Re: [Phys-l] work by magnetic fields

[John Denker <jsd@av8n.com>]

On 04/28/2011 10:13 AM, Philip Keller wrote:
> 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.

Answer #1 is not wrong.  However, in context, answer #2 would
be preferred because it is more complete.  Anybody who wanted
the simpler answer wouldn't be asking the question.

To anticipate the obvious follow-up question:  The theorem in
question applies to the motion of a charged particle in a magnetic
field.  It is a useful theorem /when it applies/.  In the present
situation, it fails to apply, because there is a whole lot more 
going on, not just a magnetic field.

Note that there are even simpler situations where the theorem
fails to apply, notably a magnetic field acting on the built-in
magnetic dipole of an electron, nucleon, et cetera.