Re: Non-conservative forces in a liquid dielectric

[Pentcho Valev <pvalev@BAS.BG>]

Perhaps I should add something about non-conservative forces. Textbooks usually define a
conservative force as one which, as you do work against it (isothermally), keeps the energy
and does not dissipate it as heat. In contrast, as you do work against a non-conservative
force, the energy is dissipated as heat. Then always friction is refered to so that many
scientists do not suspect that there could be non-conservative forces other than friction. In
fact, an important non-conservative force is gas pressure - as you do work against it,
isothermally, the energy introduced is dissipated as heat so that the energy of the gas
system remains unchanged. Due to the identification of "non-conservative force" with
friction, there is no tradition for physicists to try to falsify existing theories based on
the assumption that ONLY CONSERVATIVE FORCES act in the studied set of events. So is the
situation in electrostatics - try to imagine what would happen to its results if gas
pressure-like forces interfered with the electrical foces it deals with. Unfortunately, the
force I discuss below is gas pressure-like. If I am right, a new electrostatics will have to
be built and taught. Perhaps it will be a part of an alternative course of thermodynamics.

Pentcho

Pentcho Valev wrote:

> We are to resolve a problem physicists usually avoid, I don't know why. When two opposite
> charges (e.g. the plates of a capacitor) are immersed in water, why does the force of
> attraction between them decreases 80 times? (Of course, the answer "because the
> dielectric constant of water is 80" is unsatisfactory). If we imagine an analogous
> situation in which all participants are macroscopic (e.g. a capacitor plus some
> macroscopic "dipoles" between the plates able to rotate and "polarize") we see that
> polarization, per se, must increase rather than decrease the attraction. In other words,
> electrical forces can by no means be held uniquely responsible for the effect. Quotation:
> "Therefore the decrease in force.... which is experienced when the experiment is
> performed with a liquid that wets the plates and also completely surrounds them, cannot
> be explained by electrical forces alone." (W. Panofsky, M. Phillips, CLASSICAL
> ELECTRICITY AND MAGNETISM, Addison-Wesley, Reading, Massachusetts (1962) p. 115). On the
> same pages, Panofsky and Phillips speak of an additional pressure that develops between
> the plates, pushes them apart and so counteracts the original electric force of
> attraction. See fig. 6-7 on p. 112. When two vertical plates of a capacitor are partially
> immersed in a pool of liquid dielectric (water), this additional pressure causes the
> liquid between them to rise high above the surface of the pool. One may suspect
> that, if one punches a hole in one of the plates, water will leak through the hole
> forming an eternal waterfall outside the capacitor, in violation of the second law. Of
> course if the second law is correct water will refuse to leak through the hole.
> The first impression is that the effect (lifting of water) is similar to capillary
> action. However this is illusory. Capillary action amounts to
> attraction between the plate and adjacent water dipoles and so it increases the
> attraction between the plates. If only capillary force were acting, the attraction
> between the plates in water would be slightly greater than that in vacuum. Yet
> Panofsky and Phillips speak of a "pressure" generated inside the liquid which pushes the
> plates apart so that the original force of attraction in vacuum decreases 80 times. It is
> difficult to imagine the liquid exerting pressure on the plate and yet refusing to leak
> through the hole. Sounds like an oxymoron. In my view, the pressure is generated by the
> following mechanism. Imagine the order of water dipoles between the plates:
>
> P+ (-)(+) (-)(+) (-)(+).................(-)(+) -P
>
> P+ and -P are the plates. This perfect order would exist if there were
> no thermal motion. But there IS thermal motion and the curious thing
> is that any disturbance caused by thermal motion increases Panofsky's
> pressure inside the liquid. Assume the second dipole on the left has
> received a strong thermal stroke and has rotated:
>
> P+ (-)(+) (+)(-) (-)(+).................(-)(+) -P
>
> We can say that, microscopically, heat has been absorbed and converted
> into energy of repulsion between the dipoles. The sum of all such
> microscopic events is expressed as pressure exerted on the plates. The
> same pressure lifts the water, and lifting is performed at the expense
> of heat absorbed from the surroundings. If you punch a hole water will
> fall and dissipate the energy. But you can install a waterwheel....
>      This case can be interpreted in terms of system-doing-two-types-of-work approach I
> developed yesterday, but I think the present approach is more interesting from a physical
> point of view.
>
> Pentcho