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A beam of electrons passes undeflected through mutually perpendicular electric and magnetic fields. If the electric field is switched off, and the same magnetic field is maintained, the electrons move

1. in a circular orbit
2. along a parabolic path
3. along a straight line
4. in an elliptical orbit

Correct answer: in a circular orbit

Solution

When the electric field is switched off, the electrons experience only the magnetic force, which acts perpendicular to their velocity. This results in circular motion due to the centripetal force provided by the magnetic field.

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