Three thermal transfer mechanisms
Conduction (solids, especially metals): particles vibrate and pass energy on; in metals free (delocalised) electrons carry energy fast — say "free electrons", not just "atoms". Convection (fluids): heated fluid expands, becomes less dense and rises; cooler denser fluid sinks, forming a convection current. Radiation: energy as infrared (IR) electromagnetic waves — needs no medium, crosses a vacuum. All three can act at once.
What controls radiation: colour, temp, area
The rate an object emits or absorbs infrared (IR) radiation depends on: (1) surface colour/texture — black matt is the best emitter AND absorber; white/shiny (silvered) is the worst (best reflector); (2) temperature — the hotter the surface above its surroundings, the greater the emission rate; (3) surface area — larger area, greater total rate. Examples: dark engine radiators (good emitters), shiny survival blankets (good insulators), black solar absorber plates.
Convection cycle and everyday examples
Convection runs on density: a fluid heated near a source expands, its density drops, the warmer less-dense fluid rises, cooler denser fluid flows in beneath, and the cycle repeats as a convection current. Examples: a sea breeze (land heats faster; warm air over land rises, cool sea air flows in); a radiator (warm air rises, cool air descends). You must explain convection, not just name it — the two elements are density change AND resulting fluid movement.
Drawn from real examiner reports.
Metals conduct "because of lots of atoms"
Every material has atoms, so "metals conduct because they have lots of atoms" earns nothing. The two-element answer: (1) metals have free (delocalised) electrons that move through the lattice; (2) these transfer energy faster than vibration alone, so metals beat non-metals. Non-metals (wood, plastic) have only lattice vibration. Always write "free electrons".
June 2024 Paper 2P Q3 and November 2024 Paper 2P Q6(b): the examiner reports repeatedly noted that candidates explaining particle behaviour in materials need to include both the physical mechanism AND the specific feature (free electrons for metals) to gain full marks.
Calling radiation "heat" not infrared
"Heat" is not a form of radiation — infrared (IR) electromagnetic radiation is. "Heat rays", "thermal waves" or "heat travels through the air" score nothing; write "infrared radiation, a type of electromagnetic wave". A further mark is often available for noting IR needs no medium (travels through a vacuum). "Radiation" alone, without "infrared", may miss the mark.
June 2024 Paper 2P Q3(a) and Q3(b): the examiner noted that candidates who gave vague qualitative answers without physical reasons lost explanation marks — "gets hotter" without attributing the mechanism to infrared absorption was insufficient.
"Hot air rises" without the density cause
"Hot air rises" is only the observable effect. A two-mark convection explanation needs the cause too: (1) the heated fluid expands and becomes less dense; (2) so it rises while cooler denser fluid sinks to replace it. Stating only "hot rises, cold falls" gives the effect, not the cause, and usually scores one mark. The same holds for warm water.
Claiming convection happens in a solid
Convection cannot happen in a solid. Its particles are fixed in a lattice and cannot flow in bulk, and convection needs the fluid itself to move. A solid heating right through is always conduction. Only fluids (liquids and gases) convect — do not describe convection currents inside metal or wood.
Confusing emission with absorption
Keep emission and absorption separate. A hot black object cools fastest because black matt is the best emitter; do not explain this by "black absorbs more heat" — absorption is about incoming radiation. Good emitter = good absorber, but name the correct direction: emitting when cooling, absorbing when heated.
Vague "it gets hotter" with no mechanism
Vague answers like "it gets hotter" without a physical reason lose the explanation marks — always name the mechanism (conduction/convection/radiation) and its carrier (free electrons, moving fluid, IR waves). Also recall the general rule for all three modes: a greater temperature difference between object and surroundings gives a faster transfer rate.
Explain with Cause → Mechanism → Effect
On "explain" (not "describe") thermal questions, give the physical reason: Cause → Mechanism → Effect, and always name the mode (conduction/convection/radiation) and its carrier (free electrons, moving fluid, or IR waves).
Give both elements the mark scheme wants
Match the mark scheme's two-element demands: conduction → vibration AND free electrons (metals); convection → density change AND fluid movement; radiation → infrared AND electromagnetic wave. Missing the second element in each pair usually costs a mark.
Name the surface colour in radiation tests
In the black-vs-white radiation experiment, name the variable being changed — the surface colour. Writing about "the bottle" without saying which colour absorbs (black matt) or emits more loses marks. The black surface absorbs IR faster, so its temperature rises faster.
Draw the convection loop with density labels
For a convection-current diagram, draw a closed loop of arrows: warm less-dense fluid rising at the heat source, cooler denser fluid descending away from it. Label the density on each part, heat source at the bottom. Reversed or unlabelled arrows lose marks.
Define thermal conduction.
