Re: induced emf again

[Bob Sciamanda <trebor@VELOCITY.NET>]

There is an important difference between the magnetic dipole (eg a bar
magnet or a current loop) and an electric dipole.  In the magnetic case
the field lines are indeed endless - no beginning or end.  In the electric
case the lines begin on positive, and end on negative, charges.  Inside
the dipole, the field lines reverse direction, always going from positive
charge to negative charge.

The field lines in Ludwiks case are like the electric dipole.  They
terminate on negative surface charges and start anew in the opposite
direction inside the moving wire.

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor
----- Original Message -----
From: "William Beaty" <billb@ESKIMO.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Tuesday, April 30, 2002 12:32 PM
Subject: Re: induced emf again


On Sat, 2 Jan 1904, kowalskil wrote:

> In other
> words the rod is treated as a battery causing a
> current in the conducting loop.
>
> Is this derivation desirable?

It's a simplified model, so we'd need to describe its limits.

I think the model is analogous to the model of the "poles" of a permanent
magnet.  We know that the b-field of a bar magnet is endless loops (no
monopoles.)  The sliding bar is the electrostatic version of a bar magnet:
it seems to have positive and negative poles, and if we treat these poles
as real, it simplifies the analysis.  But in reality the V x B "field" is
made of endless loops, and our path of integration passes through the
sliding bar.  Also, if we move our reference frame along with the bar,
then the positive and negative "poles" will originate with the moving
rails.

The presence of mobile charge within the sliding bar obscures things.  As
you say, to see the motional e-field, remove the rails (or replace the bar
and the rails with insulating materials.)


(((((((((((((((((( ( (  (   (    (O)    )   )  ) ) )))))))))))))))))))
William J. Beaty                            SCIENCE HOBBYIST website
billb@eskimo.com                            http://amasci.com
EE/programmer/sci-exhibits   science projects, tesla, weird science
Seattle, WA  206-789-0775    sciclub-list freenrg-L vortex-L webhead-L