# Explain why a charged particle moving parallel to a uniform magnetic field experiences no magnetic force, whilst the same particle moving perpendicular to the field experiences a force that does no work on the particle.

> 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 a charged particle is given by F = qv × B, so the force depends on the component of velocity perpendicular to the field.
2. When the particle moves parallel to the field, the angle between v and B is 0° (or 180°), so sin θ = 0 and the force is zero.
3. When the particle moves perpendicular to the field, the magnetic force is always directed perpendicular to the velocity (centripetal direction).
4. Because the force is perpendicular to the displacement (velocity), the work done W = F · d = Fd cos 90° = 0, so the particle's kinetic energy and speed remain constant.

## Key terms

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

## 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-charged-particle-moving-39eff4a1) · Published by Druglandscape Ltd.