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Re: emf and batteries



The internal resistance is a simplified way of characterizing the internal energy losses.

Power delivered = V * I = EMF * I - (I^2) Rinternal

"Edmiston, Mike" <edmiston@BLUFFTON.EDU> wrote:
* * * On the topic of why the terminal
potential difference is less than the
cell emf...

This explanation is not suitable for
intro students, but it is pretty good
for those of us who know some
thermodynamics.

The emf for a reaction represents
the maximum work that can be obtained
=66rom that reaction. In order to obtain
the maximum work, the cell must operate
reversibly. Operating reversibly to
get the maximum work typically means
slowly, and certainly precludes
operating in a fashion that liberates
heat.

As the current (from a battery)
increases, the electrochemistry is
certainly moving from reversible
to irreversible, and heat is clearly
a factor. Therefore we cannot be
getting the maximum work. If we are
not getting the maximum work then
we must measure a potential difference
that is less than the emf.

This explanation never uses the
word "resistance."

* * * On the topic of emf...
We should remember that the emf
terminology is deeply entrenched in
physical chemistry and electrochemistry
texts and instruction. Although this
does not justify the situation, it at
least means the physicists should not
change terminology without working it
out with the pchemists and
electrochemists.

It is very frustrating to students and
professors if overlapping disciplines
try to change terminology or ideas
without working with each other. I often
see this problem with mathematicians. They
can go off into their own world and try to
change terminology and/or the way calculus
is taught without telling or consulting
those of us who depend on them to teach
calculus to our chemistry and physics
students in a manner that is consistent
with all our textbooks.



Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu