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Re: Non-conservative forces in a liquid dielectric



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