# Explain why a charged particle moving with constant velocity through a region of crossed electric and magnetic fields (a velocity selector) travels in a straight line without deflection, and state the condition on the particle's velocity for this to occur.

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

> A velocity selector consists of uniform electric and magnetic fields directed perpendicular to each other and perpendicular to the initial velocity of a beam of charged particles.

## Mark scheme (4 marks)

1. The electric field exerts a force on the charged particle (F = qE) in the direction of the field (or opposite, depending on charge sign).
2. The magnetic field exerts a magnetic force on the moving charged particle (F = qvB) in the opposite direction to the electric force.
3. The particle travels in a straight line because the electric and magnetic forces are equal in magnitude and opposite in direction, giving a zero net force.
4. The condition for straight-line travel is qE = qvB, which gives v = E/B.

## Key terms

- [velocity selector](https://www.gradenine.co.uk/glossary/velocity-selector)

## 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-e7cd8573) · Published by Druglandscape Ltd.