question_answer

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

A)  in an elliptical orbit

B)  in a circular orbit

C)  along a parabolic path

D)  along a straight line

Correct Answer: B

Solution :

 If both electric and magnetic fields are present and perpendicular to each other and the particle is moving perpendicular to both of them with \[{{F}_{e}}={{F}_{m}}.\]In this situation \[\vec{E}\ne 0\]and \[\vec{B}\ne 0.\] But if electric field becomes zero, then only force due to magnetic field exists. Under this force, the charge moves along a circle.