# Thermal physics and gases: Unit 4.1(a)

> define internal energy as sum of random kinetic and potential energies of particles

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Specification statement (Unit 4.1(a)): define internal energy as sum of random kinetic and potential energies of particles

## Explanation

1. **Internal energy** is the sum of the randomly distributed kinetic and potential energies of all the particles in a body.
2. Heating raises the **kinetic** part, which we detect as a rise in temperature. During a change of state, the energy supplied increases the **potential** part (breaking bonds), so the temperature stays constant.
3. Internal energy can be increased by heating or by doing work on the system. WJEC writes the first law as ΔU = Q − W, where Q is the heat supplied to the system and W is the work done **by** it, so work done on the system makes W negative.

## Worked example

Ice at 0 °C is heated and melts into water at 0 °C. What happens to the internal energy, the kinetic energy and the potential energy of its molecules?

1. Energy is supplied, so internal energy increases.
2. The temperature is constant, so the mean kinetic energy of the molecules is unchanged.
3. The increase is all in potential energy, as bonds between molecules are broken or weakened.

Answer: Internal energy rises; kinetic energy unchanged; potential energy rises.

## Check

Which property of a substance is directly related to the mean random kinetic energy of its particles?

- Internal energy
- Temperature
- Specific latent heat
- Density

The answer and its explanation are shown in the app once you choose an option.

## Apply it

Explain why a gas heats up when it is compressed quickly in a bicycle pump.

The model answer is shown in the app after you have tried it.

## Related

- [Thermal physics and gases lesson](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.thermal)
- [Thermal physics and gases in the library](https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.thermal)