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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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.

Model answer (4 marks)

A uniform electric field exerts a force — — — —
1. The electric field — —
2. The magnetic field — —
3. Only particles with speed — —
4. Particles with other speeds are deflected

Examiner tips

Use the symbol F= qE for the electric force and F= qvB for the magnetic force. Show that the forces are opposite when the fields are perpendicular. State the equality qE = qvB and solve for v = E/B. Mention that only particles with this speed are undeflected. Keep the answer concise and use correct SI units.

Common mistakes

Confusing the direction of the magnetic force (using the wrong hand rule). Mixing up the equality qE = qvB with qE = Bv. Writing the answer in a paragraph instead of numbered points.

Mark scheme (4 marks)

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.
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.
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.
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 in this question

velocity selector · uniform electric field · uniform magnetic field