Re: Non-conservative forces in a liquid dielectric

[Pentcho Valev <pvalev@BAS.BG>]

Robert Cohen wrote:

> On Wednesday, May 14, 2003 3:49 AM, Pentcho Valev wrote:
>
> > [snip]
> > When two opposite
> > charges (e.g. the plates of a capacitor) are immersed in=20
> > water, why does the force of
> > attraction between them decreases 80 times?
>
> I'd like to consider a different situation.  Suppose we have a
> parallel-plate capacitor, separated by a vacuum, with plate
> separation D and capacitance C.
>
>  -Q|      |+Q
> ---|      |---
>    |      |
>
> What happens if we insert into the vacuum a piece of
> something as follows:
>
>  -Q| |  | |+Q
> ---| |--| |---
>    | |  | |
>
> such that we now have two capacitors in series, each with
> plate separation D/3?
>
> Does the force of attraction between the -Q and +Q change?
> If so, in what way?
>
> Does the capacitance increase to 1.5 what it was before?
> Is this equivalent to a "dielectric constant" of 1.5?
>
> > [snip]
> > If we=20
> > imagine an analogous
> > situation in which all participants are macroscopic (e.g. a=20
> > capacitor plus some
> > macroscopic "dipoles" between the plates able to rotate and=20
> > "polarize") we see that
> > polarization, per se, must increase rather than decrease the=20
> > attraction. In other words,
> > electrical forces can by no means be held uniquely=20
> > responsible for the effect.
>
> In my case, if we imagine that the charges within my "insert"
> separate so that we have
>
>     +Q  -Q
>  -Q| |  | |+Q
> ---| |--| |---
>    | |  | |
>
> we see that the "polarization" of the insert must increase
> the attraction rather than decrease the attraction.

I agree. Panofsky and Phillips discuss the same case on p. 115 and
conclude that the attraction between the plates remains as in vacuum but I
disagree. Another interesting thought experiment is one in which we insert
a box filled with water between the plates. Again, due to polarization,
the attraction will INCREASE.

> Does this mean that electrical forces can by no means be held
> uniquely responsible for the effect?
>
> What am I missing?

Nothing except perhaps Panofsky's pressure that pushes the plates apart
and does work at the expense of heat absorbed from the surroundings. In my
view, consideration of this effect will change radically large parts of
theoretical physics and physics education. It is a shame physicists have
not considered it so far.

Pentcho