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

[Bob Sciamanda <trebor@VELOCITY.NET>]

Hi Bill,

----- Original Message -----
From: William Beaty <billb@ESKIMO.COM>
To: <PHYS-L@LISTS.NAU.EDU>
Sent: Sunday, June 27, 1999 3:33 PM
Subject: Re: "Faraday's Disk" which started it all


> On Fri, 25 Jun 1999, Bob Sciamanda wrote:
>
> > billb wrote:
> > > If a copper disk is spun on axis and immersed in a uniform b-field
> > > perpendicular to the face of the copper disk, and if the leads to
a
> > > standard voltmeter are touched to the center of the spinning disk
and also
> > > slid along its rim, the meter will measure a real potential
difference.
> > > According to the voltmeter, the "radial e-field" is real.  . . .
> >
> > The EMF mechanism here is the QVxB "generator principle" force, not
an E
> > field (I speak in YOUR lab frame).  As in any circuit, surface
charges will
> > build up and produce an E field to drive the current around the
external
> > circuit, but the EMF force acting inside the armature is the QVxB
force.
> > (The E field of the surface charges will of course also exist inside
the
> > armature, but this is a "secondary" effect, and in fact will oppose
the
> > current - all exactly as in a "standard" armature generator)  There
is no
> > "mystery" here (or there)    [:)<
>
> Aha, then I've found a big fuzzy spot in my understanding. . .

Aha, this is why I emphasized: "I speak in YOUR lab frame."

> . . . It was my
> understanding that VxB is itself an e-field: it is the same as the
> relativistic e-field that an electron sees as it flys between the
poles of
> a cyclotron magnet.  Wrong?

This (YOUR) language is not speaking from YOUR lab frame, but from a
frame attached to an anthropomorphized electron!  The lab observer
measures no such E field.  Obfuscation and confusion result from a
description which leaps from frame to frame!  Physics is designed to
describe reality from one frame at a time.  Lose this maddening habit!

> If we build a spinning disk-magnet device and sit it on the lab bench,
and
> the magnet is ceramic (nonconductor), then in the lab frame won't a
nearby
> test-charge "see" an e-field caused by the spinning magnet?  At the
> surface of the spinning disk-magnet, won't this e-field be radial?


A magnetic volume polarization M will transform into a different M' and
an electrical volume polarization P', in a different frame.  If in one
frame the disk has only an M, in the rotating frame it will have an M'
and a P', producing a radial internal E' field.  It will look like a
charged cylindrical capacitor/magnet. The EXTERNAL E' will be weak, due
only to fringing.

Forget the second disk; put brushes on the spinning magnet itself, and
you will have a generator (now using a conducting magnet).  In the lab
frame, the EMF is provided by the motional, magnetic QVxB force, inside
the magnet.  In the rotating frame the EMF is caused by the radial QE
electric force, inside the magnet.

> If we place a metal plate close to the spinning magnet and parallel to
its
> face, won't this radial e-field cause the charges of the metal plate
to
> redistribute themselves until they produce a cancelling e-field and
thus
> cease their motion?

Yes, a small effect of the weak fringe E field.
> If we reverse the situation and spin the metal plate while keeping the
> magnet still, won't the charges in the metal plate STILL redistribute
> themselves as above?  (Here neglecting any Tolman-force motions caused
by
> centripetal acceleration of mobile electrons in the metal.)

Yes, a potentially large, motional QVxB magnetic force.

> William J. Beaty                                  SCIENCE HOBBYIST
website

-Bob

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