Transfer of thermal energy
Conduction, convection, radiation, and how to reduce or increase energy transfer.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 2.3.1Describe conduction in terms of particle vibration and free electrons.
- 2.3.2Explain convection in terms of density changes in a fluid.
- 2.3.3Describe how emission and absorption of infrared radiation depend on surface colour and texture.
8 minute read
Conduction, convection and radiation
Thermal energy moves from hotter to cooler regions by three distinct mechanisms, and IGCSE questions expect you to identify which one applies and explain it in the right terms.
Conduction
Conduction happens mainly in solids. Particles at the hot end vibrate more and pass on energy to neighbouring particles through collisions. Metals are especially good conductors because they also have free (delocalised) electrons, which move through the structure carrying energy much faster than vibration alone.
Convection
Convection happens in liquids and gases (fluids), and relies on the fluid being free to flow. When a fluid is heated, it expands, becomes less dense, and rises; cooler, denser fluid sinks to take its place. This circulation is a convection current. Convection cannot happen in a solid, because the particles cannot move from place to place.
Radiation
Radiation is the transfer of energy by infrared electromagnetic waves, and unlike conduction and convection, it needs no medium — it works through a vacuum, which is how energy reaches us from the Sun.
Surface matters a great deal:
- Dull, black surfaces are good emitters and good absorbers of infrared radiation.
- Shiny, light surfaces are poor emitters and poor absorbers (they reflect radiation instead).
This is why a car's black dashboard gets much hotter in sunlight than a light-coloured one, and why vacuum flasks have a shiny inner surface to reduce radiation loss.
Think of it like this
Conduction is like a line of people passing a ball hand to hand without moving; convection is like the people themselves walking to a new spot, carrying the ball with them; radiation needs no people or ball at all — it is a beam of light carrying energy straight across an empty room.
Worked examples
Method, step by step
Explain why a radiator painted matt black warms a room more effectively by radiation than the same radiator painted white and shiny.
- 1A dull, black surface is a better emitter of infrared radiation than a shiny, light surface.
- 2So more thermal energy leaves the black radiator as radiation per second, for the same temperature.
The matt black radiator emits infrared radiation more effectively, transferring thermal energy to the room faster by radiation.
Common misconceptions
- Saying "heat rises" as if heat itself moves upward. It is convection — the hot fluid becomes less dense and rises, carrying thermal energy with it — not a property of heat.
- Believing convection can happen in a solid. Particles in a solid cannot flow, so only conduction (and, at the surface, radiation) applies.
- Assuming radiation needs a hot object to glow visibly. All objects emit infrared radiation, even at room temperature; only very hot objects emit enough for it to become visible light.
In the exam
- Explain conduction with reference to both particle vibration AND, for metals, free electrons — a full-marks answer usually needs both.
- For convection, the key phrase is "less dense, so it rises" — do not just say the fluid "gets lighter".
- When comparing surfaces for radiation, always give both emission and absorption, since a good emitter is also a good absorber.