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[Phys-L] A battery cell



On Friday, Feb 25, 2005, at 21:13 America/New_York, ALVIN BACHMAN wrote:

Two remarks. (Hope I'm not repeating someone else.)

1. I think that there are TWO issues in the one-wire task. (a) the
complete circuit - lamp construction, and (b) the substitution of
direct contact instead of a second wire. . . .

The real mystery in "a lamp and a battery" circuit is not the lamp; it
is the battery. Most authors simply avoid this mystery; they say
something like this: "accept the fact a battery is a device in which
one terminal is positive and another is negative. The device tends to
maintain a constant difference of potentials." A textbook might show a
picture in which Zn and Cu plates are immersed in sulfuric acid and
tell readers that Zn is positive while Cu is negative.

This can be presented as a paradox. Each plate, before being inserted
into acid, is neutral; electrons and protons are distributed
uniformly. Likewise positive H ions and negative SO4 ions are
distributed uniformly in water before the plates are inserted. In other
words, all three components of a battery are neutral. We insert the
plates into the electrolyte and charges are somehow separated. The
common phrase "chemical energy is converted into electrostatic energy"
is not a sufficient explanation, in my opinion. Opposite charges
attract, there must be a force responsible for their separation. What
is a good way to explain chemical separation of electric charges?
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?

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