Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

elementary emf



What exactly is an "emf"?

An emf is just the line integral of the electric field around some closed
path.

It seems to me that both of these "definitions" are inadequate in that
they are restricted to emfs that are associated with time varying magnetic
fluxes.

The concept of emf is often presented to students who have no idea of what
an integral is. Here is how I introduce it in Concepts of Science. Positive
and negative charges attract; chunks of metal are neutral because + and -
are "mixed" uniformly. Some kind of work must be done when we separat
charges of different sign. The energy, "at the expense of which", this work
is done can be of many kinds: light (photo-cells), motion (power plants),
chemical reactions (batteries), thermal, mechanical, nuclear, etc. The
processes are different and often quite complicated. But regardless of
details each source (a device for separating mutually attractive charges)
can be charactrized by the amount of work per unit charge. A car battery,
for example, is characterized by 12 J/C or 12 volts.

Then I compare separation of charges with bringing water to a higher
elevation - by human laborors, wind pumps, convenction current, etc.
The water gains potential energy on the way up (so many J/kg if you wish)
and looses it (ability to do useful work) on the way down. We can not see
electrons but we know they have a tendency to "naturally flow from higher
electric elevations to lower". The term "electromotive force" was invented
long time ago. It is a bad term for a quantity which is expressed in J/C
because the unit of force is J/m and not J/C. But everbody uses this term.

This observation gives me a chance to focus on the conventional current,
represented by the direction in which protons would flow (if they could).
Here too the old definition remains in force dispite of what we know today
about the mobility of free electrons and the immobility of bound protons.

What is emf? It is a useful charactristic of an electric source. A source
of highr emf will supply electrons with more electric potential energy
that a source whose emf is lower. The emf can be interpreted from two
persepectives: (a) how much work per unit charge is done by the source
where charges are separated and (b) how much work per unit charge is done
outside the source when already separated charges are allowed to attract
each other through wires. Heat and light from a lamp are examples of
useful effects of the "downhill" flow of electrons.

Separation of electrons from protons by the e*v*B force, for example,
when a straight wire is moved rapidly in a uniform magnetic field, is
not part of this consideration. This should come when students are
already familiar with the basic dc circuits.
Ludwik Kowalski