Re: batteries

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

John Denker wrote (in part):

> I thought somebody already had posted such an essay.
>   http://mailgate.nau.edu/cgi-bin/wa?A2=ind0007&L=phys-l&P=R27156

Thanks for reminding; John's essay clearly indicates what
students must be familiar with to understand chemical cells.

> > My impression is that electochemical cells can not be explained
> > at the elementary level;
>
> How elementary?  Elementary-school level?  Obviously this subject is
> not suitable for fourth grade.  But it would be within the scope of an
> ordinary modern-physics course.

By elementary I meant a typical introductory physics course
students take in a college, most often (I suspect 90% of cases)
it their first physics course. Chemical cells are used but they
can not be explained in terms of classical physics concepts
introduced before covering chapters on electricity. John's
essay reinforces me in claiming that the only possible approach
is the one used in textbooks; electric cells are described in
terms of what happens without trying to explain them in terms
of something that has already been learned. They are what
they are; period. [It is remarkable how much was learned from
experiments with batteries by those who did not understand
them.]

A chemical cells can be contrasted with a Van de Graaff "cell"
where principles of mechanics are used to explain the charging
process.

> > they are what they are; period.
>
> That seems like cop-out.  It's just physics, after all.
>
> > [Using big words, like Gibbs
> > potential, QM or double layer, does not contribute to clarity
> > at the level of my teaching.]
>
> Well, if you want to explain batteries in terms of 18th-century physics,
> you're going to be disappointed.  There is no ball-and-stick model that
> explains why different metals have different work functions.
>
> If you want to explain electrochemistry you're going to have to use the
> methods (or at least the results) of modern physics in some form. ...

I just consulted five Modern Physics textbooks. Topics mentioned
by John are addressed but non of the book has a chapter devoted
to electrochemistry. The word battery or cell could not be found.
One book has a diagram with arrows showing that inside the
battery electrons flow from + to - (separation of unlike charges)
but no attempt to explain (or to recognize) the "paradox" can be
found. Another textbook shows the square well potential indicating
the Fermi level and the work function. That would be an ideal
place to insert a paragraph devoted to chemical cells. By the way,
referring to the work function in metals authors say "as determined
from the photoelectric effect or from thermoionic emission."

In his essay John wrote:
> The work functions are different because of things like the Pauli
> exclusion principle.  You have a different number of fermions
> in a different-sized box, so the Fermi level will be different.
> Calculating things like work functions _ab initio_ is a real tour
> de force, and we need not discuss the details here;  it suffices
> to accept the observed work function values:
>
> Work function for aluminum: 4.2 eV
> Work function for iron: 4.63 eV
> Work function for nickel: 5.2 eV

I was not aware that the "ab-inition" calculations of such things
are possible (without introducing adjustable parameters to match
experimentally measured values). Thanks again, John.
Ludwik Kowalski