Re: Why does electrostatic attraction in water decrease?

[Pentcho Valev <pvalev@BAS.BG>]

I don't think my English is very relevant. The effect has been both
recognized and discussed - e.g. in W. Panofsky, M. Phillips, Classical
Electricity and Magnetism, Addison-Wesley, 1962, pp. 111-116. However the
discussion is based on the initial assumption that "all the forces are
conservative" (p. 96). I think this assumption is incorrect for dielectric
liquids (water) in an electrostatic field - there is a non-conservative
pressure that pushes apart the plates of a capacitor immersed in the liquid.
Perhaps my use of "non-conservative" is confusing - people tend to identify
non-conservative force with friction. In this case the paradigm for a
non-conservative force is gas pressure.

Pentcho

hERBERT H GOTTLIEB wrote:

> It seems as if  the author, Pentcho
>  Valev has some difficulty with the
> English language but it would be helpful if someone could help
> translating his equations below.
>
> Herb
>
> On Mon, 22 Apr 2002 17:42:32 +0200 Pentcho Valev <pvalev@BAS.BG> writes:
> > Let us assume that a constant-charge parallel-plate capacitor with
> > vertical plates is suspended over a pull of water. The magnitude of
> > the
> > force of attraction beween the plates is F. Then the capacitor is let
> > down and immersed in the pull - the force of attraction is greatly
> > reduced and becomes F/80. What can be the molecular mechanism behind
> > that? The situation in water can be illustrated by the following
> > picture:
> >
> > +P (-)(+) (-)(+) (-)(+)........(-)(+) P-
> >
> > where +P and P- are the positive and negative plates and (-)(+) are
> > water dipoles. If there were no thermal motion, the force of
> > attraction
> > would slightly increase, due to polarization of water. So, in my
> > view,
> > the radical decrease in attraction can only be due to thermal motion.
> > For instance, thermal motion can force the second dipole on the left
> > to
> > rotate:
> >
> > +P (-)(+) (+)(-) (-)(+)........(-)(+) P-
> >
> > Clearly, the rotation results in a local electrostatic "push", and
> > the
> > sum of all such "pushes" amounts to a kind of thermal pressure acting
> > against the force of attraction. If this pressure does exist, it
> > would
> > be NON-CONSERVTIVE - as the plates are drawn apart in water, the
> > pressure will do work at the expense of heat absorbed (somewhat
> > analogous to work a gas does on expansion).
> > The explanation is perhaps too strange but for the moment I don't see
> > a
> > more reasonable molecular mechanism. Any other explanation is
> > welcome.
> >
> > Pentcho Valev