Paper 1 Q25 — Crossed fields act on the same axis

Velocity selector — E and B forces balance only when along the same axis

What the examiners said

“Many candidates found it challenging. Those who got it wrong tended to choose option A. The fields have to be acting along the same axis for the deflection to be as shown in the diagram.”

Try the question (reconstruction)

A charged particle passes undeflected through a region containing both an electric field and a magnetic field. For this to happen, the electric and magnetic forces on the particle must be:

Examiner report: “Wrong answers had the forces perpendicular; they must act along the same axis to cancel.”

Explore — velocity selector

Tune the speed. Only when qE = Bqv (v = E/B) do the opposing electric and magnetic forces cancel and the particle flies straight. Faster or slower, and the unbalanced force deflects it.

Particle speed v: 5.0 E field: 5.0

Set E = v (in these units) for the straight-through condition. The electric and magnetic forces are both vertical — same axis, opposite directions — which is exactly why they can cancel.

Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H2 Physics 9749 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.

Made by lookang, using Claude Fable 5. More resources: iwant2study.org