Convection: the fluid itself moves
Convection transfers energy through a fluid (liquid or gas) by movement of the heated fluid itself, setting up a circulating convection current. It is the most important method in liquids and gases precisely because fluids are poor conductors. It cannot occur in a solid (particles cannot move about) or in a vacuum (no material to move). (Extended) The warmed fluid expands, becomes less dense and rises; cooler, denser fluid sinks to replace it.
Conduction: metals fast, non-metals slow
Thermal conduction moves energy through a material without the material moving as a whole. Good conductors: all metals (copper, aluminium, iron). Poor conductors / good insulators: wood, plastic, glass, rubber, air and water -- trapped air is why wool and foam insulate. Judge a material by the rate energy passes. (Extended) Hot-end particles vibrate more and pass energy on by collisions; in metals, delocalised (free) electrons also carry it quickly.
Radiation: infrared, no medium needed
Thermal radiation is energy carried by electromagnetic waves, mainly infrared. It needs no medium, travels in straight lines and can be reflected, so it crosses the vacuum of space -- this is how the Sun's energy reaches the Earth. Every object emits it, and the hotter the object the more it emits. Surface matters: black, dull is the best absorber and emitter; white, shiny is the best reflector and the worst at both.
Drawn from real examiner reports.
Warmed fluid gets LESS dense, not more
The warmed fluid becomes LESS dense and rises; cooler, denser fluid sinks to replace it. "More dense", "heat rises" and "hot air is light" all earn nothing. The reverse is examined too: a fridge's cooling element sits at the top so air chilled there becomes denser and sinks. (Extended) Name the cause -- the fluid expands, which lowers its density.
Flagged Jun 2023 P42 Q6c; Nov 2023 P11 Q33, P43 Q6ai
(Extended) "Energy is transferred" is not a mechanism
"Explain how a metal conducts" needs the particle mechanism, not a restatement that energy moves. Say that particles at the hot end vibrate more, colliding with neighbours to pass energy along -- and that delocalised (free) electrons gain energy and move quickly through the metal, carrying it to the cooler end. The electron half is the mark most often missed.
Flagged Jun 2022 P32 Q9bi; Nov 2023 P41 Q9d
(Extended) Non-metals have no free electrons
Never explain conduction in wood or plastic by delocalised electrons -- they have none. Only the slower vibration mechanism operates in a non-metal, which is exactly why its rate of thermal transfer is low and it acts as an insulator. Compare materials by how fast energy passes through them, not simply by whether it passes at all.
Flagged Jun 2022 P32 Q9bi; Nov 2023 P41 Q9d
Black emits as well as it absorbs
A black, dull (matt) surface is the best absorber of infrared and the best emitter; a white, shiny surface is the best reflector and the worst at both. Remembering only "black absorbs" costs marks: in a hot-can or Leslie-cube test the dull black face gives the highest detector reading and cools fastest, because it is the best emitter.
Flagged Nov 2022 P42 Q6c
Radiation alone crosses a vacuum
Thermal radiation needs no medium, which is how energy reaches us from the Sun across empty space. Saying radiation "needs particles" or "needs air" is wrong. Conduction and convection both require a material, so both stop dead in a vacuum -- the fact a vacuum flask exploits, leaving only radiation, which its silvered walls then reduce.
Flagged Nov 2022 P42 Q6c
Glass feels cold but is an insulator
Materials that feel cold get classified as conductors. Glass feels cold because it conducts energy away from your hand faster than air does, but next to a metal it is a poor conductor and a good insulator. When justifying a material choice, give a reason linked to conduction ("plastic is a poor conductor, so the handle stays cool"), not a vague "so it does not break".
(Extended) Earth's temperature is a BALANCE
The Earth's average temperature stays constant only when the radiation it absorbs from the Sun equals the radiation it emits into space -- not simply because "the Sun heats the Earth". Absorb more than it emits and the temperature rises; emit more than it absorbs and it falls. Atmospheric gases that reduce the emitted radiation escaping shift that balance.
Pick the transfer method first
Through a solid, or between touching objects, use conduction. Through a liquid or gas that can circulate, use convection. Across a gap, a vacuum, or from the Sun, use radiation. Devices often attack several at once -- a vacuum flask reduces all three.
Describe vs explain
State wants one fact ("copper is a good conductor"). Describe wants what happens (warm water rises, cool water sinks). Explain wants the cause chain -- vibrations plus free electrons, or expands, becomes less dense, rises. A description never scores an explain mark.
(Extended) Quote the reading, not the colour
Compare surfaces by the reading recorded -- "the dull black face gave the higher temperature, so it is the better emitter" -- not just a colour. Keep the face-to-detector distance and the water temperature the same for every surface, or the comparison is not a fair test.
Draw convection arrows as a closed loop
A convection sketch needs a complete circulating loop: warm, less dense fluid rising above the heat source, cool denser fluid sinking on the far side, back along the bottom. Label temperature and density, not just "up"/"down". Radiation arrows are straight lines.
Cambridge 0654 spec reference: Section P2 "Thermal physics", sub-topic P2.3 (Core + Extended). This leaf covers the Core ideas of good/bad thermal conductors and insulators, convection in liquids and gases, thermal radiation (infrared, needs no medium) and the effect of surface colour and texture on emission/absorption/reflection, plus everyday applications of the three methods; and the Extended ideas explaining conduction by particle vibrations and delocalised electrons, explaining convection by density change, the balance of radiation setting the Earth's temperature, and experiments distinguishing good/bad emitters and absorbers.
This topic is entirely qualitative in the 2025-27 0654 syllabus -- there are no specific-heat-capacity or specific-latent-heat calculations here. Answers are marked on correct description and explanation of the transfer processes.
Thermal conduction: the transfer of thermal energy through a material without the material moving as a whole. Metals are good conductors; non-metals, liquids and gases are poor conductors (good insulators). (Extended: energy is passed on by particle (lattice) vibrations and, in metals, by the movement of delocalised (free) electrons.)
Convection: the transfer of thermal energy through a fluid (liquid or gas) by the movement of the heated fluid itself, setting up a convection current. It cannot occur in a solid or a vacuum. (Extended: the warmed fluid expands, becomes less dense and rises; cooler denser fluid sinks.)
Thermal radiation: the transfer of thermal energy by electromagnetic waves, mainly infrared (IR). It needs no medium and can travel through a vacuum (Sun Earth). Every object emits it; hotter objects emit more.
Surface effect on infrared:
| Surface | Absorbs IR | Emits IR | Reflects IR |
|---|---|---|---|
| Black, dull (matt) | best | best | poorly |
| White, shiny (polished) | poorly | poorly | best |
(Core) What is thermal radiation, and why can it transfer energy from the Sun to the Earth?
(Core) A student has samples of copper, glass, aluminium and wood. (4 marks)
(a) Sort the four materials into good thermal conductors and good thermal insulators. (2 marks)
(b) State one everyday use of a good thermal insulator and explain why an insulator is chosen. (2 marks)
