# Explain why an electron moving in a circle within a uniform magnetic field does not gain kinetic energy, even though a magnetic force acts on it continuously.

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

> An electron travels in a circular path when it enters a region of uniform magnetic field directed perpendicular to its velocity.

## Mark scheme (4 marks)

1. The magnetic force is always perpendicular to the velocity of the electron.
2. A force perpendicular to velocity changes only the direction of motion, not the speed.
3. No work is done on the electron because work requires a force component parallel to the displacement.
4. Since no work is done on the electron, its kinetic energy remains constant.

## Key terms

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

## Related

- [Revision notes for IB DP Physics Standard 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-an-electron-moving-in-4e5df682) · Published by Druglandscape Ltd.