Conduction, convection, radiation
Three modes. Conduction: through a solid or still fluid by particle vibration; (E) in metals free (delocalised) electrons carry energy far faster, so metals conduct best. Convection: in a fluid by bulk movement from density differences — warmed fluid becomes less dense and rises, cooler fluid sinks; solids cannot convect. Radiation: infrared electromagnetic waves, needing no medium (crosses a vacuum).
Black matt: best emitter and best absorber
IR emission and absorption depend on surface colour and texture: black matt is the best emitter AND best absorber; white/shiny is the worst emitter and absorber (best reflector). Key rule: best emitter = best absorber (not opposites). A shiny saucepan is chosen because shiny surfaces are poor emitters, so energy is not radiated away and food stays hot. (E) Emission rate also rises with surface temperature and surface area.
Convection: density change AND bulk movement
Explaining convection needs two elements: (1) cause — heated fluid gains energy, expands and its density decreases; (2) effect — the less dense, warmer fluid rises while cooler, denser fluid sinks to replace it, forming a convection current. "Hot fluid rises" alone earns only the effect mark — the density-decrease cause is the missed mark. Examples: sea breeze (warm air over land rises, cooler sea air flows in); a radiator warming a room.
Drawn from real examiner reports.
Shiny = poor emitter AND poor absorber
Shiny/white surfaces are poor emitters AND poor absorbers (good reflectors) — not "good absorbers because they are like mirrors". Match the property to the question: keeping something hot needs poor emission; staying cool in sunlight needs poor absorption. Emission matters when cooling; absorption matters when heating.
(E) Metals conduct via free electrons
(E) "Metals conduct better because atoms are closer together" scores zero — every material has atoms. Mark-scheme answer: metals contain free (delocalised) electrons that gain energy at the hot end, move through the metal and collide with atoms in cooler regions, transferring energy. Lattice vibrations alone are not enough; non-metals have no free electrons.
June 2024 Cambridge 0625 Paper 41 Q5(a): "Candidates gained credit for a clear account of how delocalised electrons gain energy from atoms and move throughout the metal colliding with distant atoms. Since the question focused on the role of electrons in thermal energy transfer, there was no credit for references to lattice vibrations or general statements that metals are good conductors." Paper 41 Q5(b): "Many candidates recognised that it is the lack of delocalised electrons that makes non-metals less good — common insufficient answers simply stated that non-metals are poor conductors or that non-metals are insulators."
Name the right process (often conduction)
Two yearly errors: (1) energy through a solid (a wall or pan) is conduction, not convection — the most common wrong answer here. (2) When asked for the transfer process, do not write a change of state ("freezing", "melting"); it must be conduction, convection or radiation. Decide: through a solid → conduction; a moving fluid → convection; EM waves → radiation.
June 2024 Cambridge 0625 Paper 31/41: "Only the strongest candidates correctly stated that the thermal process transferring energy from liquid water to solid ice was conduction. Convection was the most common incorrect thermal process given. A common error was to give an answer about a change in state in the material, such as freezing or melting, rather than a thermal energy transfer process."
Only radiation crosses a vacuum
In a vacuum only radiation transfers thermal energy — infrared is an electromagnetic wave and needs no particles. Conduction needs particles in contact and convection needs a moving fluid, so both fail in a vacuum. A common error is naming conduction or convection for energy crossing the vacuum of space.
Convection arrows: warm rises, cool sinks
On a convection diagram the warmed fluid rises above the heat source and cooler, denser fluid descends at the sides, forming a closed loop. Reversing the arrows (fluid sinking at the heat source) scores zero, and the heat source must be at the bottom — a top heat source cannot drive convection downward.
Explain: Cause → Mechanism → Effect
For "Explain why…" thermal questions use Cause → Mechanism → Effect and name the mode (conduction/convection/radiation) and its carrier. "Describe" names what happens; "Explain" needs the reason: what starts the transfer, the particle or wave mechanism, then the result.
Link each feature to one transfer mode
For "explain how [device] reduces heat loss" (e.g. a vacuum flask), link each feature to a specific mode: a vacuum stops conduction AND convection (two marks); silvered surfaces are poor emitters (less radiation); plastic/air conducts poorly. "Stops heat" is too vague.
Use θ or T for temperature, not t
In thermal calculations use or for temperature — means time, and using for temperature can cost a mark. Use or for thermal energy, not (which means charge in 0625). Define any non-syllabus symbol you use.
No calculation formulas are unique to §2.3 — the physics is qualitative. One relationship holds for all three modes:
State which method(s) of thermal energy transfer can occur in a vacuum, and explain why.
Two identical metal cans — one painted black matt and one painted white shiny — are filled with the same volume of water at the same initial temperature and placed in direct sunlight.
Explain why the temperature of the water in the black matt can rises faster than the temperature of the water in the white shiny can.
[3 marks]
