# A proton enters a uniform magnetic field directed into the page. The proton initially travels horizontally to the right and follows a circular path. Explain why the speed of the proton remains constant throughout its circular motion in the magnetic field, and identify the direction of the magnetic force acting on the proton at the instant it enters the field.

> IB DP Physics Standard Level (2023 syllabus) — D.3 Motion in electromagnetic fields · Explain · 4 marks

> A proton moves horizontally to the right and enters a region of uniform magnetic field directed perpendicularly into the page.

## Mark scheme (4 marks)

1. The magnetic force on the proton is always perpendicular to its velocity.
2. A force perpendicular to velocity does no work on the proton.
3. Since no work is done on the proton, its kinetic energy does not change, so its speed remains constant.
4. The magnetic force on the proton at the instant it enters the field is directed upwards (towards the top of the page).

## Key terms

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

## 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/a-proton-enters-a-uniform-magnetic-8c18a647) · Published by Druglandscape Ltd.