Re: simple magnets question

[William Beaty <billb@ESKIMO.COM>]

On Wed, 23 Jun 1999, John Mallinckrodt wrote:

> On Wed, 23 Jun 1999, William Beaty wrote:
> > ... when an electron moves parallel to the face of a very large, flat
> > magnet where the direction of the field is perpendicular to the face.
> > The electron should "see" the relative motion of the field, and so be
> > deflected perpendicular to this motion and parallel to the magnet face,
> > resulting in a circular path.
>
> All an electron ever "sees" (in the sense of reacting to by virtue of its
> charge) are electric fields.  It is the observer who may see an electron
> "moving through" and reacting to a magnetic field.

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

To "compress" what I asked before:

  "Since an electron which moves relative to a large, flat magnet pole
  will see a perpendicular e-field, will the electron still see that
  perpendicular e-field if we reverse the situation so the electron stays
  still and the magnet moves? If so, then what happens to the shape of
  that e-field if the magnet rotates rather than moving in a straight
  line?"

If the motion is linear, I assume that it doesn't matter if the magnet or
the electron does the moving.  We'd still get a circular electron
trajectory.  Or an elliptical trajectory if the observer was moving along
with the electron.  In either case, the electron responds to the uniform
magnetic field, yet it does not encounter any changes in the field
intensity.


> > ... Relative to the electron, a moving field is not the same as a
> > stationary field.
>
> But what does it mean for a field to move?  Hannes Alfven (whose namesake
> magnetohydrodynamic waves have often been likened to the "plucking" of
> magnetic field lines) himself, warned against the danger of imagining
> fields to "move" rather than "change with time."

The motion between an electron and a uniform b-field; isn't it relative?
If so, then "moving field" is a meaningful concept.
If a moving electron is deflected sideways when it encounters a
"stationary" magnetic field, then a stationary electron must be deflected
sideways when it encounters stationary magnetic field.

I'm not talking about the effects of non-uniform fields: imagine that this
field is between two flat, closely-spaced pole pieces, like in a
cyclotron.  Shoot an electron between the poles of a cyclotron, and it
flys in a circle. Or, hold the electron still, then throw the cyclotron at
it (so that the electron goes between the pole faces).  From the fiewpoint
of a stationary observer, won't the "moving field" within the flying
cyclotron create a perpendicular e-field which deflects the stationary
electron?  If so, then a linearly-moving magnet can affect stationary
electrons, even if the electron doesn't see any changes in field
intensity.

Another way to look at it: an electron skims across the surface of an
infinitely wide, flat magnet pole.  The magnetic field is perpendicular to
the surface of the magnet.  The electron will be deflected in a circle
because there is *relative motion* between the electron and the magnetic
field, even though the electron experiences no changes in field intensity.
In other words, if motion is relative, then "field motion"  has just as
much meaning as "electron motion."



> I think these same difficulties (which ultimately bear on issues related
> to Mach's principle and the existence of absolute rotational as opposed to
> translational velocity) lurk behind the fantastic claims of the late Bruce
> DePalma for his "N-1 Homopolar Generator."

Yep.  And they relate to ongoing controversies about the physics behind
Faraday's Disk.  Long ago I drew the following .gif while thinking about
Homopolar generators.  It illustrates the same question I'm asking: does
an electron behave differently around a spinning magnet?  I think it
should!

  http://www.eskimo.com/~billb/freenrg/N-FIELD2.GIF

I suspect that any experiments would not be simple, since an electron beam
in a vacuum has a very high velocity which would swamp out any
effects from a spinning magnet.  If we could produce electron beams with
the particles moving at tens of meters per second, then perhaps they would
respond differently (or not) when a disk-magnet is made to spin.


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