food under a grill is not heated by convection: the hot air around the grill is less dense than the cooler air near the food, so the hot air rises.

Examiner insight

“Many candidates did not recognise that in this example it was cooling, as opposed to the more familiar heating which produced the current.” “Candidates needed to realise that if the bottom surface is heated, all the air will be warmed by convection and it will mask conduction effects.”

Worked example 13 marks

The storage tank and the copper pipe are filled with water. The Sun heats the water in the copper pipe.

Explain why the hot water is at the top of the tank. Use your ideas about the density of water.

  1. 1

    the heated water expands [1]

  2. 2

    so it becomes less dense [1]

  3. 3

    the less dense water rises [1]

  4. 4

    (naming a convection current is credited as one of the three)

Recap

  • Convection is an important method of thermal energy transfer in liquids and gases.
  • It does not occur in solids or in a vacuum.
  • Heated fluid expands, becomes less dense and rises.
  • Colder fluid takes its place, so a convection current is set up.
  • Cooled fluid contracts, becomes denser and falls.
  • Say the water or air becomes less dense, never the particles.
  • Through a metal wall, the process is conduction.

9. Thermal radiation

Thermal radiation is infrared radiation. All objects emit this radiation.

Thermal energy transfer by thermal radiation does not require a medium. That is why radiation can transfer energy through a vacuum.

Examiner insight

“A common misconception was to state that thermal energy transfers by convection.” “Weaker answers only referred to absorption of heat without specifically stating that it was radiation or infrared radiation.”

Worked example 11 mark

An outdoor heater warms people sitting outside. State the method of thermal energy transfer that warms the people.

  1. 1

    radiation [1]

10. Surface colour and texture

A dull black surface is a good absorber and a good emitter of infrared radiation. A shiny white surface is a poor absorber and a poor emitter.

A white or shiny surface is a good reflector of infrared radiation, so it is a poor absorber of it.

Always say what is absorbed, emitted or reflected: infrared radiation.

Examiner insight

“Some candidates did not include the quantity (radiation or thermal energy) that white was reflecting well or absorbing poorly.” “Although most candidates correctly recalled that black surfaces are better absorbers of radiation than white surfaces, many of these candidates thought that black surfaces are worse emitters of radiation, and therefore chose option C.”

Worked example 12 marks

An outdoor heater warms a table. One of the people puts a white cloth over the table. Explain why the white cloth causes the temperature of the table to decrease.

  1. 1

    the thermal energy absorbed by the table decreases [1]

  2. 2

    because white is a good reflector of radiation (or a poor absorber of radiation) [1]

11. Energy balance and temperature

An object stays at a constant temperature when it transfers energy away at the same rate that it receives energy.

If the rate of energy absorbed is greater than the rate of energy emitted, the temperature of the object increases. If the rate absorbed is less than the rate emitted, the temperature decreases.

The temperature of the Earth depends on the balance between the incoming radiation and the radiation emitted from the Earth's surface.

Examiner insight

“A few candidates thought that the rate of energy emitted by the table was greater than the rate of energy absorbed by the table.” “Some candidates provided ambiguous words such as increased, decreased and transferred instead of words that clearly describe absorption and emission.”

Worked example 12 marks

A table is warmed by an outdoor heater. Complete the sentences about energy transfers that warm the table.

The heater transfers energy to the ........ store of the table. The temperature of the table increases. The rate of energy ........ by the table is greater than the rate of energy ........ by the table.

  1. 1

    first gap: thermal (or internal) [1]

  2. 2

    second gap: absorbed. Third gap: emitted [1]

12. Rate of emission

The rate at which an object emits radiation depends on its surface temperature and its surface area.

A hotter surface emits radiation at a greater rate. A larger surface area also emits radiation at a greater rate.

Worked example 11 mark

A warm black surface emits radiation. It is decided to increase the amount of radiation emitted.

Which changes increase the amount of radiation emitted?

1 making the surface hotter

2 changing the colour of the surface to white

3 decreasing the area of the surface

A 1, 2 and 3

B 1 and 2 only

C 1 and 3 only

D 1 only

  1. 1

    1: a hotter surface emits at a greater rate

  2. 2

    2: white is a poorer emitter than black, so this decreases the emission

  3. 3

    3: a smaller area emits less, so this decreases the emission

  4. 4

    answer D (1 only) [1]

13. Experiments on emitters and absorbers

To compare absorbers, fix a white card and a black card to two thermometers and place both at the same distance from a lamp. The card whose temperature rises more in the same time is the better absorber.

To compare emitters, cover identical beakers with dull black paper and shiny foil. Fill each with the same volume of water at the same starting temperature. The faster cooling beaker is the better emitter.

Examiner insight

“Candidates need to be clear about whether surfaces are emitting or absorbing radiation.” “Many based their answers incorrectly on absorption rather than emission of thermal energy.”

Worked example 12 marks

A white card is 1.0 cm from a lamp and a black card is 2.0 cm from the lamp. After 4.0 minutes, the conclusion in(c) is that the temperature of the black card increases more quickly.

(d) The temperature of each card increases as it absorbs thermal radiation. The procedure used does not produce a fair comparison of the absorbing properties of the two different coloured cards.

(i) Suggest why the procedure does not produce a fair comparison.

(ii) State why your conclusion in 2(c) would be the same, even if the comparison were fair.

  1. 1

    (i) the distance of each card from the lamp is not the same [1]

  2. 2

    (ii) the black card was further away, but the temperature increase was still greater [1]

Recap

  • Thermal radiation is infrared radiation. All objects emit it.
  • Radiation does not require a medium.
  • Dull black: good absorber and good emitter. White or shiny: good reflector, poor absorber, poor emitter.
  • Extended: constant temperature means energy leaves at the same rate as it arrives.
  • Extended: rate absorbed greater than rate emitted, temperature rises. Rate absorbed less than rate emitted, temperature falls.
  • Extended: the Earth's temperature depends on the balance between incoming radiation and radiation emitted from its surface.
  • Extended: a hotter surface and a larger surface area both emit at a greater rate.
  • Extended: compare emitters by cooling rate and absorbers by temperature rise, keeping everything else the same.

14. Heating a pan: conduction

When a metal pan sits on a hot stove, the heat passes through the metal by conduction only.

Copper is a good conductor. Of copper, steel, rubber and plastic, copper gives the highest rate of thermal conduction.

On a cold day a metal rod feels colder than a plastic rod. The metal is the better thermal conductor.

Examiner insight

“Many candidates did not read the question carefully enough and gave convection as the answer. The question was asking about heat transfer through the beaker to the water not through the water.”

15. Heating a room: convection

A heater warms the air next to it. The heated air expands and becomes less dense, so it rises to the top of the room and displaces the colder air. This movement is a convection current.

Convection occurs in liquids and gases only. It does not occur in a solid such as wood.

Say that the air or the water becomes less dense. Do not say that its particles do.

Examiner insight

“Candidates who chose to answer in terms of particles often gave incorrect statements about, for example, hot particles being less dense.”

Worked example 12 marks

A flame under a hot-air balloon heats the air in the balloon. Hot air inside the balloon rises to the top of the balloon. Explain why the hot air rises.

  1. 1

    the heated air expands OR the air particles move apart [1]

  2. 2

    the air is less dense [1]

16. Heating by radiation

An outdoor heater warms the people near it by radiation. It is the only process that can transfer energy through a vacuum, or through space from the Sun to the Earth.

A warm surface emits radiation. Making the surface hotter increases the amount emitted.

The copper pipe of a solar water heater has a black surface, because black is a good absorber of infrared radiation.

Examiner insight

“A common misconception was to state that thermal energy transfers by convection.” “Some candidates did not include the quantity (radiation or thermal energy) that white was reflecting well or absorbing poorly.”

Worked example 12 marks

An outdoor heater warms people sitting outside. The heater also warms a table. One of the people puts a white cloth over the table. Explain why the white cloth causes the temperature of the table to decrease.

  1. 1

    the energy absorbed by the table decreases OR the rate of emission from the table is greater than the rate of absorption [1]

  2. 2

    because white is a good reflector of radiation OR a poor absorber of radiation [1]

17. Black and shiny surfaces

A dull black surface emits infrared radiation at the greatest rate. It is also a better absorber than white.

A shiny surface is a poorer emitter of infrared radiation. A white surface is a good reflector of radiation.

So a beaker covered in shiny foil cools more slowly than one covered in dull black paper.

Examiner insight

“Candidates need to be clear about whether surfaces are emitting or absorbing radiation.” “Many candidates thought that shiny white surfaces would emit radiation faster than matt black surfaces.”

Worked example 11 mark

A student uses two identical beakers and covers the outside surface of one with dull black paper and the outside surface of the other with shiny foil. The student puts the same volume of water at 80 °C into both beakers and measures their temperatures for several minutes. The cooling curve graphs for each experiment are shown. Which graph is for the beaker covered in shiny foil and what is the reason for the different rates of cooling?

A X, shiny surfaces are better absorbers of infrared radiation · B X, shiny surfaces are poorer emitters of infrared radiation · C Y, shiny surfaces are better absorbers of infrared radiation · D Y, shiny surfaces are poorer emitters of infrared radiation

  1. 1

    B: graph X, because shiny surfaces are poorer emitters of infrared radiation. [1]

18. The vacuum flask

A vacuum flask keeps a liquid warm by reducing the transfer of thermal energy.

The vacuum between the walls contains no particles. It is impossible for heat to transfer through it by conduction or convection. The vacuum does not reduce radiation. The silvered walls do that.

The silvered walls are good reflectors and poor absorbers of radiation. They reflect thermal energy back into the hot liquid.

Examiner insight

“Only stronger candidates were able to explain that, because a vacuum does not contain any particles, it is impossible for heat to transfer through the vacuum by either conduction or convection.” “Weaker answers referred in generalised terms to reduction of conduction, convection or radiation without specifying clearly which of the labelled features reduced thermal energy transfer by this method.”

Worked example 11 mark

Which thermal process can transfer energy through a vacuum?

A conduction · B convection · C evaporation · D radiation

  1. 1

    D: radiation. [1]

19. More than one process at once

Often more than one process is significant. A coal fire heats a room by three methods. Heated air moves round the room by convection, which results from changes in density. Energy passes through the solid floor by conduction. Energy also travels straight out from the fire by radiation.

A person keeps warm beside a wood fire while water in a metal kettle boils over it. Conduction, convection and radiation are all taking place.

Name each process and say where it happens.

Examiner insight

“Candidates are advised to note that when a question asks them to name the processes involved, then that implies that more than one process is needed.” “Very few candidates gained full credit for a description that included the ideas that the thermal energy is first transferred by infrared radiation through space and the atmosphere and then by conduction through the metal.”

Worked example 14 marks

All the equipment is on a roof. The storage tank and the copper pipe are filled with water. The Sun heats the water in the copper pipe.

(i) State the name of the process of thermal energy transfer from the Sun to the copper pipe.

(ii) The surface of the copper pipe is black. Explain why the surface of the copper pipe is black.

(iii) State the name of the process of thermal energy transfer through the copper.

  1. 1

    (i) radiation [1]

  2. 2

    (ii) a black surface is a good absorber [1]

  3. 3

    of infrared radiation [1]

  4. 4

    (iii) conduction [1]

Recap

  • Metal pan: heat passes through the metal by conduction.
  • Room: heated air expands, is less dense and rises, displacing colder air (convection).
  • Convection occurs in liquids and gases only.
  • Radiation can cross a vacuum. Dull black surfaces emit and absorb best.
  • Shiny and white surfaces are poor emitters and good reflectors.
  • Extended: a vacuum has no particles, so no conduction or convection. Silvered walls reflect radiation back.
  • Extended: a fire transfers energy by all three processes. Name each one and where it happens.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State whether copper is a good thermal conductor or a thermal insulator.1 mark
  2. Two rods of the same size are heated in the same way. Wax on the end of rod X melts after 2.5 minutes. Wax on the end of rod Y melts after 90 s. State which rod is the better conductor.2 marks
  3. A student compares how quickly water reaches boiling point in containers A to E. The containers are identical except that each is made from a different metal. Each is heated on an electric hotplate. State two variables that must be kept constant.2 marks
  4. A saucepan has an aluminium base and a plastic handle. Explain why this suits cooking.2 marks
  5. Describe an experiment to find which of three rods of different metals is the best thermal conductor. You do not need to say which rod it is.3 marks
  6. One end of a plastic rod is heated. Describe how thermal energy passes along the rod. Use ideas about particles.2 marks
  7. Explain why air is a poor thermal conductor.2 marks
  8. Explain why a metal conducts thermal energy better than a non-metal solid.3 marks
  9. A disc of material is placed between a hotplate and a metal block, so the block is heated through the disc. Four discs of the same thickness are tested at the same hotplate setting. The block starts at the same temperature each time. A student says P is the best conductor because it gives the greatest temperature rise. Use the data to show that the student is wrong. Then explain why Q is neither a good thermal conductor nor a thermal insulator. | disc | heating time | temperature rise of block / °C | | --- | --- | --- | | copper | 45 s | 18 | | P | 2.0 min | 36 | | Q | 4.0 min | 24 | | plastic | 90 s | 4.5 |4 marks
  10. A camping cup has two steel walls with a thin layer of trapped air between them. It holds a hot drink. Thermal energy is conducted quickly through the inner steel wall but slowly across the air layer. A student writes: "In the steel, the electrons vibrate and pass energy to their neighbours. Air is far apart, so it conducts badly." Identify the two mistakes. Then explain correctly, in terms of particles, the conduction through the steel and across the air.5 marks
  11. State the two states of matter in which convection occurs.1 mark
  12. Explain why convection cannot take place through a vacuum.1 mark
  13. A pan of water is heated on a hob. Name the process that transfers thermal energy through the metal base of the pan.1 mark
  14. A kettle heats the water at its base. State what happens to the density of this water and what it does as a result.2 marks
  15. Warm air rises above a radiator. Explain why, using the idea of density.2 marks
  16. A student writes: "The heated water particles became less dense, so they rose." Correct the mistake in this sentence.1 mark
  17. A beaker of water is heated at its base. Describe how a convection current is set up in the water.3 marks
  18. A heater is switched on at floor level in a room. After ten minutes a thermometer near the ceiling reads a higher temperature than one near the floor. Explain this, using density.3 marks
  19. A scientist investigates how well air conducts thermal energy. The air is in a sealed box between a metal plate at the top and a metal plate at the base. One plate is heated and the temperature of the other plate is measured. State which plate should be heated. Explain why heating the other plate would not test conduction, and why your choice avoids this problem.4 marks
  20. A sealed box of air has one thermometer near its top and one near its base. Both read 22 °C. In test 1, a pack of ice is placed on top of the box. After five minutes both readings have fallen. In test 2, the box starts at 22 °C again and the pack of ice is placed under the box. After five minutes the lower reading has fallen but the upper reading has hardly changed. Explain (a) the result of test 1 and (b) the result of test 2. Use ideas about density.5 marks
  21. State the name of the type of radiation that all objects emit as thermal radiation.1 mark
  22. A student says that thermal energy cannot reach the Earth from the Sun by radiation because there is no medium between them. Explain why the student is wrong.2 marks
  23. Two identical cans each hold water at 90 °C. One can is matt black and the other is shiny white. State which can cools faster and explain why.2 marks
  24. Two runners stand in bright sunshine. One wears a white shirt and one wears a black shirt. Explain why the runner in the white shirt stays cooler.2 marks
  25. A cook keeps hot food in a shiny metal pan and not in a dull black pan. Explain why the food stays hot for longer in the shiny pan.2 marks
  26. A roof tile is at a constant temperature. It absorbs energy from the Sun at a rate of 180 W. (a) State the rate at which the tile emits energy. (b) A cloud covers the Sun. The tile now absorbs energy at a rate of 120 W but still emits at 180 W. State and explain what happens to the temperature of the tile.3 marks
  27. The table gives the surface temperature and the surface area of three blocks. The blocks have identical dull black surfaces. State the order of the blocks from the greatest rate of emission of radiation to the least. Explain your answer. | block | surface temperature / °C | surface area / cm² | | --- | --- | --- | | P | 80 | 100 | | Q | 40 | 100 | | R | 80 | 200 |3 marks
  28. The temperature of the Earth is constant. (a) State how the rate at which the Earth receives incoming radiation compares with the rate at which radiation is emitted from the Earth's surface. (b) The rate of incoming radiation increases and the rate of emission stays the same. State and explain the effect on the temperature of the Earth.3 marks
  29. A student has two identical cans. One has a dull black surface and the other has a shiny silver surface. She puts 200 cm³ of hot water in each can. She records the temperature of the water at the start and after 10 minutes. (a) Calculate the rate of temperature decrease of the water in each can. (b) The student writes: 'The black can cools faster, so dull black is the better absorber of infrared radiation.' State the word that is wrong in her sentence and give the correct word. (c) Explain why her readings do not give a fair comparison of the two surfaces as emitters. (d) She then compares the two surfaces as absorbers. She uses a lamp and puts the same volume of cold water, at the same temperature, in each can. Describe what she does next and how she uses her readings. | can | starting temperature / °C | temperature after 10 minutes / °C | | --- | --- | --- | | dull black | 90 | 62 | | shiny silver | 70 | 54 |6 marks
  30. A satellite absorbs energy from the Sun at a rate of 900 W. It transfers energy away only from its radiator panels, which face away from the Sun. The panels have a total surface area of 5.0 m². When the panels first open, each 1.0 m² of panel emits energy at a rate of 120 W. (a) Calculate the rate at which the panels emit energy when they first open. State and explain what happens to the temperature of the satellite. (b) Explain why the temperature of the satellite later becomes constant. (c) The panels can be made dull black or shiny white. State which surface gives the satellite the lower constant temperature. Explain your answer.7 marks
  31. Name the process by which thermal energy passes through the metal base of a frying pan.1 mark
  32. A student lists four materials: wood, water, air and copper. State which two can carry a convection current.2 marks
  33. An electric heater warms the air next to it in a room. Explain why the warm air moves to the top of the room.2 marks
  34. On a cold day a student touches a metal gate and a plastic gatepost. The gate feels colder. Explain why.2 marks
  35. Two identical tins hold the same volume of water at 80 °C. One tin is dull black and the other is shiny silver. Which tin cools faster? Explain your answer.3 marks
  36. Two identical copper pipes stand in sunshine. One is painted black and the other is painted white. Name the process by which thermal energy reaches the pipes from the Sun. State which pipe warms faster and why.3 marks
  37. A vacuum flask has silvered walls with a vacuum between them. State which feature reduces each of conduction, convection and radiation. Explain how the vacuum works.3 marks
  38. A coal fire heats a room that has a solid floor. Name the process that transfers thermal energy (a) by moving heated air round the room, (b) through the solid floor, (c) straight out from the fire to a person facing it. For (a), explain why the heated air moves upwards.4 marks
  39. A café tests four metal teapots. Each holds the same volume of tea at 90 °C. The table gives the temperature of the tea after 10 minutes. (a) Calculate the average rate of cooling of the tea in teapot P. Give the unit. (b) A waiter says that changing the surface from matt black to shiny white keeps the tea hotter than adding a lid does. Use the data to decide whether the waiter is correct. (c) Explain why the tea in R is hotter than the tea in P after 10 minutes. | teapot | surface | lid | temperature after 10 minutes / °C | | --- | --- | --- | --- | | P | matt black | no | 54 | | Q | matt black | yes | 64 | | R | shiny white | no | 59 | | S | shiny white | yes | 68 |7 marks
  40. A space probe is in space, which is a vacuum. A hot electronics box inside it is bolted to a metal panel on the outside. The outside of the panel faces away from the Sun and is dull black. The side of the probe that faces the Sun is covered in shiny foil. (a) Explain why the probe cannot lose energy to space by conduction or convection. (b) Name the process that transfers thermal energy from the box through the metal panel. (c) Explain why the outside of the panel is dull black. (d) Explain why the side that faces the Sun is covered in shiny foil.7 marks

Go deeper

Practise and revise with member-only material for this chapter.

Workbook

Exam-style practice questions with full worked solutions.

Unlock with a plan

Chapter at a Glance

A single-page summary of every key idea for fast revision.

Unlock with a plan

Free notes are just the start.

Unlock every Workbook and Chapter at a Glance, and generate your own worksheets and predicted papers.

Explore plans

Related chapters

Physical quantities and measurement techniquesMotionMass and weightDensityForcesMomentum