Re: ORIGINAL simple magnets question

[William Beaty <billb@ESKIMO.COM>]

On Fri, 25 Jun 1999, Michael Edmiston wrote:

> I think I am beginning to understand William Beaty's original question.
>  Let me state it in a way that makes sense to me, and then William can
> tell me if I understand him correctly.

Yes, that's it exactly.  I would also add: give those magnets a large
diameter, so the reference frame becomes *almost* intertial, and also keep
the electron's velocity small (non-relativistic) so we perhaps could build
such a spinning magnet in the real world.

> For the explanation of the problem, let's assume we are observers in
> the lab frame.
>
> (1) Let's take a very homogenous magnetic field, perhaps that between
> the pole faces of an NMR magnet.
>
> (2) Let's trap an electron (or other ion) in a cyclotron orbit inside
> the magnet's gap.  This is something that can be done experimentally.
>  People have trapped charges in cyclotron orbits for long times.
>
> (3) Once the charge is trapped in it's orbit, lets begin rotating the
> magnet about the same axis and in the same rotational direction as the
> cyclotron orbit.
>
> An intuitive view would be that the relative motion between the magnet
> and charge has changed.

Exactly.  In earlier messages when I said "relative to what?", I was
hoping the answer might be "electron motion relative to the iron atoms
which are creating the magnet's b-field in the first place."  Maybe I was
mistaken, but I thought that people were answering: "relative to the
stationary b-field of the non-spinning magnet."  But does "stationary
b-field" have meaning?  The problem is solved if the answer involves the
atoms' relative motion.

> In fact, if the magnet rotation could be
> instantly brought to the same angular velocity as the charge's original
> cyclotron orbit, we would say the relative motion between the magnet
> and charge has ceased.

Right, and if the electron happened to initally have a tangential velocity
of 1 meter per second, then it might be physically possible to rotate the
entire magnet and observe what actually happens to the trapped electrons.
Do they see a transverse e-field and therefor alter their circular orbit?
Or do they ignore all rotary motions of the magnet?  (Separate question:
if we *suddenly* start rotating the magnet, what will the electrons think
of the acceleration^2 of the dipoles in the magnet's surface adjacent to
the electron? )


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