# Explain why a proton moving in a circular path in a uniform magnetic field does not gain kinetic energy, even though a magnetic force acts on it continuously.

> IB DP Physics Higher Level (2023 syllabus) — D.2 Electric and magnetic fields · Explain · 4 marks

## Mark scheme (4 marks)

1. The magnetic force on the proton is always perpendicular to its velocity.
2. A force perpendicular to the velocity changes only the direction of the velocity, not its magnitude.
3. No work is done by the magnetic force because work requires a force component along the displacement.
4. Since no work is done on the proton, its kinetic energy remains constant and cannot increase.

## Key terms

- [magnetic force](https://www.gradenine.co.uk/glossary/magnetic-force)
- [circular path](https://www.gradenine.co.uk/glossary/circular-path)
- [kinetic energy](https://www.gradenine.co.uk/glossary/kinetic-energy)

## 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-a-proton-moving-in-01dea66a) · Published by Druglandscape Ltd.