# Explain how a uniform electric field and a uniform magnetic field can be combined to act as a velocity selector for charged particles moving through the region.

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

> A beam of charged particles travelling at different speeds enters a region where a uniform electric field and a uniform magnetic field are applied simultaneously and perpendicularly to each other and to the initial direction of particle motion.

## Mark scheme (4 marks)

1. The electric field exerts a force on the charged particle in the direction of (or opposite to) the field, depending on the sign of the charge.
2. The magnetic field exerts a force on the moving charged particle perpendicular to both the velocity and the magnetic field (F = qvB), in the opposite direction to the electric force.
3. Only particles with a specific speed pass through undeflected when the two forces are equal and opposite, giving the condition qE = qvB, so v = E/B.
4. Particles with speeds different from E/B experience a net force and are deflected out of the beam, so only the selected speed passes straight through.

## Key terms

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

## 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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