Re: simple magnets question

[William Beaty <billb@ESKIMO.COM>]

On Wed, 23 Jun 1999, paul o johnson wrote:

> If I am correctly visualizing your magnetic disk, Bill, I believe it
> would have a simple magnetic dipole field around it. Assuming uniform
> magnetization of the disk, the field would be the same whether the disk
> was stationary or spinning on axis.

That was my first guess, but then I considered what happens in a similar
situation: 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. Obviously this applies to the pole faces in
a Cylotron.  However, in this case the electron's motion is a relative
motion between the electron and the uniform field.  Relative to the
electron, a moving field is not the same as a stationary field.  Therefor
I imagine that if the electron was initially stationary and the magnet was
moving in a straight line, then the electron would experience a force
perpendicular to the magnet's motion.

So, if that same large, flat magnet is rotated rather than moved linearly,
won't it create a force on any stationary electrons which happen to be
hovering near the magnet's surface?  As the magnet rotates, I'd expect
that an electron would see the local fields moving almost in a straight
line, and so it would experience a force directed across the magnet face,
pointing either towards or away from the magnet's axis of rotation.

I've never encountered this thought-experiment before, and I'm wondering
if a rotating disk-magnet *can* attract a cloud of stationary electrons
towards a line running through its axis.

> Out of this plane, the field has a radial component so any axial
> component of velocity would cause a tangential force, accelerating the
> particle in its ecliptic path.

To simplify things, we could assume that the electron was very close to
the face of a very large magnet, where the field lines would be
approximately parallel to each other and perpendicular to the magnet face.



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