20.3.5—The motion of a charged particle moving in a uniform magnetic field

With velocity perpendicular to a uniform B, magnetic force provides centripetal force, so the particle follows circular motion with r=mv/(BQ).

Speed remains constant because magnetic force is perpendicular to velocity; reverse charge or field reverses curvature.

A faster particle follows a larger-radius path in the same field, while a more highly charged particle curves more tightly.

Magnetic field changes direction of velocity, not speed, and the path is helical if velocity has a component parallel to B.