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Re: [Phys-l] Conceptual question about magnetic fields and work



Philip Keller wrote:
This is a discussion question from "University Physics" (Sears and Zemansky):

"Two parallel conductors carrying current in the same direction attract each other. If they are permitted to move toward each other, the forces of attraction do work. From where does the energy come? Does this contradict the assertion that magnetic forces on moving charges do no work?"



Here is what the solution guide said:

"The energy comes from the electrical energy of the currents, which in turn comes from the emf that drives the current through the conductors. There is no contradiction. The magnetic force is perpendicular to the direction of the drift velocity of the current-carrying electrons in the conductor so doesn't affect the magnitude of the drift velocity. The motion of the wires is perpendicular to the motion of the current carrying charges in the wires. It is the motion of the wires, in the direction of the magnetic force, that causes work to be done by this force."

A similar question also came up in my class -- essentially, in a linear motor, if the force on the wire accelerates the bar, didn't it do work? Since I am up against the approaching AP exam, for now I have provided an answer that I am not happy with: that the rule which says magnetic fields do no work applies to individual charges, but not charges in a current-carrying wire.

I suspect that the work is done by the electric force that holds the electrons in the wire as the wire moves. I don't understand the solution guide.

Any help would be appreciated.
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As conductor moves due to the force of the magnetic field, the induced emf in the conductor opposes the current in the conductor and the source of emf has to do work.
Force on a length l of the conductor is Bil and rate of work done by the magnetic field is (Bil)v. The emf in the wire is Bvl and the rate of work done by the cell is (Bvl)i
which are equal. (Magnetic field can not be seen in isolation and the electric field induced in the wire has to be accounted for)

Surendranath