Re: Batteries

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

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:

http://blake.montclair.edu/~kowalski/elec/flash.html

It was an addendum to a comment made by William Betty last
summer. The analogy helps to figure out "how does a battery
know what to do?" The windmill stops turning when the
pressure of water matches the constant wind (and it turns
very rapidly when a lot of water is escaping lowering the
pressure. But the analogy has nothing to do with "forces
responsible for separation of unlike charges."

I am not criticizing what we do, I am only recognizing a
situation in which an explanation is not possible in terms
of what students already know in an introductory course.
There is nothing tragic in this; describing what happens is
a valuable preliminary step in learning something new. The
reason I was referring to "work function" has to do with
what JohnD wrote after my draft was posted in August.
Ludwik Kowalski

Chuck Britton wrote:

> I would suggest that students ARE familiar with 'electrification by
> contact' (often erroneously limited to FRICTIONAL charging). There is
> no reason to use the term 'work function' until the time is right.
> Only when the time is right (it comes up on the syllabus) is the term
> used and THEN it can be related BACK to the PAST discussion of
> charging by contact.
>
> At  1:19 PM -0500  on  2/10/02,  Ludwik Kowalski   wrote
> > in order to
> > follow your advice I would have to violate a basic rule of
> > good teaching; I would have to explain something in terms
> > of something to be learned later.
>
>         <snip>
>
> >  but the concept of "work function" does not exist in
> > the minds of my students. Therefore they have nothing to
> > link battery cells with. The concept of the DOP produced
> > by a cell can not be linked with something which may
> > or may not be introduced to them later. But the link must
> > be made when the work function is introduced. Lean on
> > the past, do not lean on the future. That should be the
> > basic teaching principle.