As a charged particle enters a uniform magnetic field B, its path is bent into a circle whose radius r is proportional to its linear momentum, mass times velocity (mU). The particle’s velocity does not change, so its kinetic energy is unchanged, and the field does no work on the particle. This is analogous to gravity’s exerting a continuous centripetal force on an orbiting satellite in space. THe magnetic field direction is shown by a blue axial line; particle entry angle by a green radial line.