# Thermal physics and gases

> Internal energy, specific heat capacity and latent heat, the ideal gas laws, kinetic theory and the link between temperature and mean molecular kinetic energy.

Canonical: https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.thermal
Last updated: 2026-08-01

Flagship subject: topics and specification statements are authored to the WJEC specification. No question in it has been through human review yet.

- Subject: [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- Specification reference: Unit 4.1–4.2
- Difficulty: 4/5
- Assessment objectives: AO1, AO2
- Status: authored · checked · 2026-08-01

## Specification coverage

| Reference | Statement | AOs |
| --- | --- | --- |
| Unit 4.1(a) | define internal energy as sum of random kinetic and potential energies of particles | AO1 |
| Unit 4.1(b) | apply Q equals m c delta T for temperature changes and Q equals m L for changes of state | AO2 |
| Unit 4.1(c) | use the ideal gas equation pV equals nRT and pV equals NkT with temperature in kelvin | AO2 |
| Unit 4.2(a) | relate pressure to molecular momentum change and derive kinetic theory result | AO2 |
| Unit 4.2(b) | use half m c squared average equals three halves kT to link temperature and molecular kinetic energy | AO2 |
| Unit 4.2(c) | interpret p-V and heating curves including latent heat plateaus | AO2, AO3 |

## Summary

Internal energy, specific heat capacity and latent heat, the ideal gas laws, kinetic theory and the link between temperature and mean molecular kinetic energy.

## What earns marks

- Q = mcΔT for a temperature change; Q = mL for a change of state at constant temperature.
- pV = nRT = NkT, with temperature always in kelvin.
- Kinetic theory: ½m⟨c²⟩ = (3/2)kT, so absolute temperature is a measure of mean molecular kinetic energy.
- Internal energy is the sum of the random kinetic and potential energies of the molecules.

## Where marks are lost

- Working in °C in a gas law.
- Applying Q = mcΔT during melting or boiling.
- Saying 'heat' when internal energy or temperature is meant.

## Related

- [WJEC · A Level · Physics](https://revise-gray.vercel.app/library?subject=wjec-alevel-physics)
- [Thermal physics and gases lesson](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.thermal)