Re: simple magnets question

[brian whatcott <inet@INTELLISYS.NET>]

At 13:20 6/25/99 -0700, Bill wrote:

> The thread "simple magnets question" started as below:
>
> ------
> Date: Wed, 23 Jun 1999 12:05:17 -0700 (PDT)
> From: William Beaty <billb@eskimo.com>
> To: Forum for Physics Educators <PHYS-L@LISTS.NAU.EDU>
> Subject: simple magnets question
>
>
> I was recently reminded of something that's been bugging me.  When an
> electron moves at right angles to a uniform b-field in a vacuum, it
> follows a circular path.  What happens if we turn this situation
> inside-out? Suppose we place an electron in the field of a thin
> disk-shaped permanent magnet which has poles on the opposite faces of the
> disk, and then we briefly rotate the disk on axis.  Assume that everything
> is stationary at the beginning, then reach out and give the disk-magnet a
> twist.  What does the electron do?
>
> Or if the sudden change in motion is a problem, then instead what will
> happen if an electron approaches a disk-magnet which is already spinning?

> William J. Beaty


Ahhh, Bill is having so much fun with this mind experiment,
it must be time for me to introduce another physical mechanism that
reminds me of his setup.
   It's called a mechanical speedometer.

We might want to think of it as an electrodynamic coupling.
There's a ring magnet facing an aluminum disk.
 The magnet rotates.
The disk follows, restrained by an opposing torque from a hair spring.
This disk is attached to a pointer. This indicates speed.

Conversely:
If the magnet is stationary but the conductive metal disk moves,
that's called an eddy current damper of the kind you will see on your
Ohaus triple beam balance (which nobody cares to discard despite the
attractions of electronic scales...)

Back to the speedo:
Now, I need to move to a higher level of abstraction, where physicists
perhaps feel more at home   :-), by taking a particle of this metallic
disk, so small that it includes just one mobile electron.

   When the magnetic disk moves (like when you move off in your car)
the electron proceeds in a macroscopic circle, providing an opposing
magnetic field to that of the magnet, in my frame.

 Well, waddaya know?    :-)

What I have said describes the speedo quite well (I believe),
but I offer no guarantee of its magnetic field homegeneity.

I suppose that this uniformity is at the heart of the debate?
  -Possibly, but at some point we WERE using a horseshoe magnet
spinning on an axis through its middle,  between its poles.

This WILL drag a nearby metallic disk into rotation,
I am sure you will agree. And that implies a charge carrier
 describing a circle, and thereby providing an opposing
magnetic field.

(Sorry if relativity, cyclotrons, current sheets, vacuum,
Gauss, Maxwell an' all that good stuff do not figure in
my demonstration.      Around here we have speedos.
You might obtain one from a salvage yard...)
brian whatcott <inet@intellisys.net>
Altus OK