I keep getting “Magnetic force on a moving charge” questions wrong in Moving Charges & Magnetism. What's the trap?
using qvB when velocity is parallel to the field.
The usual trap: using qvB when velocity is parallel to the field. How to avoid it: only the perpendicular velocity component bends. Go back to the definition — A charge moving in magnetic field feels q(v x B). The force is perpendicular to both velocity and field, so a magnetic field changes direction of motion but does no work and cannot change speed by itself.
Key point
Magnetic force on a moving charge
Common mistake
using qvB when velocity is parallel to the field.
Memory tip
only the perpendicular velocity component bends.
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