Force and deflection of moving charges

Repair the common break

Common misconception: A magnetic field changes a charged particle’s speed.

Correct model: Use F = BQv sinθ perpendicular to motion; combine electric and magnetic forces for deflection and velocity selection.

Worked transfer

A magnetic force Qv × B is perpendicular to velocity, so it changes direction but does no work. In crossed fields, undeflected motion requires qE and qvB to oppose, giving v = E/B.

Guided practice

Explain why “A magnetic field changes a charged particle’s speed.” fails. Then construct one example that correctly applies force and deflection of moving charges.

Independent practice

A proton enters crossed fields E = 3.0 × 10⁴ V m⁻¹ and B = 0.20 T. Find the undeflected speed and state how an electron at that speed behaves.

Return to the recorded repair plan when the explanation and application are secure enough to re-test.