Re: Why does electrostatic attraction in water decrease?

[Bob Sciamanda <trebor@VELOCITY.NET>]

Panofsky is using the simple parallel plate capacitor as a paradigm.
Experimentally the net effect of immersing charges in a dielectric is
always to simply change the dielectric constant in Coulomb's law, willy
nilly the geometry.  Panofsky seeks the "cause" of this "universal" effect
by considering a simple (calculable) case.  Non-linear dielectrics and
bizarre situations would add other considerations, but he is uncovering
the role of the electrostriction effect on the liquid pressure.

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor
----- Original Message -----
From: "John S. Denker" <jsd@MONMOUTH.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Wednesday, April 24, 2002 3:03 PM
Subject: Re: Why does electrostatic attraction in water decrease?


Bob Sciamanda wrote:
> ... Panofsky says that the effects of the dielectric field (modelled
> by the fields of equal and opposite bound surface charges) on each
> capacitor plate cancel.

That's true in the narrow context of a parallel-plate
capacitor (i.e. uniform field).  But here we're talking
about non-uniform fields, in which case the effect of
the induced charges certainly doesn't cancel.

To see this, consider the force on a dielectric in
the inhomogeneous near a point charge.  I would think
the dielectric causes an equal-and-opposite reaction
(force) on the point charge, wouldn't you?  Conservation
of momentum and all that.