Re: EMF +- battery speculation

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

1) "John S. Denker" reminded us about the essay he posted
last summer; a very important contribution.

http://www.monmouth.com/~jsd/physics/battery.htm#sec-actual-battery

2) Ludwik:
> > Many textbooks do refer to "a non-electric" force (agent)
> > doing positive work on ions inside a battery cell.

JohnD:
> Which textbooks are those?

I should have said "non-electrostatic" force instead of the
"non-electric" force, in previous messages. But I found at
least one reference at home (Fundamentals of Electricity
and Magnetism", by A.F. Kip from University of California,
Berkeley, 2nd edition) in which the term non-electric is used.

3) As for the electric field inside a battery cell let me show
what is written about it in "Univeristy Physics", Young and
Freedman, 10 the edition. Sears and Zemansky are also listed
as authors, plus two contributing authors: Sandin and Ford.

The electromotive force term is introduced on page 809
(with a little apology about the word force, as in most
textbooks). On the next page there is a box representing
a source of emf. It shows two terminals (positive "+" and
negative "-"). There is a vector E going directly from + to -
INSIDE THE BOX. Not a single word about the "thin layers"
near the electrodes can be found.

Two additional arrows appear in this box, they represent
forces acting on charge carriers inside the source. The Fe
arrow is pointing in the same direction as E and it is called
the electrostatic force. The second arrow, Fn, has the same
length as Fe but is pointing in the opposite direction, is is
called the non-electrostatic force. This is Figure 26-10; it
represents the "open circuit" situation.

The next Figure (26-11) shows the same box in the
"complete circuit" situation. This time the Fn arrow
is longer than the Fe arrow. Referring to Fn the authors
write: "this force, operating inside the device, pushes
charges from "-" to "+" in the "uphill" direction against
the electric force Fe. Thus Fn maintains the potential
difference between the terminals. If Fn were not present,
charge would flow between the terminals until the potential
difference was zero. The origin of the additional influence
Fn depends on the kind of source."

How does all this differ from my speculation (see below)?
That speculation was not written after reading the above.
But Sears and Zemansky was the first textbook I used as
a teacher (in 1969) and my ideas were probably strongly
influenced by it.

If the above model of a battery cell is wrong then it should
be criticized at the national level, for example, in The
Physics Teacher or in American Journal of Physics.
Textbook authors would see the criticism of their model
and would have a chance to comment. Most of us are not
knowledgeable enough to decide who is right and who is
wrong. It is hard enough to be a teacher; expecting each
physics teacher to be a scientist is not realistic today.
Ludwik Kowalski
---------------------------------------------------------------
Let me speculate about the origin
of the word "force."  Suppose a primitive cell is made
from two plates in an acid. Plates made from Zn and Cu
have the same size and are parallel to each other. It would be
a nearly ideal capacitor in empty space. But in an electrolyte
the chemical equilibrium will be established. Cu will become
positive (source of E lines) and Zn will become negative
(sink of E lines).

1) There is no load to begin with. Is it true that most of the
electric flux is in the acid between the plates, as in a charged
capacitor? I think so. But the electric field between the plates
does not produce a current inside the electrolyte. Free carriers
are available but they do not drift. Why not? Because in
addition to the electric force (Fe=q*E) each carrier experiences
another force, equal and opposite to Fe. This non-electostatic
force was labeled as emf. What does it move? It moves nothing,
unless a load resistor is connected. What is a gravitational
analogy? A basket with steel balls at some elevation. The balls
do not roll down because there is a non-gravitational force (from
the basket) which is equal and opposite to their weights.

2) Let me continue to speculate. A load resistor (a wire loop)
is then connected to the terminals. Is it correct to say that most of
the flux of E (from Cu to Zn) is now in the wire and not between
the plates of the cell? The answer seems to be "yes". If it was a
capacitor then the plates would discharge quickly. But, unlike in
a capacitor, the mysterious non-electric force, perhaps of chemical
(or QM) origin,  is still present between the plates. It is no longer
balanced by the electric force and each ion is acted upon with a
net force. This net force is responsible for the drift of ions; it does
work on them, it increases their potential energy. How else would
ions move uphill (electrically speaking) between the two plates
inside the battery cell? The current in the outer loop is due to the
electric force (Fe=q*E) but the current inside the cell is due to the
net force which is predominately non-electric. That is how energy
is gained inside a battery and lost outside of it.

A historian of science would look into old books and papers to
trace the origin of the emf term. But I am not a historian. The
only thing I can do is to speculate. What is wrong with the
above scenario? Who invented the emf term and how was it
originally explained? Is it possible to associate the electromotive
force with an electromotive potential energy per unit charge?
Ludwik Kowalski