Question

In a region of space, a uniform magnetic field B exists in the ydirection. A proton is fired from the origin, with initial velocity v making a small angle α with the y-direction in the yz plane. In the subsequent motion of the proton, 

(A) Its X-coordinate can never be positive 

(B) Its X- and z-coordinates cannot both be zero at the same time. 

(C) Its z-coordinate can never be negative. 

(D) Its y-coordinate will be proportional to the square of its time of flight. 

Answer 1

So its x coordinate cannot be positive. Its x- and z- coordinate will be zero when particle will complete one revolution.

y - Coordinate = v cos α t