A charge 'q' moving along the X-axis with a velociyt `vecv` (fig) is subjected to a uniform magnetic field `vecB` acting along the Z-axis as it crosses the origin O.
(i) Trace its trajectory.
(ii) Does the charge gain kinetic energy as it enters the magnetic field? Justify your answer.

Explanation

The trajectory is a circualr path in X - Y plane as shown in fig.

(ii) The charge does not gain kinetic energy as it enters as it enters the magnetic field `vecB`. Force `vecF ( = q vecv xx vecB)` due to magnetic field is in a perpendicular direction and changes only the direction of motion of charge. There is no change in magnitude of velocity of charge and hence no change in its kinetic energy.