Understanding Work Done in a Bfield on an Electron

[chiddler]

Homework Statement

Magnetic forces do no work because all the force is perpendicular to movement.

A bar is moving in a Bfield. "How much work is done on an electron moving across the bar?"

Why is there work in this case?

Homework Equations

F = qvBsin(θ)
W = Fd = qvBd*sin(θ)

The Attempt at a Solution

Thanks very much.

 

[chiddler]

My friend provided a helpful response:

Because the particle is constrained to the bar. Magnetic forces do no work on particles because the force on the particle is always perpendicular to the field. When the particle is trapped in a bar and is forced to move in one direction only, the magnetic field creates a constant force in one direction only, that means it DOES create work. Think of it this way, in a spherical field, a particle will always be pushed away from the curve of the field itself, meaning its always a centripetal force, meaning its always perpendicular to the movement of the particle. Now if you restrain that particle into a bar, the force may come at various angles to it, but now the particle can't escape and thus moves along the bar path, meaning the forces start to apply some work.

:-3