Re: Why does electrostatic attraction in water decrease?

[Bob Sciamanda <trebor@VELOCITY.NET>]

Panofsky is here adding the caveat that he has calculated the
electrostatic energy changes on the assumption that the dielectric
constant k is fixed in time. Since k may be a function of temerature, and
applying the E field may heat the material, one would have to impose a
constant temperature resevoir to enforce this condition.  His calculation
of the energy change (6-13)  would not then include these heat transfers
with the resevoir and therefore would not be the TOTAL energy change, only
the electrostatic effects (the item of interest in his discussion).

I don't see how this affects your problem.  Whatever you want to make of
this, it equally affects the two effects which you want to distinquish
(pressure increments vs charge interactions)  They both come from the same
energy balance considerations. Panofsky is merely pointing out that any
heat exchange, imposed to maintain a constant k, may add some other
energetics, not included in his result.

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor

----- Original Message -----
From: "Pentcho Valev" <pvalev@BAS.BG>
To: <PHYS-L@lists.nau.edu>
Sent: Friday, April 26, 2002 6:03 AM
Subject: Re: Why does electrostatic attraction in water decrease?


Bob Sciamanda wrote:

> Read the material in Panofsky which precedes the section you are quoting
> (at least in my [first] edition copy).  He deduces the pressure effect
> from energy considerations which depend on polarization.  His
conclusions
> are nicely summarized in terms of the Maxwell stress tensor.  Bottom
line:
> the two effects which you distinguish (pressure increments vs charge
> interactions) are the same.  Panofsky stresses, more than once, that it
> all reduces to Coulomb's law.

I read it carefully but bumped into a serious problem. Panofsky does
indeed
stress that it all reduces to Coulomb law, but, on the other hand,
recognizes
the existence of heat effects: "Thus we cannot equate the increment of
work
done, dW of eq. (6-13), to the increase in total energy, since HEAT
TRANSFERS
ARE ALSO INVOLVED" (p. 100). . . .