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Re: EM Induction - A conceptual question



"Fakhruddin, Hasanbhai" wrote:

Imagine a circular loop of wire of radis R. A uniform magnetic field is
distributed perpendicular to its surface and confined to a concentric
circle of radius r < R.

I had a hard time figuring out what this meant by "the surface".
I assume this meant to say "perpendicular to the plane containing
the loop" or some such. The surface of the loop itself, strictly
speaking, is a torus ... but context indicates that's not what was
meant.......

Will the change in the magnetic field strength induce a current (emf)
in the loop?

A current is by no means synonymous with an EMF.

If the aforementioned magnetic field changes then yes,
a voltage will be induced, !!provided!! it is not cancelled
by changes in other magnetic fields.

The foregoing proviso is nontrivial; for instance a
superconducting ring will produce a field of its own
of just exactly the right size to make sure that there
is no voltage drop around the ring. So in this case
we get an induced current with no voltage.

In the opposite limit, of a nonconducting ring, we get
a voltage with no current.

By the Faraday's law there should be an emf induced becuse of changing
flux.

Sure.

However, my contention is that B-field lines are not "physically"
linked to the loop. Why should changing field affect anything in the
loop?

1) Well, in the topological sense, the magnetic field lines
most certainly are "linked" to the loop.

2) To increase the amount of flux inside the loop requires
bringing in a new flux line from outside, so there must
be a time and a place where the flux line crosses through
the material of the loop. This picture is 100% consistent
with our notion of _local_ laws of physics.

3) On the other hand, we must keep in mind that flux-lines
are only an artifice, invented by humans, and do not 100%
accurately represent the known laws of physics. People have
spend innumerable man-years trying to construct a "mechanical"
model of the electromagnetic field, without success. For
instance:

a) if you fly past a magnetic field at just the right
(relativistic) velocity, it disappears altogether -- so the
field lines can't be 100% real.

b) The laws of physics predict magnetic interactions in
places where there is *no* B-field. In some sense, the
A-field knows something the B-field does not.
http://scienceworld.wolfram.com/physics/Aharonov-BohmEffect.html