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Topic 2 · 2.3

Transfer of thermal energy

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe experiments on good and bad thermal conductors; explain conduction by lattice vibrations and free electrons EXTENDED.
  • Explain convection in liquids and gases in terms of density changes, and describe experiments to show it.
  • Know that thermal radiation is infrared, needs no medium, and depends on surface colour and texture; describe experiments on emitters and absorbers EXTENDED.
  • Explain the energy balance of an object and of the Earth, and how emission depends on temperature and area EXTENDED.
  • Explain everyday applications, including cases where more than one method matters EXTENDED.

📚The physics

Conduction

Rods of copper, aluminium, iron and glass stick out of a tank of hot water, with pins held on by wax along them. All the pins have fallen off the copper rod, fewer from aluminium and iron, and only the nearest from glass. A particle view of a metal shows atoms vibrating and passing on vibrations, and free electrons moving between them.
Metals are good thermal conductors; plastic, wood, glass and air are poor conductors (insulators).

Conduction is the transfer of thermal energy through a material without the material itself moving. EXTENDED In all solids, hotter atoms vibrate more and pass vibrations on to their neighbours through the lattice. Metals also have free (delocalised) electrons that move through the metal and carry energy quickly, so metals conduct far better. Gases and most liquids conduct badly because their particles are far apart and collide less often. Many solids lie between good conductors and insulators.

Convection

A beaker of water heated at one side of its base. A purple convection current, traced by a crystal of potassium manganate(VII), rises above the heat, flows across the top, sinks down the far side and returns along the bottom. Warm water expands, becomes less dense and rises; cooler, denser water sinks to take its place.
Convection happens in liquids and gases, never in solids.

Convection is an important method of thermal energy transfer in liquids and gases. The fluid near the heat source expands, becomes less dense and rises; cooler, denser fluid sinks to replace it, setting up a convection current. That is why a room heater warms the whole room and why the freezing compartment is at the top of a fridge.

Thermal radiation

Thermal radiation is infrared radiation; all objects emit it, and it needs no medium — it crosses the vacuum of space from the Sun. Dull black surfaces are the best emitters and absorbers; shiny white or silver surfaces are the worst emitters and absorbers and the best reflectors.

Left: a cube of hot water with a dull black face and a shiny silver face at the same temperature; many more infrared arrows leave the black face. Right: a heater midway between a dull black plate and a shiny plate, each with a coin stuck on the back with wax; the coin falls off the black plate first.
EXTENDED Keep the distances and temperatures equal so the test is fair.
  • EXTENDED An object stays at a constant temperature when it transfers energy away at the same rate as it receives it. If it receives energy faster than it loses it, its temperature rises; if slower, it falls.
  • EXTENDED The Earth’s temperature depends on the balance between incoming radiation from the Sun and the infrared emitted from its surface. Greenhouse gases absorb some outgoing infrared, so more gas means a warmer balance.
  • EXTENDED The rate of emission increases with the surface temperature and the surface area.

Consequences and applications

A vacuum flask: an insulating stopper reduces conduction and stops convection from the top; a vacuum between two glass walls stops conduction and convection because there are no particles; silvered surfaces reflect infrared, reducing radiation; a protective outer case surrounds it.
A vacuum flask reduces all three methods of energy transfer.
  • Cooking pans: metal base (good conductor) with a plastic or wooden handle (insulator).
  • Heating a room: convection currents carry warm air from a heater around the room.
  • EXTENDED A fire burning wood or coal: radiation heats you in front of it; convection carries hot gases up the chimney and draws in fresh air; conduction heats the grate.
  • EXTENDED A car radiator: coolant conducts energy away from the engine, is pumped to the radiator, and the thin metal fins pass it to air driven through by the fan (forced convection), with some radiation.

✏️Worked example

The diagram shows a solar water heater on a roof. (Modelled on 0625/32 June 2026 Q5.) (a) Name the process by which energy travels from the Sun to the pipes. [1] (b) Explain why the pipes are painted black. [2] (c) Name the process by which energy passes through the copper walls into the water. [1] (d) Explain why hot water collects at the top of the tank. [2]
The Sun shines on a collector of black copper pipes under glass on a sloping roof. Heated water flows up a red pipe into the top of a storage tank, where the hot water sits above cool water; cool water returns from the bottom of the tank along a blue pipe to the bottom of the collector. Hot water leaves the top of the tank for the house and cold mains water enters lower down.

(a) (Thermal) radiation — infrared, which needs no medium.

(b) Dull black surfaces are the best absorbers of infrared radiation, so the pipes (and water) gain energy faster.

(c) Conduction.

(d) Hot water expands and is less dense than cold water, so it rises (floats on the denser cold water): a convection current.

Check it. Radiation from the Sun, conduction through a solid, convection in a liquid: each answer names the only method possible in that medium.
“Heat rises.” Heat does not rise — hot fluid rises because it is less dense. Always mention density.

📝Practise

In the style of the multiple-choice, theory and practical papers. EXTENDED marks Supplement content.

1. (Multiple choice.) In which states of matter can convection occur? A: solids only. B: liquids only. C: liquids and gases. D: solids, liquids and gases.
C.
2. (Multiple choice.) EXTENDED A warm, dull black surface emits infrared radiation. Which changes increase the rate of emission? 1: raising its temperature. 2: painting it shiny white. 3: increasing its surface area. A: 1 only. B: 1 and 2. C: 1 and 3. D: 1, 2 and 3.
C. A shiny white surface is a worse emitter.
3. (Theory.) EXTENDED Explain why metals are much better thermal conductors than non-metals. [3]
In all solids, energy passes by atoms vibrating and passing vibrations to neighbours (lattice vibrations). Metals also contain free (delocalised) electrons, which move through the metal, gain kinetic energy at the hot end and transfer it quickly to cooler parts by collisions.
4. (Theory.) Explain how a vacuum flask keeps a drink hot. [4]
Vacuum: no particles, so no conduction or convection across it. Silvered walls: reflect infrared, reducing radiation. Stopper: an insulator that reduces conduction and stops convection currents and evaporation at the top.
5. (Practical.) Describe an experiment to show that a dull black surface is a better absorber of infrared radiation than a shiny surface. [4]
Place a heater midway between a dull black plate and a shiny plate, the same size and at the same distance. Stick a coin (or place a thermometer) on the back of each with wax. Switch on and time how long each coin takes to fall (or compare temperature rises). The black plate’s coin falls first.
6. (Theory.) EXTENDED A satellite in sunlight stays at a constant temperature. Explain, in terms of energy, why. [2]
It absorbs energy from the Sun at the same rate as it emits (radiates) energy into space, so its internal energy, and temperature, stay constant.
7. (Theory.) Explain why the heating element of an electric kettle is at the bottom. [2]
Water heated at the bottom expands, becomes less dense and rises; cooler water sinks to be heated: a convection current heats all the water. At the top, hot water would stay at the top.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • Institute of Physics — demonstrations of conduction, convection and radiation
  • PhET “The Greenhouse Effect” — the Earth’s radiation balance