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Re: motional emf



Kenny Stephens asked about:
"magnetic forces can do no work"

The first 17 times I read this, I didn't see the point
of the question, but I now see that there are indeed
tricky issues involved. I now see there are two ways
of looking at it:

1) According to the Lorentz force law for a charged
particle, the magnetic force is perpendicular to the
velocity. So the motion of a charged particle through
a magnetic field does not do any work. (When I speak
of the "_motion_ of a charged particle" I mean to
exclude any effects due to flipping the nonclassical
spin of the particle, which are perfectly real but not
relevant to this discussion.)

If there were nothing but magnetic forces, no work would
ever get done by the motion of charges.

2) But we can turn the last sentence on its head. In
an electric motor or in a dynamo (which is the context in
which the question was raised), if there were no magnetic
field, no work would get done.

That is, you could say that the work happens "because of"
the magnetic field. This passes the test for causality,
according to one reasonable formal definition: if you
remove the cause, the effect goes away.

====

Combining points (1) and (2), we see that the causality
relationship for the motor or dynamo roughly agrees with
the following Boolean expression:

(magnetic forces) AND (other factors) IMPLIES (work)

But ... analyzing the chain of causation is not the same
as analyzing the physics.

For starters, among the "other factors" involved, the
on/off switch must be turned on for the motor or dynamo
to do work. So from a macroscopic point of view, the
switch "causes" work to be done, in the same sense that
the magnetic field "causes" work to be done. Alas, that
doesn't describe the physics, certainly not the micro-
scopic physics. If you look closely, you find that
the switch is in one place, while the work happens in
another place.

The magnetic field is somewhat like the switch:
macroscopically speaking, it is causative, but
microscopically speaking, it is never the _proximate_
cause of the work. If you analyze the physics in
sufficient microscopic detail, you find that there
is always some non-magnetic process doing the work.

It is worth noting that the mechanical work done *on*
the dynamo is in the X direction (hand pushing the rod)
while the electrical work done *by* the dynamo is in
the Y direction (electrons pushed out one end of the
rod). That tells you something.

In the lab frame, the magnetic field is necessary for
steering the electrons into places where nonmagnetic
forces can do work on them.

Bottom line: macroscopic chain-of-causation is not
the same as microscopic physics.