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, June 25, 1999 5:43 PM
Subject: "Faraday's Disk" which started it all


> For those who haven't encountered it before, the "simple magnets question"
> stems from a real-world experiment called "Faraday's Disk."
>
> 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.  . . .
> William J. Beaty                                  SCIENCE HOBBYIST website

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)    [:)<

-Bob

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