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.
----- Original Message -----
From: "Pentcho Valev" <pvalev@BAS.BG>
To: <PHYS-L@lists.nau.edu>
Sent: Thursday, April 25, 2002 8:06 AM
Subject: Re: Why does electrostatic attraction in water decrease?
. . .In the former case,
according to Panofsky, the decrease in force is due to liquid pressure
between
the charges, whereas in the latter the effect is due to polarization. So
the
question "Why is the same quantity used?" is reasonable, although someone
may
prove that using the same quantity is correct.