The magnetic force on a moving charge is given by the Lorentz force: \(\vec{F} = q(\vec{v} \times \vec{B})\). The cross product means the force is always perpendicular to both the velocity \(\vec{v}\) and the magnetic field \(\vec{B}\). Since the force is perpendicular to velocity, it does no work, so speed (and hence momentum magnitude and kinetic energy) remains unchanged.