Re: simple magnets question

[Chuck Britton <britton@ODIE.NCSSM.EDU>]

Let's see if this model might help things along. We all appreciated Leigh's
infinite current sheet, uniform B field. Feynmann has a similar discussion
on the E and/or B field(s) around a straight conductor. The nub of the
arguement seems to be the 'charge neutrality' of the wire in 'our' rest
frame. When the reference frame is moving (perhaps at the drift velocity)
then the 'background' charges are contracted into a higher charge density
which produces the E field felt by the 'stationary' external charge.

Let's see if we can create a similar model for Bill's cyclotron pole piece.
I certainly won't go through the details of the charge distributions, but
maybe SOMEBODY will.

Let's say that the pole piece is actually made of two equal sized disks.
One covered with an (unknown) positive charge distribution and the other
with an identical. but negative, distribution.

When the two disks are are at rest relative to each other we have the
required charge neutrality. If we spin the positive disk while the other
disk remains stationary (or vice versa)we generate a B field with the
circulating charges on the MOVING disk. OK?)

If we 'ride the rim' of the rotating disk we will see the OTHER disk as
contracted (locally) and it must be this contraction of the 'background'
disk that produces the required E field. (I'm sure there may be some land
mines strategically placed here, but I'm not going beyond the locallity of
GR)

> The problem here arises from your model. I don't know anything about
> tiny close packed dipole magnets; the only sources of which I am aware
> are electric charges, and I believe that the question must be answered
> using a model that involves only electric charges. A simple amperean
> current won't suffice because you've brought in edge effects and those
> can only be dealt with numerically. The argument I used for the
> infinite current sheet won't translate to the spinning magnet even far
> from the edges; I need a different, more complicated model.
>
> The very fact that you recognize the electric field you envision as
> being "very weird" should tell you that there is something wrong with
> your analysis. It could, perhaps, be turned into a *reductio ad
> absurdum* demonstration of the proposition that the motion of the
> spinning magnet can be ignored.
>
> Leigh

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Chuck Britton                                   Education is what is left when
britton@odie.ncssm.edu                          you have forgotten everything
North Carolina School of Science & Math         you learned in school.
(919) 286-3366  x224                            Albert Einstein,        1936