# Explain why a charged particle moving at a constant velocity in a uniform electric field and a uniform magnetic field can travel in a straight line without deflection, and outline what condition must be satisfied for this to occur.

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

> A beam of charged particles enters a region where a uniform electric field and a uniform magnetic field are directed perpendicular to each other and to the initial velocity of the particles.

## Mark scheme (4 marks)

1. The electric field exerts a force on the charged particle (electric force = qE).
2. The magnetic field exerts a force on the moving charged particle (magnetic force = qvB) directed opposite to the electric force.
3. When the two forces are equal in magnitude, the net force on the particle is zero, so it continues in a straight line at constant velocity.
4. The condition for straight-line travel is qE = qvB, which simplifies to E = vB (the electric field strength equals the product of the particle's speed and the magnetic flux density).

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- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
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