# Explain why the internal energy of an ideal gas depends only on its temperature and not on its volume.

> IB DP Physics Higher Level (2023 syllabus) — B.1 Thermal energy transfers · Explain · 4 marks

> An ideal gas is allowed to expand freely into a vacuum (Joule expansion) so that no work is done on or by the gas and no heat is transferred to or from the gas.

## Mark scheme (4 marks)

1. In an ideal gas, intermolecular forces between particles are assumed to be zero (negligible).
2. Because there are no intermolecular forces, there is no potential energy associated with the separation of the molecules.
3. The internal energy of the gas consists entirely of the (random) kinetic energy of its molecules.
4. The mean kinetic energy of the molecules depends only on (absolute / kelvin) temperature, so the internal energy depends only on temperature.

## Key terms

- [internal energy](https://www.gradenine.co.uk/glossary/internal-energy)
- [ideal gas](https://www.gradenine.co.uk/glossary/ideal-gas)

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-internal-energy-of-5a8a2527) · Published by Druglandscape Ltd.