Re: Batteries

["John S. Denker" <jsd@MONMOUTH.COM>]

Ludwik Kowalski wrote:
> A clever student may observe that "electrification by contact, as
> far as we know, involves dielectric materials.

Or the student may not observe any such thing.

1) Most electrical devices (contact electrification or
otherwise) involve dielectric materials.  Try designing,
say, an electric motor or even a flashlight, using  _only_
conductive metal parts.  You can maybe sorta do it, but
the result would be highly impractical to say the least.

2) I can devise a contact-electrification machine (a very
powerful one) where all the critical contacts (the ones
that produce charge separation) are metal-to-metal.

> 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?

The key idea in both cases is that different materials
sit at different voltages in equilibrium.  In the same
way that people think "nature abhors a vacuum" they seem
to think that having nonzero electric fields running around
can't be the natural state ... but it is.

> 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?"

If you insist on a ball-and-stick model, you are
guaranteed to be disappointed.  If you adhere to
19th-century physics, you will confidently predict
that atoms (if they even exist) will all behave the
same.  Helium is just a heavy isotope of hydrogen.
And lithium the same, only heavier.  And oxygen the
same, only heavier still.  The alternative is to find a way
to accept that electrons stick to some atoms differently
from others.  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.

> 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.

That's option (a) in the list above.

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

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.

> 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.

OK, that's an example of something that can be
described in terms of 19th-century physics.
But ONE thing being possible doesn't prove
that ALL things are possible.

> 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.

I've never seen a decent explanation in any textbook.
I had to figure it out for myself.  Try this:
  http://www.monmouth.com/~jsd/physics/battery.htm