# 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 maximum magnetic force.

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

> A proton moves through a region of uniform magnetic field.

## Mark scheme (4 marks)

1. The magnetic force on a charged particle depends on the component of velocity perpendicular to the magnetic field (F = qvB sinθ).
2. When the particle moves parallel to the field, θ = 0°, so sin 0° = 0 and the magnetic force is zero.
3. When the particle moves perpendicular to the field, θ = 90°, so sin 90° = 1 and the force is at its maximum value F = qvB.
4. The magnetic force acts perpendicular to both the velocity and the field (by the right-hand rule / cross-product nature of the force), so it changes the direction of the particle but does no work on it.

## Key terms

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

## 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-a-charged-particle-moving-5a1ca879) · Published by Druglandscape Ltd.