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Detailed notes on Thermal Physics for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
Heat transfers by conduction, convection, and radiation. Cambridge tests the mechanism of each process, factors affecting their rate, and how insulators and emitters are used in real-world applications.
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
Heat travels through solids by particle vibration and (in metals) by free electrons.
Conduction: transfer of thermal energy through a material without bulk movement of the material itself.
Mechanism in non-metals:
Mechanism in metals:
Thermal conductivity comparison:
| Material | Conductor/Insulator |
|---|---|
| Silver, copper, aluminium | Excellent conductors |
| Glass | Poor conductor |
| Wood | Insulator |
| Air | Very poor conductor / good insulator |
| Wool, expanded polystyrene | Good insulators (trap air) |
Factors affecting rate of conduction:
Applications:
Hot fluids rise, cold fluids sink — creating circular convection currents.
Convection: transfer of thermal energy by the bulk movement of a fluid (liquid or gas).
Mechanism:
Why can't convection occur in solids?
Applications of convection:
| Application | How convection works |
|---|---|
| Radiator in a room | Hot air rises from radiator; cool air descends; room heats by convection |
| Heating water in a tank | Heater at bottom: hot water rises; cool water sinks to heater |
| Sea breezes | Land heats faster than sea; warm air over land rises; cool air from sea moves in |
| Refrigerator freezer at top | Cold air descends from freezer, cools food below |
Convection currents in the atmosphere:
Factors affecting convection rate:
All objects emit and absorb infrared radiation. Black matt surfaces are best emitters AND absorbers.
Radiation: transfer of thermal energy by infrared (IR) electromagnetic waves.
Key features:
Emission and absorption:
| Surface | Emitter | Absorber |
|---|---|---|
| Black, matt | Best (most) | Best (most) |
| White, shiny | Poor (least) | Poor (least) — reflects most IR |
Factors affecting rate of radiation:
Practical applications:
Vacuum flask — all three processes minimised:
Verbatim phrases and definitions Cambridge mark schemes credit.
Paper 4: 'Explain how heat is transferred from a hot radiator to the air in a room' (3 marks — conduction through radiator surface; air in contact heats and expands → less dense → rises; cool air falls and is heated → convection current). 'State why the inside surfaces of a vacuum flask are silvered' (2 marks — shiny surface is a poor emitter and poor absorber of IR radiation; reduces energy transfer by radiation). 'Explain why a black car becomes hotter in sunlight than a white car' (2 marks — black surface absorbs more IR radiation than white surface).
Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (P3); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.
Step-by-step solutions to past-paper-style questions on thermal processes, written exactly the way a tutor would explain them at the board.
Question
Explain why metals are better thermal conductors than non-metals such as wood.
Step-by-step solution
Step 1
In a metal, thermal energy is transferred by two mechanisms: lattice vibrations and by free (delocalised) electrons.
Step 2
Free electrons gain kinetic energy from the hot region, move rapidly through the lattice, and transfer energy to cooler regions by collisions. This is much faster than vibration alone.
Step 3
Non-metals (e.g., wood) have no free electrons; energy is transferred only by slower lattice vibrations between neighbouring atoms.
Answer
Metals have free electrons that transport thermal energy rapidly; non-metals rely only on slower lattice vibrations.
Question
Explain, in terms of density changes, how convection currents form in a liquid heated from below.
Step-by-step solution
Step 1
The liquid at the bottom is heated; its particles move faster and spread out more, so the liquid expands and becomes less dense.
Step 2
The less dense, warmer liquid rises because it experiences a greater upthrust than the denser cooler liquid above it (buoyancy).
Step 3
Cooler, denser liquid from above sinks to replace it. This sets up a circulation (convection current) that transfers thermal energy throughout the liquid.
Answer
Warm liquid expands, becomes less dense, rises. Cooler denser liquid sinks. This convection current transfers thermal energy throughout the fluid.
Examiner tip
Must explain density changes — simply saying 'hot rises, cold sinks' without explaining why earns few marks.
Question
Explain how a vacuum (Thermos) flask minimises heat loss by all three thermal processes.
Step-by-step solution
Step 1
Conduction: The vacuum between the double glass walls prevents conduction, since there are no particles to transfer energy. The glass stopper minimises conduction through the top.
Step 2
Convection: The vacuum also prevents convection, since there is no fluid medium for convection currents.
Step 3
Radiation: The silvered walls reflect infrared radiation back into the flask, reducing energy loss by radiation in both directions.
Answer
Vacuum stops conduction and convection. Silvered walls reflect infrared, reducing radiative transfer.
Examiner tip
Questions often ask you to 'explain' rather than 'describe'. Always link the feature to the mechanism it reduces.
The formulae you need to memorise for thermal processes on the Cambridge IGCSE 0654 paper, with every variable defined in plain English and a note on when to use it.
P∝T4×A
When to use
Qualitatively comparing radiation from surfaces at different temperatures (quantitative form not required at IGCSE).
Definitions to memorise and the exact keywords mark schemes credit for thermal processes answers — sharpened from recent examiner reports for the 2026 0654 sitting.
The transfer of thermal energy through a material by particle-to-particle collisions (and, in metals, by free electrons), without bulk movement of the material.
The transfer of thermal energy through a fluid (liquid or gas) by the bulk movement of the fluid itself. Warmer fluid expands, becomes less dense, rises; cooler fluid sinks.
The transfer of thermal energy by infrared electromagnetic waves, which do not require a medium. All objects above absolute zero emit thermal radiation.
A matt black surface is a good emitter and good absorber of infrared radiation. A shiny white surface is a poor emitter and poor absorber (good reflector).
Example
Cooling fins on a car radiator are painted black to maximise radiation of thermal energy.
Short-wave solar radiation passes through the atmosphere and is absorbed by Earth's surface. The surface re-emits longer-wave infrared radiation, which is absorbed by greenhouse gases (CO₂, methane), warming the lower atmosphere.
The traps other students keep falling into on thermal processes questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.
Why it happens
Confusing radiation with conduction or convection.
How to avoid it
Radiation is electromagnetic and travels through a vacuum. It is the only method that can transfer energy through space — this is how we receive energy from the Sun.
Why it happens
Students apply convection to all materials.
How to avoid it
Convection requires a fluid (liquid or gas) that can move in bulk. Solids are rigid; thermal energy in solids transfers only by conduction.
Why it happens
Simplified recall from primary school.
How to avoid it
Always explain the mechanism: heating causes expansion → lower density → buoyancy force → rise. Cold dense air sinks to replace it. Examiners want the density link.
The things students keep getting wrong in this sub-topic, answered.