H1 Physics 8867 · Nov 2023PhysicsPaper 2Widespread errors

Paper 2 Q4 — Magnetic force: the angle, the charge, and zero work

Electromagnetism — F = BIL sinθ and F = Bqv sinθ; why the magnetic force does no work

What the examiners said

“Some candidates neglected the angle between the current and the magnetic field… Another occasional error was considering the electron charge to be −1 C. A significant number argued that there was work done since the electron moved.”

Item 1 — force on a moving electron

An electron moves at 6.0 × 10⁵ m s⁻¹ at right angles to a uniform magnetic field of flux density 0.45 T. What is the magnitude of the force on it? (e = 1.6 × 10⁻¹⁹ C)

Examiner report: “Occasional errors: neglecting the angle, or taking the electron charge to be −1 C.”

Item 2 — does the magnetic force do work?

The magnetic force deflects the electron into a circular path. How much work does the magnetic force do on the electron?

Examiner report: “A significant number argued that there was work done since the electron moved.”

Explore — steering without speeding

Watch the electron circle in the field (field into the page). Vary B and v. The force arrow stays perpendicular to velocity — check the kinetic-energy readout as it turns.

Flux density B (T): 0.45 Speed v (×10⁵ m s⁻¹): 6.0

Increase B: the circle tightens but the KE readout never moves. Motion ≠ work — work needs displacement along the force.

Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H1 Physics 8867 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.

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