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Re: Non-conservative forces in a liquid dielectric



In considering this putative violation of conservation rules,
this line of reasoning is helpful:
it is found that water rises between the plates of a charged capacitor,
half submerged. So if there were a small vertical slit in one plate,
the water
might leak through the slit....but there are already two half slits in each
plate,
which define the vertical edges of the plate.

So it would be interesting to see the water elevated at the edge of the
plate,
and fall away from the edge, to the un-elevated level of the water.

But wait, where the water level is elevated between the plates, there is
a smooth transition near the plate's edge down to the major water level.

So one can confidently predict that the elevated level smoothly transitions
down at a slit, and for similar reasons, curves smoothly below the lower edge
of holes of particular sizes and heights, or forms a containing meniscus
across the others.

Still, this is gedanken, not science: nothing is demonstrated, only a certain
plausibility is examined in this way.... All the same there is a distinct
family
resemblance between this gedanken and several perpetual motions.

One is the beautiful round flask
with the thin stem at the top through which water rises through capillary
action (originally explained as due the much greater weight of water in
the body of the flask), and which allows drops to fall back into the flask
from the u bend at the top.

Or put it more simply: if a water capillary rises 3 centimeters in a tube,
will not a side hole at 1 cm elevation permit a little water fall?

Of course not: this is the same answer given confidently for the last
150 years or so. The problem is to conform ones imagination,
the stuff of gedankens - to reality.
Reality is subtle at times...

Brian Whatcott


At 10:56 AM 5/22/2003 +0300, Pentcho, you wrote:
/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