[Phys-L] Re: A battery cell

["John S. Denker" <jsd@AV8N.COM>]

On 02/26/05 10:51, Ludwik Kowalski wrote:

> ... this is not at all obvious.

That's for sure.  Batteries are highly non-obvious.

I know more about batteries than most people do, yet
I don't have a consistent end-to-end understanding of
the whole process.  Bits of the process yes, but not
the whole package.

> I
> am thinking in terms of a model of a conductor described in textbooks.

Don't get me started about wrong things in textbooks.

> That model states that excess charges, if any, exist only on surfaces,
> and that they can be removed by momentary grounding.

Baloney.  That's totally inconsistent with the well-known
physics of work functions.  If that were true, we wouldn't
have batteries.  We wouldn't have static electricity, either.

> What prevents me
> from discharging (neutralizing) a metallic plate before immersing it
> into an electrolyte?

Just what do you intend by that?
   a) Bringing them to equilibrium, i.e. zero difference
    in electrochemical potential?
   b) Or perhaps just zero difference in chemical potential?
   c) Or zero difference in electric potential?

The foregoing options are seriously inequivalent.

> A plate suspended by a nylon thread, for example,
> can be lowered into a grounded metallic cavity (touching it from
> inside) before being inserted into the acid.

That is truly an excellent Gedankenexperiment.  I've
used that one myself to great advantage on several
occasions.

You'll find that it doesn't generally result in
electrical neutrality, for reasons having to do with
the work functions of the two materials.

By shuttling the plate back and forth between two
dissimilar cavities you can make a powerful electrostatic
generating machine.

A highly imperfect writeup of what I understand about
these two closely-related topics (batteries and
contact electrification) can be found at
   -- http://www.av8n.com/physics/contact-electrification.htm
   -- http://www.av8n.com/physics/battery.htm