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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (4 marks)
A charged particle entering the selector experiences two forces.
1. The electric force qE acts perpendicular to the particle’s velocity.
2. The magnetic force qvB acts in the opposite direction to the electric force.
For the particle to emerge undeflected the two forces must cancel: qE = qvB.
Cancelling q gives v = E/B – the speed that satisfies the balance.
Thus the selector allows only particles with speed E/B to pass straight through; particles with other speeds feel a net force and are deflected away and do not exit the aperture.
1. The electric force qE acts perpendicular to the particle’s velocity.
2. The magnetic force qvB acts in the opposite direction to the electric force.
For the particle to emerge undeflected the two forces must cancel: qE = qvB.
Cancelling q gives v = E/B – the speed that satisfies the balance.
Thus the selector allows only particles with speed E/B to pass straight through; particles with other speeds feel a net force and are deflected away and do not exit the aperture.
Examiner tips
- Show the two forces and their directions clearly; use the symbol q for charge.
- Write the balance equation qE = qvB and cancel q to give v = E/B.
- Explain that the result is independent of charge or mass, so the selector works for any particle type.
- Mention that particles with v ≠ E/B are deflected and miss the exit aperture.
Mark scheme (4 marks)
- The electric force and magnetic force act in opposite directions on the moving charged particle.
- For a particle to pass undeflected, the electric force must equal the magnetic force in magnitude: qE = qvB.
- 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.
- 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 in this question
velocity selector · charge-to-mass ratio
Related
- All IB DP Physics Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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