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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.