Re: Why does electrostatic attraction in water decrease?

[Pentcho Valev <pvalev@BAS.BG>]

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). Heat effects are an unmistakable sign that
non-conservative forces akin to gas pressure are involved. One may assess
their influence by repeating the initial procedure of assembling the charges
(moving them from infinity to their places in the system) on the assumption
that there is an additional ideal gas subsystem that expands or compresses in
the process. With respect to the gas subsystem, there is certainly heat
exchange but there is also WORK different from the work done against Coulomb
forces. Panofsky is able to deal with the heat - he introduces the (Helmholtz)
free energy that successfully eliminates the heat effects. However he is not
able to eliminate the non-conservative work and makes the following wrong
statement: "This means that in the presence of dielectrics the expression U =
(1/2) INT (E.D)dv  cannot be identified with the total energy of the system,
but can only be identified with the thermodynamic free energy" (p. 100). In
fact, the change in free energy is given by

(1/2) INT (E.D)dv  +  Wgas

where Wgas is work done on the gas subsystem.

I implicitly assumed that the gas subsystem is both closed and inert, but
generally this may not be the case - it may exchange matter with other
subsystems or undergo chemical changes. So, if "heat transfers are also
involved", the only reasonable approach is to identify and study the repective
heat-producing (or heat-absorbing) processes, both theoretically and
experimentally. There is absolutely no way to deduce their properties (e.g. to
deduce the gas pressure) from the electrostatic interactions in the system,
although these interactions should be taken into account.

Pentcho