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



On 04/30/2009 07:48 AM, 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?"

A magnetic field is like a conventional sliding constraint,
in the sense that the force is perpendicular to the motion (*)
and therefore the constraint cannot do any work (*)

(*) Provided the constraint doesn't move!

If you have a moving constraint, all bets are off. To my
mind, at this level of detail, that's all that need be said.
It's not really a magnetic concept; it's just a matter of
remembering the proviso about the non-moving constraint.

====================

If for whatever reason you do want to delve into the conceptual
foundations concerning the magnetic field of a current-carrying
wire, here is my explanation, with pictures:
http://www.av8n.com/physics/magnet-relativity.htm

It is related to the note I wrote earlier today in another thread,
insofar as it involves "being at home in four dimensions".

The notion of "field lines" is partly helpful and partly unhelpful
in three dimensions. The corresponding four-dimensional objects
are much better behaved.