Question

A beam of electron 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 a circular orbit
• B. along a parabolic path
• C. along a straight line
• D. in an elliptical orbital

Answer 1

The correct option is A in a circular orbit

Since, direction of motion of electron must be perpendicular to the both electric and magentic field therefore, when electric field is switch off then the magnetic field is perpendicular to the direction of motion and therefore, the electrons starts doing circular motion.
Hence option(A) because magnetic force will be always acting perpendicular to the direction of motion of electron giving necessary centripetal force.