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Re: Lenz's Law



Bernard Cleyet wrote:

when in doubt, use the energy principle. The current direction must
be in the direction that doesn't violate conservation of energy. i.e.
so one doesn't have a perpetual motion machine, i.e. so one does work
on the system. I'm assuming your example is pushing (or pulling) a
magnet into a coil, or passing a magnet past a conductor producing an
eddy current. Same principle applies in the case of a transformer
where the coils are fixed.

Hmmmm.

I agree that conservation of energy is good.
I agree that perpetual motion machines are bad.

But alas energy arguments are not a reliable
method of solving problems of this sort.

The direction of the induced current in the
typical Lenz's law situation depends on the
fact that the conductor, by virtue of its
conductivity, is diamagnetic, at least over
smallish timescales. A superconductor can
be considered a perfect diamagnet.

So, in contrast, consider what happens if you've
got a paramagnetic material. The induced Amperian
currents go in the _same_ direction as the
inducing current ... just the opposite of
what Lenz's law would tell you.

This does not violate conservation of energy.
This does not mean that you can build a
perpetual motion machine by exploiting
paramagnetism. It just means that the inductance
you seen when looking into the inducing coil is
higher than you might otherwise have expected.

===========

This goes to show that explaining stuff is a
tricky business. Just because you have an
argument that leads to the right answer doesn't
mean the argument _logically_ leads to the
right answer.

Many teachers are so familiar with the right
answer that they don't need to invoke any
physics at all to get the right answer; they
can just say roses are red, violets are blue,
therefore the current goes that-a-way... without
noticing that the conclusion, while correct, does
not follow from the alleged explanation.

I've seen lots of textbooks that feed students
a steady diet of roses-are-red non-sequitur
"explanations". It is little wonder that the
students have trouble applying these "ideas" to
new situations.