IGCSE Physics: Thermal Physics — Revision Notes

Condensed for the final weeks. For the full explanation, use the Thermal Physics study guide.

The kinetic particle model — the one idea behind everything

State	Arrangement	Separation	Motion
Solid	Regular, fixed positions	Very close	Vibrate about fixed points only
Liquid	Random, close together	Close	Move around each other, sliding
Gas	Random, far apart	Far apart	Move fast, randomly, in straight lines between collisions

• Absolute zero = −273 °C = the lowest possible temperature, where particles have the least possible kinetic energy.
• T (K) = θ (°C) + 273
• Gas pressure is caused by gas particles colliding with the walls of their container — more frequent or harder collisions mean higher pressure.
• pV = constant (Supplement), for a fixed mass of gas at constant temperature (Boyle’s law) — pressure is inversely proportional to volume, so halving the volume doubles the pressure.
• Brownian motion: the random, erratic motion of visible smoke/pollen particles suspended in a gas or liquid, caused by collisions with the much smaller, fast-moving gas/liquid particles — evidence for the particle model.

Thermal expansion

• Solids, liquids and gases all expand when heated (at constant pressure) — gases expand the most, then liquids, then solids, because of the differences in how strongly particles are held together and how far apart they already are (Supplement).
• Everyday consequences: expansion gaps in bridges/railway tracks, bimetallic strips in thermostats, a tight jar lid loosening when warmed.

Specific heat capacity (Supplement)

c = ΔE / (mΔθ)

Specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1 °C (or 1 K). A higher specific heat capacity means a substance needs more energy for the same temperature rise — this is why water (very high c) is slow to heat up and slow to cool down compared to metals.

Core-only idea: a temperature rise means an increase in an object’s internal energy — you don’t need the equation for this, just the link.

Changes of state

• Melting/boiling: energy is put in, but temperature stays constant while the state changes — the energy is used to overcome the forces between particles (increasing potential energy), not to speed the particles up.
• Evaporation happens at any temperature, only at the liquid’s surface, and is the escape of the most energetic particles — this removes energy from the remaining liquid, which is why evaporation has a cooling effect.
• Boiling happens at one fixed temperature (the boiling point), throughout the liquid, not just the surface.
• Factors that increase the rate of evaporation (Supplement): higher temperature, larger surface area, air movement (draught) over the surface, lower humidity.

The three ways heat transfers — keep them strictly separate

Process	Needs	Mechanism
Conduction	A medium (solid best)	Particle vibrations pass energy to neighbours; in metals, free electrons carry energy too (Supplement)
Convection	A fluid (liquid or gas)	Heated fluid expands, becomes less dense, rises; cooler, denser fluid sinks to replace it
Radiation	Nothing — works through a vacuum	Infrared emitted by all objects; dull black surfaces are the best emitters and absorbers, shiny/light surfaces are the worst emitters and best reflectors

Common mistakes

• Saying gases “conduct badly” without the reason — particles are far apart, so there’s little particle-to-particle contact to pass vibrations along.
• Confusing evaporation and boiling: evaporation is surface-only, any temperature, slower; boiling is throughout the liquid, one fixed temperature.
• Forgetting that during a change of state, temperature does not rise even though energy is still being supplied.
• Mixing up which surfaces are good vs poor radiators: dull/black = good emitter and absorber; shiny/white = poor emitter, good reflector.
• Writing pV = constant without the condition “at constant temperature, for a fixed mass of gas.”

Examiner report insight

• Gas pressure and volume (at constant temperature) are inversely proportional, not directly proportional – as volume decreases, pressure increases, and vice versa; this is one of the most commonly guessed relationships in the whole syllabus.
• Thermal radiation can travel through a vacuum – it is the only one of the three transfer methods (conduction, convection, radiation) that can, which is precisely why it is the mechanism by which energy reaches Earth from the Sun.
• Brownian motion: smoke or pollen particles move randomly because they are struck by fast-moving, randomly-moving gas or liquid molecules – not because of the particle’s own density relative to the surrounding fluid.

Source: Cambridge International, 0625 Physics Principal Examiner Report, June 2024 series, Papers 11, 12, 13, 21, 22, 23, 42 (verified 2026-09-02).

Self-test

1. Describe, in terms of particle arrangement and motion, the difference between a liquid and a gas.
2. Explain why touching a metal spoon feels colder than touching a wooden spoon at the same room temperature.
3. A fixed mass of gas at constant temperature has its volume halved. What happens to its pressure?
4. Explain why sweating cools the body.
5. State which type of surface is the best absorber of infrared radiation, and why this matters for choosing the colour of a car roof in a hot country.

Answers: 1. In a liquid, particles are close together, in random arrangement, and move around/slide past each other; in a gas, particles are far apart, in random arrangement, and move fast in random directions between collisions. 2. Metal is a much better thermal conductor than wood, so it carries heat away from your hand faster, making it feel colder even though both are at the same temperature. 3. Pressure doubles (pV = constant, so halving V doubles p). 4. Sweat evaporates from the skin; evaporation removes energy from the most energetic water particles as they escape, which lowers the remaining liquid’s (and skin’s) temperature. 5. A dull black surface is the best absorber; a white/light, shiny car roof is chosen instead because it’s a poor absorber (and good reflector) of radiation, keeping the car cooler.