Re: Why does electrostatic attraction in water decrease?

[Pentcho Valev <pvalev@BAS.BG>]

"John S. Denker" wrote:

> Pentcho Valev wrote:
> > Four hypotheses seem relevant:
> >
> > 1. Panofsky gives a wrong picture - the effect does not exist.
> >
> > 2. If we punch a hole in the plate, below the surface of the liquid inside the
> > capacitor but above the surface of the liquid outside the capacitor, no liquid
> > will leak out through the hole.
> >
> > 3. The liquid will leak out in violation of the first law.
> >
> > 4. The liquid will leak out in violation of the second law.
>
> That's a good summary.
>
> So why not do the experiment?  That ought to be
>  a) easy, and
>  b) the most effective way to settle the issue.
>
> Suggestions:
>  -- Use carefully deionized water, so conductivity doesn't
> cause problems.
>  -- Don't make the apparatus larger than it has to be.
>  -- Put a resistor in series with the voltage source,
> so no harm is done if there is breakdown (spark) in the
> air above the liquid, or if somebody accidentally touches
> something they shouldn't.
>  -- Observe the height at a couple of different voltages.
>
> BTW I still predict (2), i.e. electrostatic attraction with
> no leakage.

I cannot do the experiment, for many reasons. However there is another instructive
thought experiment - let me describe it. Initially, the capacitor is suspended over
the pool - no contact with the water. Then an operator performs the following 4
steps:

1. The plates are slowly drawn together. Through a pulley, the movement can be
harnessed to lift a weight - the operator GAINS work from the process.

2. The capacitor is slowly let down and completely immersed. Again, the operator
GAINS work from the process.

3. In water, the plates are slowly drawn apart until the initial distance between
them is restored. The operator SPENDS work in the process, but this work is 80 times
smaller than the work gained in step 1.

4. The operator slowly lifts the capacitor until the initial state of the system
(capacitor + pool + earth) is restored.

Now if only steps 1 and 3 are taken into account, there is obviously a large GAIN in
work. Where does the energy for this net work gained come from? There are two
possible sourses:

A) The net work gained in steps 1 and 3 is done at the expense of heat absorbed from
the surroundings. If so, the second law is of course violated.

B) The net work gained in steps 1 and 3 is done at the expense of WORK THE OPERATOR
SPENDS IN STEPS 2 and 4. This saves the second law, but is somewhat
counterintuitive. Roughly speaking, this means that in step 4, as the operator lifts
the capacitor, the capacitor is MUCH HEAVIER than in step 2 when the operator lets
it down.

Pentcho