Re: volume polarization vs. surface charge

[Bob Sciamanda <trebor@VELOCITY.NET>]

-----Original Message-----
From: Donald E. Simanek <dsimanek@EAGLE.LHUP.EDU>
To: PHYS-L@LISTS.NAU.EDU <PHYS-L@LISTS.NAU.EDU>
Date: Thursday, March 04, 1999 12:47 PM
Subject: Re: volume polarization vs. surface charge


> . . . and these real charges are the ones which are relatively mobile,
they
> can be added or removed from the insulator, especially if on the surface
> of it. While the induced charge, due to a field polarizing the dielectric,
> acts as a surface charge, but is bound, and not free to migrate to the
> conducting plates of a capacitor and become current, at least not with the
> size fields of this demo. You'd have to break up molecules to do that.  I
> think that the distinction between induced charge resulting from
> polarization of the dielectric, and the excess charges (mobile, loosely
> bound) was not clearly brought out in our discussion of the dissectable
> capacitor demo.
> . . .
> Donald E. Simanek
> dsimanek@eagle.lhup.edu                 http://www.lhup.edu/~dsimanek


Donald has made the crucial, clarifying point; let me just expand a little:

A neutral but polarized (either permanent or induced by an applied field)
dielectric produces a weak 1/r^3 dipole field.  This dipole field is
numerically equivalent to the 1/r^2 field produced by a fictitious volume
density of charge equal to (minus) the divergence of the polarization volume
density;  this is the mathematical origin of the so-called "bound" charges
in and on a polarized dielectric.  These fictitious charges are introduced
only as a convenience for calculation.  The neutral dielectric produces no
monopole field, but its dipole field is calculationally equivalent to the
monopole field of this distribution of fictitious bound charges.

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