Re: Gauss' law and displacement current

[Bob Sciamanda <trebor@VELOCITY.NET>]

In the general case static charges arise in order to direct the E field (and
so the current) along the conducting path.  If the original field caused by
the emf already does this, there will be no need for such charge
accumulations.  I think John is describing one such special case.

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
www.velocity.net/~trebor
----- Original Message -----
From: "John S. Denker" <jsd@MONMOUTH.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Monday, April 23, 2001 12:26 PM
Subject: Re: Gauss' law and displacement current


> At 12:08 PM 4/23/01 -0400, Eugene Mosca wrote:
> > > > For example, the field inside the
> > > > current carrying wire is sourced by surface charges on the wire.
> > >
> > > Is this true even for portions of a very long, straight wire well
> > > away from any corners?
> >
> > In a word, yes.  This is discussed in Chapter 6 of Chabay and Sherwood's
> > "Electric & Magnetic Interactions," Wiley (1995).
>
>
> I don't have that reference at hand, but I find the result to be highly
> implausible.
>
> In particular, consider a large _loop_ of wire.  Run some flux through the
> middle of the loop.  (Consider the wire to be a one-turn short-circuit
> secondary on a transformer if you like, and ramp up the current in the
> primary.)
>
> Then there will be a current in the wire.  By symmetry, it is exceedingly
> hard to see how the field could be "sourced" by surface charges (or any
> other kind of charges).
>
> Make the loop polygonal (piecewise straight) if straightness is important
> to you.  The same argument applies.