Question

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

Answer 1

Answer: (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 $\vec{B}\ne 0$
But if electric field becomes zero, then only force due to magnetic field axists. Under this force, the charge moves along a circle.