Question

A charged particle moves through a region of magnetic field pointing in the direction perpendicular to its direction of velocity. Then during its subsequent motion,

(1)	Its momentum changes but the kinetic energy remains constant.
(2)	Its momentum and kinetic energy both changes.
(3)	Its momentum and kinetic energy both remains constant.
(4)	Its kinetic energy changes but the momentum remains constant.

Answer 1

Answer
(1) Its momentum changes but the kinetic energy remains constant.

As momentum is a vector, the magnetic field will exert a force at a right angle to both the the velocity and the magnetic field, which will cause a change in direction and therefore a change in momentum. Though the velocity changes, the speed doesn't so kinetic energy stays the same.