[Phys-L] Re: A battery cell

[Ludwik Kowalski <kowalskil@MAIL.MONTCLAIR.EDU>]

On Saturday, Feb 26, 2005, at 02:55 America/New_York, John S. Denker
wrote:

> On 02/25/05 23:19, Ludwik Kowalski wrote:
> > Each plate [of a battery] before being inserted
> > into acid, is neutral;
>
> How sure are you that they are neutral?
> How do you know?
>
> I claim that in equilibrium, two dissimilar chunks
> of metal are not electrically neutral.  They will
> carry a charge that depends on the materials, and
> on the (inverse) distance between them.
>
> There is a device called a Kelvin bridge that is
> based on this.  It is used to measure the work
> function of materials.
>
> The fact that things have work functions, and
> therefore are not neutral in equilibrium, is a
> key part of the physics of batteries.

You are probably correct, as usual. But this is not at all obvious. I
am thinking in terms of a model of a conductor described in textbooks.
That model states that excess charges, if any, exist only on surfaces,
and that they can be removed by momentary grounding. What prevents me
from discharging (neutralizing) a metallic plate before immersing it
into an electrolyte? 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. A zinc plate, according to
Giancoli ("Physics for Scientists and Engineers," third edition, page
635) becomes negative in the acid when Zn atoms are dissolved.
Referring to an illustration (a negative Zn rod and positive C rod,
both in sulfuric acid), he writes: "The acid reacts with the zinc
electrode and tends to dissolve it. Each zinc atom leaves two electrons
behind and enters the solution as a positive ion. The zinc electrode
thus acquires a negative charge." The author probably thinks that Zn
was initially neutral.

Ludwik Kowalski
Let the perfect not be the enemy of the good.