Re: EMF

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

Shawn Knudsen wrote:

> I think you should stick with "electromotive force" yet explain the
> appropriate concept to your students.  The reason I say this is simply
> that after your students leave your classroom, the rest of the world
> (barring a few on this Phys-L list) will refer to EMF as "electromotive force."

You are probably correct; telling students that the word
"force" is not appropriate is useful, even when "frog" or
"food" is used instead. But 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-electric 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