I know I have asked this in the past, but I still don't understand it and my AP students have asked again so...
This is about what some books call a linear motor: a long, u-shaped conducting rail with a conducting bar resting on the rails, free to slide without friction, completing the circuit. There is a battery in the circuit and a magnetic field into the plane of the setup. The battery causes a current to flow and the magnetic field exerts a force on the bar which causes the bar to accelerate (at first -- the induced back emf then causes the current to drop and the bar attains a terminal velocity, but that is not the issue at hand).
The question is: while the bar is accelerating, what force is doing the work that increases the bar's kinetic energy?
Possible answer #1: The magnetic force, F=ILB, that is exerted by the field on the current-carrying bar. That force is in the same direction as the bar's displacement. There is no rule that says magnetic fields can never do work.
Possible answer #2: The magnetic force is not the one that does the work. That force acts on the individual charge carriers in the bar and it is always perpendicular to their motion so it can never do work. But the individual charge carriers are not free to follow the circular orbits that the magnetic field would otherwise cause. They are held by electrostatic forces exerted by the kernels of the lattice atoms. It is that electrostatic force that ultimately does the work.