Re: Non-conservative forces in a liquid dielectric

[Pentcho Valev <pvalev@BAS.BG>]

Initially, the distance could be kept constant by placing e.g. a rigid rod
between the plates which then is removed. But I don't think discussing such
technicalities would be fruitful. In practical terms, the system is too
clumsy so only the idealized model needs to be discussed. Of course, in a
situation other than an e-mail discussion, I would need more space or time to
define the idealized model so that no confusions could arise.
     The curious thing is that, although the model is idealized, it would
stimulate students to think of both theoretical and experimental verification
of the second law. The solution (A) predicts that, as the capacitor is
partially immersed (its height with respect to the ground is fixed), its
"weight" (the net force that pulls it downwards) depends on the distance
between the plates - the greater the distance, the greater the "weight". In
contrast, the solution (B) predicts that the "weight" is independent of the
distance between the plates.
    That is what I have been fighting for all along. Not a fanatical
rejection of the second law (which would just be the negative of its
fanatical support) but rather showing ways in which people could reach
clarity for themselves. In the above case, there is an even easier
experimental verification. As the capacitor is half-immersed and water
between the plates has risen above the surface of the pool, one can punch a
small hole in one of the plates. If water leaks through the hole, the second
law is violated. I live in difficult conditions and am unable to do any
experiment but a colleague from UCSD did the experiment, hoping to prove the
validity of the second law in this way. First he informed me that water did
indeed rise above the surface of the pool, and then he had to punch the hole
the next day. That was a year ago - so far he has been silent and has not
replied to my messages.

Pentcho

Ludwik Kowalski wrote:

> The vertical plates (before being immersed) attract each
> other. A mechanical force, for example from compressed
> nylon springs, keeps the distance constant. You say: "We
> slowly draw them together (step 1) and so gain some work."
>
> Are you ignoring work that has to be done to further
> compress the springs before the immersion?
> Ludwik Kowalski
>
> Responding to Brian Whatcott (May 21, 2003) Pentcho Valev wrote:
>
> > The source is disconnected before the plates are immersed - I should
> > have
> > made this assumption explicit. This suggests another interesting
> > thought
> > experiment. The VERTICAL plates are suspended above the pool, ready for
> > immersion but the immersion has not started yet. We slowly draw them
> > together
> > (step 1) and so gain some work - e.g. one of them, through a pulley,
> > lifts a
> > weight. Then we slowly and completely immerse them into the pool (step
> > 2).
> > Under water, we slowly draw them apart until the initial distance
> > between them
> > is restored (step 3). Since the attraction between them under water is
> > much
> > smaller than the attraction in step 1, the work we spend in step 3 is
> > much
> > smaller than the work we gain in step 1. (Since the movements are very
> > slow,
> > friction can be neglected). Finally, in step 4, we slowly withdraw the
> > plates
> > until their initial position is restored.
> >      Now if only steps 1 and 3 are taken into account, the net work we
> > gain is
> > large. The question is: At the expense of what energy is this net work
> > (work
> > gained in step 1 minus work spent in step 3) done? This will answer
> > your last
> > question. There are ONLY TWO possibilities. (A) As we immerse the
> > plates in
> > step 2 and then withdraw them in step 4, we SPEND net work which, at
> > the end
> > of the cycle, appears as net work gained from steps 1 and 3. This
> > would mean
> > that the capacitor is "lighter" in step 2 and "heavier" in step 4. This
> > possibility is in accordance with the second law. (B) The net work
> > gained from
> > steps 1 and 3 is done at the expense of heat absorbed in step 3, as
> > Panofsky's
> > pressure pushes the plates apart and absorbs heat in the process. This
> > contradicts the second law of course.
> >      Note that, even if (B) is the right answer and the second law is
> > violated, this by no means suggests that a machine converting heat
> > into work
> > under isothermal conditions can be built. Isothermal heat engines,
> > although
> > possible in principle (in my view), are extremely slow and ineffective
> > and for
> > that reason it has never occured to serious engineers to try to build
> > one.
> > Speculative scientists however have found it advantageous to raise a
> > postulate
> > which can be translated like that: "What has not been built and has not
> > brought money is impossible". Serious efforts are needed to eradicate
> > anthropomorphism from physical sciences.
> >
> > Pentcho