Re: Batteries

[William Beaty <billb@ESKIMO.COM>]

On Sun, 10 Feb 2002, Ludwik Kowalski wrote:

> 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?

Rubbing?   Electrification by contact only involves contact, not rubbing.

> I do not know why charges are separated through electrification
> by friction;

Not friction.  Just touching, followed by separation (or not!)

> how can my ignorance help me to understand what
> happens in the dish?"

Analogy: I don't know why the apple is attracted to the Earth and falls
downwards.  How can my ignorance help me to understand how the moon stays
up there?  (Diverse phenomena explained by a single concept...  but when
the explanatory concept itself is then NOT explained.  Isn't this a common
thing in physics?  Do we have reason to complain about it? )


> 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.

But do you normally explain gravitational attraction in terms of something
else?  Or do you refuse to teach the concept of gravitational attraction
because you're only teaching a description, not an explanation?

Isn't it highly desirable to notice the connection between a falling apple
and an orbiting planet?

My point is about the immense explanatory power of noticing connections.
The "knowledge network" takes its power from connections between
apparantly separate phenomena. In 1600, who would have thought that a
falling apple had any connection to the motions of the "celestial
spheres."  And in the world of a child, who would have thought that the
spontaneous electrostatic charging of dissimilar materials would have
anything to do with flashlights?

PS, when I say "electrification by contact", I don't mean the rubbing of
silk on rubber.  Instead I'm thinking about having an extremely sensitive
electroscope to detect the polarity of charged objects.  (Perhaps connect
an electronic electrometer to a charge-integrating bucket.  Touch a pair
of dissimilar objects together, then insert each one into the bucket. )

PPS, I harp about "electrification by contact" because that concept
greatly helped me understand physics.  I stumbled over the idea in some
really ancient pre-QM textbook, where the mystery of contact-charging was
being discussed.  It gave me a really massive "AHA!", and suddenly I was
able to connect thermocouples with batteries, LEDs, solar cells, diodes,
transistor explanations, and fur-rubbed balloons.  "Contact
electrification" is like discovering a new kind of screwdriver, and
suddenly gaining the ability to manipulate screws which my other tools
couldn't touch.  I've always wondered if my enlightening experience could
be canned and distributed to students for consumption.


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William J. Beaty                            SCIENCE HOBBYIST website
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