question_answer

A beam of electrons passes undetected through mutually perpendicular electric and magnetic fields. If the electric field is switched off, and the same magnetic field is maintained, the electrons move:     [AIPMT (S) 2007]

A)              in an elliptical orbit

B)                   in a circular orbit

C)  along a parabolic path               

D)       along a straight line

Correct Answer: B

Solution :

[b] 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 \[B\ne 0\].

But if electric field becomes zero, then only force due to magnetic field exists. Under this force, the charge moves along a circle.