20.3.1—The direction of the force on a charge moving in a magnetic field

For charge q moving at speed v through field B, magnetic force magnitude is F=Bqv sinθ and direction is perpendicular to v and B.

Use the right-hand rule for positive charges and reverse the direction for negative charges. A stationary charge feels no magnetic force.

A charged particle entering perpendicular to a uniform field follows circular motion because the force continually turns its velocity.

Magnetic force does no work on an isolated moving charge because it is perpendicular to velocity, so speed stays constant.