Re: simple magnets question

[Leigh Palmer <palmer@SFU.CA>]

> Yes, so the question becomes: "does an axially-spinning disk magnet create
> a motional e-field which can attract/repel a stationary charged particle?"

I guess the answer is, simply, no, it does not. If the field and its
source are axially symmetric then spinning the source about its axis
will affect nothing outside. As John Mallinckrodt emphasizes, one
should never think of a field (electric or magnetic) as "moving".
That picture can yield no result differing from the result obtained
by considering a time varying field, and it can frequently lead one
to erroneous conclusions such as the one you recognized yourself.

I find that it is frequently illuminating in such questions to
consider the interactions directly in terms of the sources. Given
the fact that there exist no monopoles, the only sources possible
are the elementary electric charges which make it up. If the
disc magnet is spun, the only source that can change is the current
source. If the magnet is electrically neutral and nearly homogeneous
then the resulting currents will also vanish, and with them any
magnetic fields which might have otherwise resulted. Even if the
magnet bears a net charge or is inhomogeneous with respect to charge
density, the resulting magnetic fields will, indeed, produce a
magnetic field differing from that of the stationary magnet. However
even in that case there would be no net force acting on a stationary
charge in the field. In this latter case, however, there could be a
net force acting on the charge *during angular acceleration* of the
disc.

Consideration of source interaction is, I find, an excellent way to
derive the relativistic electromagnetic field transforms.

Leigh