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A clever student may observe that "electrification by contact, as
far as we know, involves dielectric materials. What does a set
of metallic plates, for example, Cu and Zn, immersed in a dish
with salty water, have to do with rubbing a glass rod with silk?
I do not know why charges are separated through electrification
by friction; how can my ignorance help me to understand what
happens in the dish?"
And I do not remember any explanations of the electrification
by contact, only a description of it. Therefore I still feel that
I have nothing to lean on when trying to explain the nature
of something that takes electrons away from Cu and delivers
them to Zn via salty water. I can introduce proper vocabulary
and use it to describe what happens, I can measure how much
of it happens in different situations, I can describe chemical
reactions taking place, etc. All this is highly desirable and we
do it. But I can not explain batteries, or contact electrification,
in terms of something else. Compare this with the P*V=n*R*T,
for example. We can not only describe this relation, we can also
explain it in terms of molecular collisions.
May I suggest that somebody who has a good explanation of
electric batteries (for example, based on a textbook) posts it here,
or on a website. Then we can discuss its appropriatness for a
first physics course. The best I was able to do so far was to use
the water pump analogy, as described at the very end of my
handout: