Re: "Faraday's Disk" which started it all

[Bob Sciamanda <trebor@VELOCITY.NET>]

----- Original Message -----
From: William Beaty <billb@ESKIMO.COM>
To: <PHYS-L@LISTS.NAU.EDU>
Sent: Friday, July 02, 1999 3:31 PM
Subject: Re: "Faraday's Disk" which started it all
> . . .
> I'm still leery of the the rotating motion that each small bit of the
> donut-magnet experiences, but perhaps this doesn't produce any EM effects.
I believe each magnetic bit m acquires an electrical polarization p.  But,
in the rotational situation the numerous little vectors p are so arranged
that the net P presented to the outside world is zero.  This is due to an
ORDERED, cylindrical arrangement; but the external result might be compared
(and contrasted) to a ferromagnet above its Curie point, where the zero net
external effect (magnetic in this case) is due to a RANDOM arrangement.

> If the donut-magnet is not a thin shell, then the tangential speed of the
> inner rim will be smaller than the tangential speed on the outer rim.  As
> a result, wouldn't the polarization give some bound layers of charge where
> the charge on the inner rim is *less* than the charge on the outer rim?
> Suddenly I see an obvious way out of this problem.  The mismatched charges
> might simply mean that the charge on the outer rim is balanced by opposite
> charges distributed throughout the rest of the magnet, rather than a layer
> of opposite charge existing exclusively on the surface of the inner rim.

I'm not sure I follow these interesting speculations, but let me repeat the
substance of a discussion we had on an earlier thread.  These "surface
layers of bound charge" are calculational artifices invented to ease the
evaluation of the field/potential produced by a volume distribution of
dipole polarization.  They not only ease calculations, they also allow an
attractive EQUIVALENT conceptual model.  But it should be remembered that
they arose as a clever mathematical re-arrangement of the result of
evaluating the field/potential of a volume distribution of dipoles, thereby
expressing the results as the field/potential of an equivalent monopole
charge distribution. Any models upon which you may speculate to give the
same result may be equally useful - the actual volume distribution of
polarization in this rotational case may be equally served by a myriad of
"equivalent monopole charge distributions".

> William J. Beaty                                  SCIENCE HOBBYIST website

-Bob

Bob Sciamanda
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor