Re: Explaining things (was battery)

[Mark Sylvester <msylvest@XNET.IT>]

I've found the datalogging interface to be useful here. I hook it up to log
the terminal p.d. of the cell and the current, measured as the p.d. across
a large 1 ohm resistor. Now I can do a quick excursion of the rheostat in
the circuit - down to minimum ohms and up again - and compare the V-I plot
with what I get for a slow tour along the same route. The quick excursion
gives the textbook-like straight line, while the slow tour shows very
nicely how the battery "gets tired". I too have forgotten more chemistry
than I ever knew, but I believe it's possible to relate what we see to the
textbook model by saying that the accumulation of waste products changes
the emf. The battery has a "depolarising agent" that mops up these
substances so it recovers after resting. This seems to leave the emf plus
internal resistance model intact while acknowledging the chemical
complications.

Is this problem an argument for teaching Physical Science (phys & chem
combined)?

Mark

At 20:28 20/03/04 -0500, Ludwik Kowalski wrote:
> 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.

Mark Sylvester
UWCAd
Duino Trieste Italy