Re: simple magnets question

[Leigh Palmer <palmer@SFU.CA>]

Bill Beaty says:

> I think this might not work, because I want to apply the concepts to a
> spinning disk-magnet in the real world, and an infinite current-sheet
> won't fit.  Therefor I'm trying to imagine what goes on near the surface
> of a very large, very thin, disk-shaped permanent magnet which spins on
> axis.

I understand; I was just reading your last message. My point is that I
can't analyze the problem you pose, or at least I can't analyze it
easily (and my consultancy fee you can't afford!) in terms of steady
sources. It's shmutzig, as I said, and I want to make sure that the
simpler problem is understood by others in the discussion. The
existence of an electric field in a moving frame is very often hard
for a student to accept, and I believe my argument makes it more
acceptable.

It is my belief that the detailed source of a uniform field is not
important. As I indicated, variations in the parameters of my model do
not propagate through to the final result. That this is true for a
rotating magnet I can't defend; but that's what Maxwell's equations
say, and I believe them.

> Perhaps the disk-magnet could instead be modeled as an array of many tiny,
> close-packed dipole magnets, with their N poles on one face of the planar
> array, and their S poles on the other.  What happens when such a magnet
> moves relative to an electron?  Or when it *spins*?

The problem here arises from your model. I don't know anything about
tiny close packed dipole magnets; the only sources of which I am aware
are electric charges, and I believe that the question must be answered
using a model that involves only electric charges. A simple amperean
current won't suffice because you've brought in edge effects and those
can only be dealt with numerically. The argument I used for the
infinite current sheet won't translate to the spinning magnet even far
from the edges; I need a different, more complicated model.

The very fact that you recognize the electric field you envision as
being "very weird" should tell you that there is something wrong with
your analysis. It could, perhaps, be turned into a *reductio ad
absurdum* demonstration of the proposition that the motion of the
spinning magnet can be ignored.

Leigh