question_answer

Cathode rays are passed between the poles of a magnet as shown in figure. The effect of magnetic field is

A) to increase velocity of rays

B) to deflect the rays towards S-pole

C) to deflect the rays towards N-pole

D) to deflect the rays perpendicular to the plane of paper, upwards

Correct Answer: D

Solution :

Key Idea: The magnetic lens of force emerge from north pole and enter south pole. Cathode rays are fast moving electrons and direction of current is taken opposite to direction of motion of electrons. In the given set up magnetic field is from north pole to south pole, direction of current is opposite to direction of cathode rays, hence from Flemings left hand rule, force acts perpendicular to plane of paper upwards. Note: Flemings left hand rule is if the fore-finger, middle finger and thumb of left hand are stretched mutually at right angles to one another such that the fore finger points in the direction of magnetic field and middle     finger in direction of current, then thumb will point direction of force acting on the charged particle.