Response to Ludwik's 1), below:
We are here concerned with two distinct phenomena typically used to
generate an emf:
a) If a conductor moves "through" a B field, the force qVxB drives charge
carriers ("the generator effect");
b) A B field which changes value in time is the source of an E field which
can drive (initially stationary) carriers (the "transformer effect").
This E field comes from Curl(E) = -dB/dt, and has closed, continuous field
lines with no beginnings or ends (Just as the B field generated by
currents in the isomorphic equation Curl(B) = mu*j ).
c) It happens that these two distinct effects result in an emf which, for
many circuits, can be calculated by the flux rule emf = - dPhi/dt.
Read Feynman Vol II 17-1 and 17-2. RPF points out that the flux rule is a
not completely reliable calculational tool and (when it does work) only
gives you the resulting emf, not the underlying physics. The
unmistakeable physics is in a) and b), along with F=q(E + VxB).
....................
Response to Ludwik's 2), below:
The experiment is to cut a wire in two while sweeping it through a B
field. The two halves will be equally and oppositely charged. This has
been done (successfully). I can't now give a reference, though - stand by.
Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net http://www.velocity.net/~trebor
----- Original Message -----
From: "Ludwik Kowalski" <kowalskiL@MAIL.MONTCLAIR.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Wednesday, May 01, 2002 8:19 AM
Subject: Re: induced emf again
1) What still bothers me is this. The emf=B*v*L can
also be derived from Faraday's law of induction.
d(flux)/dt = B*d(area)/dt=B*L*dx/dt = B*L*v
This derivation does not imply any gradients of
charge density; it contradicts the existence of such
gradients. The other derivation, also in the laboratory
frame, implies the existence of charge density
gradients. Both can not be correct.
2) Does anybody know of an experiment in which
voltage along a rod moving in the B field (for example,
airplane wings) was actually measured? I am referring
to a rod which is NOT sliding along a rigid wire frame
perpendicular to B lines. (The wires connecting the rod
to the voltmeter would have to be parallel to B lines in
order to keep the flux = 0; I suppose.)
Ludwik