A charged particle moves through a magnetic field perpendicular to its direction. Then

When a charged particle moves through a magnetic field perpendicular to its direction, it attains a circular path. The magnetic field exerts force in a direction perpendicular to its direction of velocity so its changes at each point but the magnitude is same. Thus work done by magnetic force is zero so kinetic energy of charge particle remains same. As external force is acting on the system momentum will not conserve.