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Re: Batteries



Responding to this:

But I can not explain batteries, or contact
electrification, in terms of something else.

JohnD wrote (in part):

If "something else" is restricted to pre-20th-century
physics, neither Ludwik nor anybody else is going to
explain chemistry or atomic physics or materials science.

John gives three possible solutions of the dilemma:

You can (a) accept this as an observed fact (based on
experiments), or you can (b) accept it as an article of
faith (revealed to you in a dream or whatever), or you
can (c) do the quantum mechanics.

In my opinion it is not the matter of 19th versus 20th
century physics; it is a matter of sequencing the learning
path. There are many possible ways of going from known
to unknown; some are likely to be better than others. The
sequence in most textbooks today is essentially the same it
iwas a century ago. New books simply imitate older books,
in that respect. A radical departure is needed, I think. For
the time being option (a) seems to be the best way of
dealing with batteries in an introductory physics course.

I would like to know what other teachers think about the
EXPLANATION of batteries on John’s website:

http://www.monmouth.com/~jsd/physics/battery.htm

Is it suitable for an introductory physics course under the
current sequence?
Ludwik Kowalski (P.S. was added below)

******************************************
Here is a good description of WHAT happens; it was sent
to me in private, by XXX. Thanks.

Put two metals in an acid bath and they begin do dissolve.
In that process ions move into the solution and electrons are
left behind. The result is that the metal is negatively charged
relative to the solution. After a while, depending on the
chemistry of the dissolution, a steady state is reached in
which the rate of dissolution matches the rate of redeposition.
[In other words, two competing tendencies, ions going away
from the metal and returning to it from the solution.] If the
metals are different, the dissolution rates are different, and
one electrode will be more negative than the other. The less
negative we call the positive electrode. This chemical reaction
has, in effect, charged a capacitor made of the two metal
plates, which then have a potential difference because of
the difference in charge state.

If a wire is connected to the plates, the potential difference
results in a field in the wire and all the electrons start
drifting. As electrons slowly leave the more negative plate,
its dissolution reaction starts again because the steady state
condition has been violated, the field opposing dissolution
has been reduced at one of the electrodes.