A magnet can exert a force on a moving charged particle, but it cannot do work on it (changing its kinetic energy). Why not?...

Question

A magnet can exert a force on a moving charged particle, but it cannot do work on it (changing its kinetic energy). Why not? (a diagram might help).

Answer 1

If a force does not displace a body in the direction of the force, it does no work on the body.  The magnetic force is always perpendicular to the velocity vector, hence it does no work.  It only deflects the moving charge.  

 

Recall 

 

F = qv x B

 

so F is perpendicular to both v and B, meaning there is no projection of magnetic force along the direction of motion.