Re: induced emf again

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

> Is this derivation desirable? It seems to imply
> that the electric current in the rigid frame is
> due to static charges residing at the ends of
> the sliding rod. But we know that this is not
> true; we know that electric field lines in the
> closed loop have no beginning and no end.
>
> Suppose I remove the rigid frame over which
> the rod was sliding. The same rod is moved in
> a vacuum, still perpendiculary to the magnetic
> field B. Are there going to be static charges of
> opposite sign at the ends of the rod or not?
> Ludwik Kowalski

Here are a couple of thoughts that may help. (Disclaimer: The topic
of motional emf also often confuses me. Gene Mosca has shared some
helpful insights. Maybe he'll have time to add a few remarks also.)

1. You may wish to distinguish in your thinking between the
*conservative* electric field set up by static charges and the
*nonconservative* field due to the dynamics (Faraday's law). You may
find a similar conceptual division to be helpful in thinking about
inductors in answering questions like: Why can we apply the Voltage
Loop Rule to circuits containing inductors in light of the fact that
induced emf's are not potentials?

2. Who says the same amount of charge piles up at the ends with the
frame there as when it is not there? I would certainly expect less
(depending on the sharpness of the angle between the rod and frame)
in the former case.
--
Carl E. Mungan, Asst. Prof. of Physics  410-293-6680 (O) -3729 (F)
      U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
mungan@usna.edu    http://physics.usna.edu/physics/faculty/mungan/