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[Phys-L] Re: A battery cell



On Friday, Feb 25, 2005, at 23:19 America/New_York, Ludwik Kowalski
wrote:

P.S.
I suspect that the term EMF was initially introduced as a
real force working on electric charges (to separate them
against Coulomb forces of attraction). Then somebody
redefined the word to stand for something that is not a
force. The name EMF now stands for the "no-load
difference of potentials," for example between Zn and
Cu. Why is this confusing terminology tolerated?

1) It would be useful if somebody with an easy access to very old
physics texts could trace the history of the term "electromotive
force," and share the result with the rest us. For the time being let
me speculate.

2) Suppose that two cubes, of the same size but made from different
materials, are floating in an aquarium. Our free body diagram shows
that each cube is in equilibrium (buoyant force=m*g). The blocks are
our platforms, for example dry wood and wet wood. The center of gravity
of the dry wood platform, in equilibrium, is above the center of
gravity of the wet wood platform. Suppose the difference of elevation
is h. Tiny blocks, called ylektons, are allowed to slide from the top
of the higher platform to the top of the lower platform along an
inclined plane. The potential energy is thus converted into thermal
energy, as in a wire between the terminals of a battery. Ylektons
emerging from the inclined plane are then acted upon by some
non-gravitational force, for example, in a wind tunnel, and brought
back to the initial higher elevation. It is natural to say that the
force bringing ylektons back is "ylecto-motional." Why not? It is
responsible for the constant flow of "ylektons" in our mechanical loop.
Instead of wind one may think about an army of ants (biological force)
or of tiny helicopters (chemical force).

Ignoring frictional losses along the uphill path (like in a battery
whose internal resistance is negligible) one can say that work done by
the ylekto-motive force, on each ylekton, must be equal to g*h, a
change in the potential energy of a sliding ylekton. We know that work
done by the ylekto-motive force is not the same thing as the "working
force." But this is a linguistic trap. Suppose we decide to abandon
the idea of explaining things in term of forces; we prefer to explain
them in terms of energies. To accomplish this we eliminate the concept
of force but forget to change the "ylekto-motive force" into
"elekto-motive energy." After all the adjective "ylekto-motive" is more
appealing than "anti-gravitational," or "biological" or "chemical,"
etc. Is it conceivable that something like this really happened in a
field dominated by logical consistency? How else can one explain the
fact that the term "electromotive force" is still used to represent the
amount of energy needed to bring a positive probe charge, arriving to
the negative electrode (lower potential energy), to the positive
electrode (higher potential energy) inside the battery? Yes, "chemical"
forces are working (to overcome electrical forces) inside the
electrolyte.

3) What can be done to prevent authors of textbooks from even
mentioning the term "electromotive force?" The common abbreviation
"emf" should be replaced by something like "pd," "dp," or "dop"
(difference of potentials).

Ludwik Kowalski
Let the perfect not be the enemy of the good.
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