Launching your learning experience…
Detailed notes on Thermal Physics for Cambridge IGCSE Physics, covering key concepts, explanations, examples, and exam-focused revision points.
Three modes of heat transfer: conduction (solids), convection (fluids), radiation (any). Plus how to reduce each — vacuum flasks are the classic example.
Mapped to the Cambridge IGCSE 0625 syllabus (2026-2028).
Vibrations passed particle to particle. Metals are best (free electrons).
Mechanism. In solids, particles vibrate. A heated end has particles vibrating MORE. They jostle their neighbours, who jostle theirs — vibration energy passes through.
In metals (Extended). A second mechanism dominates: free (delocalised) electrons. The electrons gain KE at the hot end and travel quickly to cooler regions, transferring energy with them. This is why metals conduct heat MUCH better than non-metals.
Order of conductors.
Worked qualitative. Why does a metal handle on a saucepan get hot quickly, but a wooden one stays cool? Metal has free electrons → fast conduction. Wood has none → slow.
Hot fluid expands, becomes less dense, rises. Cold fluid sinks. Cycle = convection current.
Mechanism (fluids only).
This sets up a continuous convection current — a loop that carries heat through the fluid.
Worked qualitative. A radiator at the bottom of a room heats the air. Hot air rises to the ceiling, cools, sinks back down on the other side. The room warms evenly through this cycle.
Why no convection in solids. Particles can't flow → no bulk movement → no convection.
Sea breezes. During the day, land heats up faster than the sea. Air over land rises; cooler sea air rushes in to replace it. At night the reverse happens (land cools faster).
Infrared electromagnetic waves. Needs no medium. Works through vacuum.
Thermal radiation is electromagnetic waves emitted by all warm objects, predominantly in the infrared band. Doesn't need a medium — that's how heat from the Sun reaches Earth through space.
Surface effect.
| Surface | Emits | Absorbs |
|---|---|---|
| Black, dull | Well | Well |
| White, shiny | Poorly | Poorly (reflects) |
Practical applications.
Rate of radiation depends on:
Worked. Why are houses in hot countries painted white? White reflects most of the Sun's radiation, keeping the inside cooler.
Vacuum stops conduction & convection. Silvered walls reduce radiation.
A vacuum flask keeps drinks hot or cold using all three principles:
Even so, no flask is perfect. Heat slowly escapes (or enters) through the stopper, and the small amount of radiation that does cross the silvered walls.
Real applications. Coffee thermos, cryogenic dewars, double-glazed windows (vacuum or low-conductivity gas between panes plus reflective coatings).
Verbatim phrases and definitions Cambridge mark schemes credit.
Heat transfer appears every Paper 2 (3-4 marks: identify which mode dominates) and most Paper 4s (5-6 marks: explain a vacuum flask, sea breeze, or solar panel). Examiner reports flag students writing "heat rises" — Cambridge wants "hot air rises because it is less dense".
Sources: Cambridge IGCSE Physics 0625 syllabus 2026-2028 (2.3); 0625/22 May/Jun 2024 — Q9 (vacuum flask); 0625 Examiner Reports 2022-2024. Last reviewed 2026-05-06.
Step-by-step solutions to past-paper-style questions on transfer of thermal energy, written exactly the way a tutor would explain them at the board.
Almost every transfer of thermal energy exam question is one of these shapes. Learn to spot each one and you will always know how to start.
Recognise it by
The stem says explain why / how — why a metal feels hotter, how a convection current forms, how a vacuum flask works. A mechanism, not a number, is wanted.
How to approach it
Name the mechanism (conduction, convection or radiation), then explain it at particle level: free electrons for metal conduction; density change and rise/sink for convection; absorb-and-re-emit for radiation.
Common trap
Examiner reports flag 'hot air rises' with no mention of density, and saying the atmosphere 'reflects' heat — the marking point for the greenhouse effect is absorbs and re-emits.
Recognise it by
The question gives a list of scenarios or asks you to state the factors / name the main mode of thermal transfer.
How to approach it
Match each case to its mechanism by asking what carries the energy — a solid path (conduction), a moving fluid (convection) or empty space / IR (radiation).
Common trap
Naming convection for heat travelling through a solid metal rod — solids do not flow, so that is conduction.
Recognise it by
You are asked to sketch or interpret cooling curves — typically two objects cooling on the same temperature–time axes.
How to approach it
Draw both curves falling and levelling off toward room temperature; place the better emitter (matt black) below, since it loses energy faster, and annotate why.
Common trap
Drawing straight-line cooling or curves that cross — cooling curves are concave and the better emitter stays below the poorer one at all times.
Question
Explain why a metal spoon feels hotter than a wooden one when both are placed in hot soup.
Step-by-step solution
Step 1
Metals have FREE (delocalised) electrons that move quickly through the lattice.
Step 2
Free electrons transfer kinetic energy to cooler regions much faster than vibrating atoms alone.
Answer
Metals conduct via free electrons; non-metals only via slower atomic vibration.
Question
Explain how a convection current forms when water is heated from below.
Step-by-step solution
Step 1
Water at the bottom warms, expands, becomes less dense.
Step 2
Less dense water rises; cooler, denser water sinks to take its place.
Step 3
Continual rising-and-sinking forms a convection current.
Answer
Heated water expands, becomes less dense and rises; cool water sinks → circulation current.
Question
Why is a teapot painted matt black better at radiating heat than one with a shiny silver finish?
Step-by-step solution
Step 1
Matt black surfaces are good EMITTERS and ABSORBERS of infrared radiation.
Step 2
Shiny silver surfaces are POOR emitters and absorbers (and good reflectors).
Answer
Black, dull → high IR emission. Shiny silver → low IR emission.
Question
Explain how a vacuum flask reduces all three types of thermal energy transfer.
Step-by-step solution
Step 1
Vacuum gap eliminates conduction AND convection (no particles to transfer energy).
Step 2
Silvered inner walls reduce radiation by reflecting IR back inside.
Step 3
Plastic stopper and outer casing reduce conduction further.
Answer
Vacuum stops conduction/convection; silvering reduces radiation; plastic limits residual conduction.
Question
Compare conduction in a metal and an insulator (e.g. wood) in terms of particle behaviour.
Step-by-step solution
Step 1
In all solids, atoms vibrate. Energy is transferred from hotter (faster-vibrating) atoms to cooler neighbours through the lattice — this is slow lattice conduction.
Step 2
Metals additionally have free (delocalised) electrons. These move quickly through the structure, carrying KE from hot regions to cold regions much faster than lattice vibration alone.
Step 3
Non-metals such as wood have NO free electrons, so only the slow lattice mechanism operates → they are poor conductors / good insulators.
Answer
Metals: fast electron-based conduction + lattice vibration. Non-metals: lattice vibration only → much slower.
Examiner tip
The examiner report flags candidates often write 'metals have more particles' or 'metals contain heat'. Marks come from naming free/delocalised electrons explicitly.
Question
State four factors that affect how quickly an object emits infrared radiation, and for each say how an increase changes the emission.
Step-by-step solution
Step 1
Surface temperature: hotter object → more IR emitted (very strong dependence on temperature).
Step 2
Surface area: larger area → more IR emitted (proportional).
Step 3
Surface colour: matt black → high emission; shiny silver/white → low emission.
Step 4
Surface texture: dull/rough → high emission; smooth/polished → low emission.
Answer
Higher temperature, larger area, darker colour, duller texture each increase the rate of IR emission.
Question
For each scenario, name the MAIN mode of thermal energy transfer: (a) a metal saucepan handle becoming hot, (b) warm air circulating in a room, (c) heat reaching Earth from the Sun, (d) a marshmallow held near (not in) a campfire.
Step-by-step solution
Step 1
(a) Conduction along the metal — free electrons and lattice vibration.
Step 2
(b) Convection — warmer air rises (less dense), cool air sinks.
Step 3
(c) Radiation — EM waves through the vacuum of space.
Step 4
(d) Radiation — IR from the flames travels through the air without heating it appreciably (air is a poor conductor and convection currents rise above the marshmallow).
Answer
(a) Conduction. (b) Convection. (c) Radiation. (d) Radiation.
Question
Two identical kettles hold the same volume of boiling water in the same room. Kettle A is painted matt black; kettle B is highly polished silver. Sketch on the same axes how the water temperature in each kettle changes with time, and explain.
Step-by-step solution
Step 1
Both kettles cool: temperature drops smoothly toward room temperature (curved, levelling off).
Step 2
Matt black A is a BETTER emitter of IR radiation → loses energy faster → its curve falls more steeply, especially while hot.
Step 3
Shiny silver B is a POOR emitter → cools more slowly → its curve is above A throughout.
Answer
Both curves fall toward room temperature; black-kettle curve A is below the silver-kettle curve B at all times (A cools faster).
Question
Explain how the Earth's atmosphere keeps the surface warmer than it would otherwise be, in terms of incoming and outgoing radiation.
Step-by-step solution
Step 1
The Sun emits short-wavelength radiation (visible/near-IR) which mostly passes through the atmosphere and is absorbed by the Earth's surface, warming it.
Step 2
The Earth's surface re-emits radiation, but at a much LOWER temperature → longer-wavelength infrared.
Step 3
Greenhouse gases (CO2, methane, water vapour) absorb this long-wavelength IR and re-emit it in all directions, so part of the energy is sent back toward the surface.
Step 4
Less energy escapes to space → equilibrium temperature is higher than without the absorbing atmosphere.
Answer
Greenhouse gases trap long-wavelength IR re-emitted by the Earth, raising the equilibrium surface temperature.
Examiner tip
The examiner report flags candidates often say 'the atmosphere reflects the Sun's heat back' — the marking point is ABSORBS and RE-EMITS, not reflects.
Question
A house owner adds a layer of fibreglass loft insulation. (a) Which mechanism does it primarily reduce, and how? (b) The owner also fits foil-backed boards under the roof tiles. Which mechanism does the foil mainly reduce? (c) Explain why thick double-glazing reduces heat loss through windows more than a single sheet of glass of the same total thickness.
Step-by-step solution
Step 1
(a) Fibreglass traps small air pockets. Air is a poor conductor and the trapped pockets stop convection currents forming → reduces convection AND conduction through the ceiling.
Step 2
(b) Foil is a shiny surface → poor emitter and good reflector of IR → reflects radiation back into the loft, reducing radiative loss.
Step 3
(c) Double-glazing has a sealed air (or argon) gap between two thin panes. The trapped gas is a poor conductor, and the small gap suppresses convection. A single thick glass sheet still conducts through its full thickness — glass is a much better conductor than air.
Answer
(a) Fibreglass: conduction and convection (trapped air). (b) Foil: radiation (poor emitter, reflects IR). (c) Double-glazing uses a trapped-gas gap that is a worse conductor than glass, while solid glass conducts throughout its thickness.
High-scoring sample answers for transfer of thermal energy on the Cambridge IGCSE 0625 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
Name the three ways in which thermal energy can be transferred.
Model answer
Conduction, convection and (infrared) radiation.
Why this scores
One mark for all three named correctly. Convection occurs only in fluids (liquids and gases); radiation is the only mode that can cross a vacuum.
Explain why metals are much better conductors of thermal energy than non-metals such as wood.
Model answer
Metals contain free (delocalised) electrons that can move quickly through the structure. When one part is heated, these electrons gain kinetic energy and carry it rapidly to cooler parts, in addition to the slower transfer by vibrating atoms. Non-metals such as wood have no free electrons, so thermal energy can pass only by the slow vibration of atoms — making them poor conductors.
Why this scores
One mark for naming free/delocalised electrons in metals, one for explaining they transfer energy quickly (and that non-metals lack them). 'Metals contain more heat' or 'metals have more particles' scores nothing.
Explain how a convection current is set up when a beaker of water is heated from the bottom.
Model answer
1. The water at the bottom is heated, so it expands and becomes less dense than the cooler water above it. 2. The less dense warm water rises, and cooler, denser water sinks to take its place. 3. This sinking water is then heated in turn, so a continuous circulation — a convection current — is set up, transferring thermal energy through the body of the water.
Why this scores
Three marks: heated water expands and becomes less dense (1); less dense water rises while denser water sinks (1); continuous circulation transfers the energy (1). 'Hot water rises' alone is not enough — the density change must be stated.
A solar water heater has a panel that should absorb as much radiation as possible. A car radiator should lose heat to the air as fast as possible. A vacuum flask should lose as little as possible. State and explain the best surface (matt black or shiny silver) for each.
Model answer
Matt black surfaces are good absorbers and good emitters of infrared radiation; shiny silver surfaces are poor absorbers and poor emitters (good reflectors).
Why this scores
Four marks: the general matt-black good / shiny poor principle (1); correct surface with reason for each of the three cases (1 each). Good absorbers are also good emitters — the symmetry is the key idea examiners test.
A vacuum (thermos) flask keeps a hot drink hot for many hours. Explain how its design reduces heat loss by conduction, convection and radiation.
Model answer
1. The flask has a double wall with a vacuum between the two layers. A vacuum has no particles, so thermal energy cannot be transferred across it by conduction or convection (both need a material/fluid). 2. This removes the two largest loss routes. 3. The two surfaces facing the vacuum are silvered (shiny), making them poor emitters and good reflectors of infrared radiation, so radiation losses are greatly reduced — heat radiated from the hot contents is reflected back. 4. The stopper and outer case are made of plastic (or cork), which is a poor conductor, reducing the small amount of conduction through the top and walls. 5. Together these features cut all three transfer modes, so the drink cools very slowly.
Why this scores
Five marks: vacuum stops conduction (1) and convection (1) because there are no particles; silvered surfaces reduce radiation by being poor emitters/reflecting IR (1); plastic stopper/case is a poor conductor (1); overall all three modes reduced (1). Linking each design feature to a specific mechanism is what gains the marks.
A homeowner wants to reduce heat loss from a house. Explain how each of the following works: (a) fibreglass loft insulation, (b) double glazing, (c) shiny foil fixed behind radiators on outside walls.
Model answer
(a) Loft insulation (fibreglass): the fibreglass traps small pockets of air. Air is a poor conductor, and because the air is trapped it cannot form convection currents — so heat loss through the ceiling by both conduction and convection is reduced.
(b) Double glazing: two panes of glass have a sealed gap of air (or argon) between them. The trapped gas is a poor conductor and the narrow gap suppresses convection, so much less heat is conducted/convected through the window than through a single pane.
(c) Shiny foil behind radiators: the foil is a shiny surface, so it is a good reflector (and poor emitter) of infrared radiation. It reflects radiated heat back into the room instead of letting it warm the outside wall, reducing radiative loss.
Why this scores
Six marks: loft insulation traps air → poor conductor (1) and stops convection (1); double glazing's trapped gas reduces conduction (1) and convection (1); foil reflects infrared radiation back into the room (1) because shiny surfaces are good reflectors/poor emitters (1). Each feature must be tied to the correct mechanism — naming the wrong mode loses the mark.
The formulae you need to memorise for transfer of thermal energy on the Cambridge IGCSE 0625 paper, with every variable defined in plain English and a note on when to use it.
Conduction→solids; Convection→fluids; Radiation→any
When to use
Identifying which mode applies in a question.
Definitions to memorise and the exact keywords mark schemes credit for transfer of thermal energy answers — sharpened from recent examiner reports for the 2026 0625 sitting.
Transfer of thermal energy through a material WITHOUT bulk movement of the material. Mainly via lattice vibration; in metals, via free electrons.
Transfer of thermal energy in a fluid by bulk movement of warmer, less-dense regions of the fluid.
Transfer of thermal energy by electromagnetic waves; requires no medium.
Matt black surfaces emit and absorb IR well; shiny silver surfaces are poor at both (good reflectors).
The traps other students keep falling into on transfer of thermal energy questions — taken from recent Cambridge IGCSE 0625 examiner reports and mark schemes — and how to avoid them.
0625/42 — recurring
Why it happens
Confusing with sound or convection.
How to avoid it
EM waves (including IR) travel through a vacuum — that's how the Sun reaches us.
Why it happens
Convection is the most familiar.
How to avoid it
Solids do not flow — heat in a metal rod is CONDUCTION (mainly free electrons).
Why it happens
Folk science.
How to avoid it
Hot air rises BECAUSE it is LESS DENSE than cooler surrounding air.
Why it happens
Forgetting symmetry.
How to avoid it
Good absorbers are also good emitters; shiny is bad at BOTH.
The things students keep getting wrong in this sub-topic, answered.