A magnetic field deflects an electron beam, but it can not do any work on the beam. Why?
It does not change the kinetic energy, only the direction.
The gotcha is that kinetic energy is frame dependent, but this feature is probably not of primary interest to you at this point.
Another way of saying what turin just said is that the magnetic force is always perpendicular to the electron's velocity: No work is done since there is no displacement in the direction of the force.
thank you for the help. :)
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