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Charge interactions in media



I am returning to a problem the group abandoned but which
nevertheless is one of the most important (unresolved) problems in
physics education. Why is the attraction between two opposite charges
weaker in water? Please see an explanation in the text accompanying
fig. 20.2 on p. 373 in

http://www.garlandscience.com/mdf/MDF%20ch20%20.pdf

Dipoles surrounding the charges somehow SHIELD the charges and this
shielding allegedly causes the decrease of the force of attraction.
As I look at the picture, I see the opposite - the main contribution
is made by dipoles BETWEEN the two charges and these dipoles INCREASE
rather than decrease the attraction between the charges. If I am
right (this is still a subjective view of course), the real decrease
of the attraction which is experimentally confirmed is not due to
shielding, i.e. to the action of electrical forces.
The following thought experiment (which could be real as well)
will help us to resolve the problem. A metal slab is inserted between
the plates of a (charged and disconnected) parallel plate capacitor
(without causing discharge of the plates of course) and students are
to say what happens to the voltage (V) and force of attraction
between the plates in the presence of the slab. They may decide to
apply

V_metal = V_vacuum / D /1/

where D is the dielectric constant of the intervening material. The
inspection of the physical picture or an experiment would convince
them that V_m is practically zero, i.e. D is extremely great. But
then they may find that, in accordance with Coulomb's law

F = k(q1*q2)/Dr^2 /2/

the attraction between the plates has also vanished. (After all, the
plates can be ragarded as two charged bodies that obey Coulomb's
law). But this would be a wrong conclusion - the same physical
picture that showed vanishing of the voltage now shows INCREASE of
the force of attraction:

Attraction_m >> Attraction_v /3/

I am afraid sooner or later physics teachers will have to answer
the following question: Are we right in placing, in the Coulomb's
law, the same constant D that we place in the expression for the
voltage of a capacitor. Then they may return to the bothering idea of
the action of NON-CONSERVATIVE forces in electrostatics.

Pentcho Valev