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Re: Why does electrostatic attraction in water decrease?



Pentcho Valev wrote:

Perhaps [conductivity] is not the essential problem.

I agree.

to define the energy in an electric field, one considers a
set of charges as they are moved, isothermally, from infinity to their
places in the system.

I wouldn't "define" the energy that way, but you can
_measure_ the energy that way, subject to restrictions
discussed below.

If this assembling is infinitely slow (to avoid
friction), the work of assembly is identified with the energy of the
system.

OK, keeping in mind that the energy is "in" the fields
not "in" the electrons.

An essential condition is that all of the forces are CONSERVATIVE

That means we must neglect magnetic effects; this is
one of the restrictions alluded to above.

- if, for instance, two opposite charges are drawn apart at some stage,
the energy of the system must increase at the expense of the work you
spend but NOT at the expense of heat absorbed from the surroundings.

OK. But making a fuss about thermal effects is probably not
the essential issue either. One could take a microscopic
view and include the work (defined as F dot dx) done by the
environment, including it on the same footing as the macroscopic
forces.

This
IS the case when you draw two opposite charges apart in a vacuum, but does
NOT seem to be the case when you draw them apart in a dielectric liquid
(e.g. water).

Really? What's the evidence of that?

In the latter case, conductivity is irrelevant - the
essential factor is the increased liquid pressure between the charges that
pushes them apart.

Really? What's the evidence of that?

The effect of a dielectric can't be summarized as a pressure
in any reasonable way; consider this:
-- Scenario 1: I bring a charge to point A and another charge
to point B (bringing them in from infinity as described above)
in the absence of any dielectric.
-- Scenario 2: Same, except that there is a slab of _solid_
dielectric material in some of the space between A and B.

There's no pressure on the charges in either scenario, but
the energies and forces are quite different.

Now the problem: is this pressure analogous to gas
pressure with respect to the fact that, as the gas expands slowly and
isothermally, it absorbs heat from the surroundings? If the analogy does
exist, I am not sure about the consequences - somehow non-conservative
forces of this type will have to be included in the general theory.

This question is predicated on false assumptions.

======

Also: The energy budget of a dielectric can be described
quite nicely in terms of the charges and fields inside the
dielectric. This is discussed in the standard texts e.g.
Feynman volume II section 10-5 especially figure 10-8 and
figure 10-9.