# Explain why a proton moving in a straight line at constant speed through a region of space where both a uniform electric field and a uniform magnetic field are present is possible, and outline what this tells us about the relative magnitudes and directions of the two fields.

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

> A proton travels in a straight line at constant speed through a region where a uniform electric field and a uniform magnetic field both exist simultaneously.

## Mark scheme (4 marks)

1. For the proton to travel in a straight line at constant speed, the net force on it must be zero.
2. The electric force (qE) and the magnetic force (qvB) must be equal in magnitude.
3. The electric force and magnetic force must act in opposite directions to each other (so that they cancel).
4. This arrangement acts as a velocity selector: only particles with speed v = E/B travel undeflected, regardless of their charge-to-mass ratio.

## 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)
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