Re: motional emf

["Carl E. Mungan" <mungan@USNA.EDU>]

John Denker made some excellent clarifying comments:

> If there is an error, it comes from misapplying this
> Maxwell equation.  The equation properly applies to
> changing the flux through a given loop.  In contrast
> if you try to apply it to changing the loop around a
> given flux, all bets are off.  Counterexamples abound.

I agree. Another way of saying the same thing -- the point where the
error comes in is converting:
     integral of {partial derivative of B wrt time * area element dA}
into
     total time derivative of integral of {B * dA}
and then applying the latter to circuits in which the area changes.

> Are we getting tangled up in the terminology?  I have
> never figured out what "EMF" is supposed to mean.  I
> prefer discussing things in terms of "voltage".

I have no problem with using the word "voltage". I only call it
"motional emf" because that's the conventional name.

I usually think of emf as meaning a "generated voltage" - example:
what we measure across the terminals of an ac generator or battery
(ignoring the internal resistance), or across the ends of our bar
cutting across magnetic field lines. I agree this is a rather flimsy
nomenclature and my meaning doesn't fit well with the fact that the
voltage across an inductor is commonly glorified in texts with the
equation emf = - L*dI/dt, whereas that across a resistor is simply
voltage = R*I.
--
Carl E. Mungan, Asst. Prof. of Physics  410-293-6680 (O) -3729 (F)
      U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5040
mailto:mungan@usna.edu       http://usna.edu/Users/physics/mungan/