Explaining things (was battery)

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

COMMENTS ON WHAT JOHND WROTE (see below)

1) The term EMF can be found in most textbooks.
     The authors make it clear that EMF is not a force.
2) Most teachers of introductory physics courses,
    myself included, either never learned about the
    electrochemical potential or have only a very
    vague idea about what it is.
3) Students are also totally unprepared for an
    explanation based, for example,  on changes in
    the "rates of shifting the electrochemical potential."
    And I doubt that they learn about Nernst equation
    in an introductory chemistry course.
4) What to do in a situation in which something
     can not be explained in terms of what is
     already known to students? Yes, I know that
     this is not an easy to answer question. But it
     worth addressing, I think.
Ludwik Kowalski

On Saturday, Mar 20, 2004, John Denker wrote:

> Ludwik Kowalski wrote:
> > The textbook I am using states:
> > "The real battery, however, always has some
> > internal resistance r. As a result, the terminal
> > voltage is not equal to the emf."
> >
> > I do not like the "as a result" phrase.
>
> I dislike several things about the quoted statement.
>
> For starters, the thing they seem to be calling "emf"
> has for the last jillion years or so been called the
> "open-circuit voltage" or "Thevenin equivalent voltage"
> or some combination of the two, such as "Thevenin
> open-circuit voltage".
>
> Also Ludwik is quite right to be suspicious of the alleged
> origin of the observed Thevenin-equivalent impedance.
> Batteries are remarkably tricky little creatures.  The
> I/V characteristic is nowhere near linear.
>   -- For small currents, the dominant effect has to do
> with the chemical rate constants, and how much you shift
> the rates by shifting the electrochemical potential.
>   -- For larger currents, the dominant effect is diffusion
> through the electrolyte.  Ionic mobility and all that.
>   -- I suspect that in any halfway-well-designed battery,
> ohmic losses in the metal parts is a quite small effect.
>
> > The change of resistivity
> > of wires (due to ohmic heating) is small
>
> yes.
>
> > and
> > the same is probably true for the electrolyte,
> > unless the number of free carriers drops
> > significantely.
>
> I disagree.  Ions move a lot slower than electrons.
> The ionic conductivity of liquids is remarkably poor
> compared to the electronic conductivity of ordinary
> metals.