IGCSE Physics: Transfer of Thermal Energy Practice Questions
Thermal energy transfers by conduction through vibrating particles and free electrons, by convection through the movement of a heated fluid, and by radiation as infrared waves that need no medium at all.
Sub-topic 2.3 of Cambridge IGCSE Physics 0625 is entirely explanation. Every question wants the mechanism named and described, and answers that simply say heat rises score nothing. The questions below train the full mechanism for each mode of transfer.
What you need to know for Transfer of Thermal Energy
- ConductionParticles at the hot end vibrate more and pass energy on through collisions with neighbouring particles. In metals, free delocalised electrons also carry energy rapidly through the structure, which is why metals are the best conductors.
- ConvectionA heated region of a fluid expands, becomes less dense and rises. Cooler, denser fluid sinks to take its place, creating a convection current. Convection cannot occur in solids or in a vacuum.
- Thermal radiationInfrared electromagnetic waves emitted by all objects. Radiation requires no medium, so it is the only way energy reaches the Earth from the Sun.
- Good emitters and absorbersDull black surfaces are the best emitters and the best absorbers of infrared radiation. Shiny silver surfaces are the poorest emitters and absorbers, and the best reflectors.
- Rate of radiationThe rate at which an object emits radiation increases with its surface temperature and its surface area, and depends on the nature of its surface.
- Vacuum flaskA vacuum between the walls stops conduction and convection, and silvered surfaces reflect radiation. A stopper reduces evaporation and convection at the top.
IGCSE Physics Transfer of Thermal Energy questions and answers
4 exam-style questions written to the 0625 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Explain how thermal energy is conducted along a metal rod, and explain why metals conduct much better than non-metals.
Show the worked answer
- Particles at the heated end gain energy and vibrate more vigorously.
- They collide with neighbouring particles and transfer some of that energy, so the vibration spreads along the rod. This happens in all solids.
- In a metal there are also free delocalised electrons that are not bound to any one atom.
- These electrons gain kinetic energy at the hot end and move rapidly through the metal, colliding with particles at the cooler end and transferring energy to them.
- This electron mechanism is far faster than vibration alone, so metals conduct thermal energy much better than non-metals, which have no free electrons.
Explain how a convection current forms in a beaker of water heated from below.
Show the worked answer
- Water near the heater at the bottom gains energy, so its particles move faster and move further apart.
- The water in that region therefore expands, and since the same mass now occupies a greater volume, its density decreases.
- Less dense water rises through the denser water above it.
- Cooler, denser water from the top sinks to take its place, is heated in turn, and the cycle repeats. This circulation is a convection current.
Two identical metal cans are filled with equally hot water. One is painted dull black and the other is polished silver. Predict which cools faster and explain why.
Show the worked answer
- Both cans lose energy by conduction and convection at the same rate, because they are identical in shape, size and material.
- The difference is in the rate at which they emit infrared radiation, which depends on the nature of the surface.
- A dull black surface is a good emitter of infrared radiation, while a polished silver surface is a poor emitter and reflects radiation back.
- The dull black can therefore radiates energy away faster and its water cools more quickly.
Explain how a vacuum flask keeps a hot drink hot, referring to all three methods of thermal energy transfer.
Show the worked answer
- The flask has double walls with a vacuum between them. A vacuum contains almost no particles, so there is nothing to pass energy on by collision and conduction is prevented.
- There is also no fluid in the vacuum to circulate, so convection cannot occur across the gap.
- The surfaces facing the vacuum are silvered. A shiny silver surface is a poor emitter of infrared radiation and reflects radiation back towards the drink, so energy loss by radiation is greatly reduced.
- The insulating stopper reduces conduction and convection through the top, and reduces evaporation, which would otherwise cool the drink.
Common mistakes in this topic
- Writing that heat rises, without explaining the density change.
- Saying convection occurs in solids. Convection needs a fluid whose particles can move.
- Claiming radiation cannot travel through a vacuum.
- Confusing emission with absorption when an object is cooling.
- Describing conduction in metals without mentioning free electrons.
Exam tips
- Name the mode of transfer first, then describe its mechanism in particle terms. That structure matches the mark scheme.
- For convection, use the chain: heated, expands, less dense, rises, cooler fluid sinks.
- Remember dull black is best for both emitting and absorbing, and shiny silver is worst for both.
- In any comparison experiment, identify what has been kept the same before explaining the difference.
- Link insulation applications to the specific transfer they block: loft insulation traps air to stop convection, foil behind a radiator reflects radiation.
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Transfer of Thermal Energy FAQs
Why are metals good conductors of heat?
In addition to energy passing between vibrating particles, metals contain free delocalised electrons that are not bound to any one atom. These electrons gain energy at the hot end and travel rapidly through the metal, transferring energy by collision at the cool end. This mechanism is far faster than vibration alone.
How does a convection current form?
A region of fluid is heated, so its particles move further apart and the fluid expands. The same mass now occupies more volume, so its density falls and it rises through the denser fluid above. Cooler, denser fluid sinks to take its place and is heated in turn, creating a circulating current.
Can thermal radiation travel through a vacuum?
Yes. Thermal radiation is infrared electromagnetic waves, which need no medium at all. This is why energy from the Sun reaches the Earth through empty space, and why a vacuum flask must be silvered as well as evacuated.
Which surfaces are the best emitters and absorbers of radiation?
Dull black surfaces are the best emitters and the best absorbers of infrared radiation. Shiny silver surfaces are the poorest emitters and absorbers, and the best reflectors. This is why solar panels are dark and why survival blankets and vacuum flasks are silvered.
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Written to the published Cambridge IGCSE Physics (0625) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.