Pentcho,
Not wishing to re-hash this topic, I suggest you please go to the archives
for the last time we covered this question ( raised by you then, also).
Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net http://www.velocity.net/~trebor
----- Original Message -----
From: "Pentcho Valev" <pvalev@BAS.BG>
To: <PHYS-L@lists.nau.edu>
Sent: Monday, May 19, 2003 8:41 AM
Subject: Re: Non-conservative forces in a liquid dielectric
| Carl Mungan wrote:
|
| > Pentcho wrote:
| >
| > >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.
| >
| > I'm not very happy with these definitions.
| > I would prefer to replace "heat" with "thermal energy" and take your
| > 3rd sentence as defining a "dissipative" force rather than a
| > "nonconservative" one.
|
| The problem is much more serious than that. In 1962, two renowned
| authors write in a renowned textbook:
|
| "A factor 1/k is frequently included in the expression for Coulomb's law
| to indicate this decrease in force. The physical significance of this
| reduction of force, which is required by energy considerations, is often
| somewhat mysterious. It is difficult to see on the basis of a field
| theory why the interaction between two charges should be dependent upon
| the nature or condition of the intervening material, and therefore the
| inclusion of an extra factor 1/k in Coulomb's law lacks a physical
| explanation" (W. Panofsky, M. Phillips, CLASSICAL ELECTRICITY AND
| MAGNETISM, Addison-Wesley, Reading, Massachusetts (1962) p. 114)
|
| The authors do not call k "dielectric constant" and their interpretation
| of the effect leads to the following conclusion: The decrease of the
| force of attraction between two capacitor plates immersed in a liquid
| dielectric, mysterious or not, surely has NOTHING TO DO with the
| decrease in voltage between the plates. The former effect is due to the
| mysterious pressure developing between the plates and pushing them apart
| whereas the latter can easily be explained in terms of polarization.
| For the next 40 years, no physics teacher has found it suitable to
| ask him/herself: If Panofsky and Phillips are right and the two effects
| are essentially different, why don't I explain this to students? Why do
| I mislead them so fatally by introducing the same quantitative measure
| (the dielectric constant) for two essentially different effects? As
| Panofsky's pressure does work (e.g. pushes the plates apart), at the
| expense of what energy is this work done? Since this could not be the
| electrical potential (the pressure counteracts it), could the energy
| source be heat absorbed from the surroundings? In what other cases do
| electrochemical systems absorb heat and convert it into work? How about
| batteries? They are known to convert CHEMICAL energy into work, but
| could that be just one of the numerous illusions in physics education?
| The simplest electrochemical cell
|
| Zn2+ (high concentration), Zn II Zn, Zn2+ (low concentration)
|
| surely involves NO CHEMICAL REACTION and yet it could do work. At the
| expense of what energy?
| Note that the problems raised concern material taught to freshmen
| and college students. Before resolving these problems it would be
| premature to argue which force is conservative and which is not so
| conservative.
|
| Pentcho