# Explain how a velocity selector uses crossed electric and magnetic fields to allow only particles of a specific speed to pass through undeflected, regardless of their charge-to-mass ratio.

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

> A velocity selector consists of two parallel plates creating a uniform electric field, with a uniform magnetic field applied perpendicular to both the electric field and the initial direction of particle motion.

## Mark scheme (4 marks)

1. The electric force and magnetic force act in opposite directions on the moving charged particle.
2. For a particle to pass undeflected, the electric force must equal the magnetic force in magnitude: qE = qvB.
3. Cancelling the charge q from both sides gives v = E/B, showing the selected speed depends only on field strengths and not on the charge or mass of the particle.
4. Particles with speeds different from E/B experience a net force and are deflected away from the straight-through path, so they do not exit through the selector aperture.

## Key terms

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

## 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-how-a-velocity-selector-uses-d8672aa1) · Published by Druglandscape Ltd.