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/snip/
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.
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 ....
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 step
> > 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