Thermal Energy Transfer: Conduction, Convection and Radiation

Thermal energy moves by three mechanisms: conduction, convection and radiation. This subtopic carries the most marks in Thermal Physics because it powers the 6-mark extended-response question. Mark schemes here reward mechanisms, never just names.

How do conduction, convection and radiation actually work?

Conduction transfers energy through a material without the material moving. Particles vibrate more vigorously when heated and pass energy to neighbouring particles through collisions. In metals, free electrons also move through the lattice, carrying energy quickly. This is why metals conduct far better than non-metals. The free-electron mechanism is Extended content; Core candidates need the vibration story.

Convection happens only in liquids and gases. Heated fluid expands, becomes less dense and rises; cooler, denser fluid sinks to replace it. This sets up a convection current. The key scoring word is density: “hot air rises” alone earns nothing.

Radiation is the transfer of energy by infrared waves. It needs no medium, which is how the Sun’s energy crosses the vacuum of space. Surfaces matter:

Surface	Emits infrared	Absorbs infrared	Reflects infrared
Dull black	Best	Best	Worst
Shiny silver/white	Worst	Worst	Best

Extended candidates also learn that emission rate rises with surface temperature and surface area, and that Earth’s temperature stays stable only when energy radiated away balances energy absorbed from the Sun.

Which mechanism applies in which situation?

Solids: conduction only. Liquids and gases: mainly convection, plus radiation. Vacuum: radiation only. A vacuum flask demonstrates all three at once: the vacuum gap stops conduction and convection, while silvered surfaces reduce radiation. That single object answers half the application questions Cambridge sets.

Worked Exam Question

A black metal saucepan of water sits on an electric hotplate. Explain how thermal energy is transferred from the hotplate to all of the water. [6]

Model answer: The hotplate heats the pan base by conduction. Particles in the metal vibrate more vigorously and pass energy through collisions; free electrons in the metal also transfer energy quickly. Conduction through the base heats the water touching it. This water expands, its density decreases and it rises. Cooler, denser water sinks to take its place, forming a convection current that heats all the water. The hot black surface also emits some infrared radiation to the surroundings.

Mark scheme (any six):

• B1: conduction through the metal base
• B1: particle vibrations passed on by collisions
• B1: free electrons transfer energy (Extended)
• B1: heated water expands / becomes less dense and rises
• B1: cooler denser water sinks, forming a convection current
• B1: convection current circulates energy through all the water

Common Mistakes

• Writing “heat rises”. Hot fluid rises; energy itself has no direction preference. Examiners reject the phrase outright.
• Explaining convection without density. Expansion → lower density → rises is the required chain.
• Claiming convection or conduction happens in a vacuum. Both need particles; only radiation crosses empty space.
• Saying black surfaces “attract heat”. They absorb and emit infrared radiation well, so use absorb and emit.
• Naming the mechanism without describing it. “Conduction occurs” scores zero on an explain question.

Exam Technique Tip

On the 6-mark question, structure the answer as one paragraph per mechanism, in the order the energy actually travels. Tick off the chain words as you write: vibrate, collide, electrons (conduction); expands, less dense, rises, sinks, current (convection); infrared, emits, absorbs (radiation). Mark schemes are built from exactly these words, so an answer containing the chains rarely drops marks.

How This Is Examined

Every paper uses this subtopic. Papers 1 and 2 test surface comparisons and which-mechanism-where questions. Papers 3 and 4 carry the structured and 6-mark explanations; the Core/Extended split is the free-electron mechanism and the temperature-area dependence of radiation, which appear only on Papers 2 and 4. Papers 5 and 6 regularly use cooling experiments: identical beakers with different surfaces or insulation, cooling curves plotted against time. Malaysian contexts come up naturally: sea breezes are giant convection currents, and white-painted houses stay cooler because white surfaces absorb infrared poorly. Worried about writing six full sentences under time pressure? The chain-word method above cuts planning time to under a minute.