Practice question · Multiple choice
An electron moves through a uniform magnetic field. The magnetic force is always perpendicular to its velocity. What can this force change?
Hints
- Work requires a force component along the displacement: .
- Zero work means zero change in kinetic energy. What is left for the force to do?
Show the answer
C. The direction of motion only
Why
A perpendicular force does no work, so the kinetic energy, and hence speed, is frozen. All the force can do is bend the path, which is why charges in uniform magnetic fields travel in circles and spirals rather than speeding up.
Practise Magnetic Field and Ampere's Law
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