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Re: induced emf again



Since the rod is moving at constant velocity, the net field in the metal
would be zero. The only way to get that is to allow a charge
distribution to occur which would cancel the induced field. It
certainly wouldn't be just charges at the end, since the induced field
is constant, and the field due to the charges would, in the extreme be 1
over r squared.
Anyway, thats the way I imagine it.

joe

On Tue, 30 Apr 2002, kowalskil wrote:

THIS MESSAGE WILL NO LONGER APPEAR
AT THE BOTTOM OF YOUR LIST. THE DATE
WILL BE CORRECT THIS TIME, I HOPE.

It has often been emphasized here that the induced
emf and the static emf (from a battery) are very
different. In one case the E lines are endless loops
in another they begin on positive charges and they
end on negative charges.

I am puzzled by motional emf (v*B*L) produced
when a metal rod of length L slides with the speed
v along the rigid wire frame perpendicular to the
uniform magnetic field B. The v*B*L formula
can be derived in two ways: (a) from Faraday's
law of induction and (b) from balancing two
forces. The second derivation bothers me it goes
like this:

The free carriers in the rod will be subjected to
F'=q*v*B force. Thus one end of the rod will
tend to become positive while another will tend
to become negative. The process will continue
till the electrostatic force F"=q*E (acting on free
carriers) becomes equal and opposite to F'. This
leads to E=v*B and emf=E*L=q*v*B. In other
words the rod is treated as a battery causing a
current in the conducting loop.

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


Joseph J. Bellina, Jr. 219-284-4662
Associate Professor of Physics
Saint Mary's College
Notre Dame, IN 46556