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Re: Thermodynamics and Lenz's Law



Hi all-
I would guess that most of us have heard this kind of argument
made when we learned Lenz's law because it is an excellent mnemonic for
the sign. My problem lies in the purported connection with thermodynamics
which, in most formulations, connects the second law with statistics.
I think that the distinction between the two becomes clearer if
we consider what happens when we run time backward. The magnetic field
changes sign (it is proportional to a current) and so does dE/dt (partial
derivative). Lenz's law is still valid.
On the other hand the increase of entropy of closed systems
specifies the direction of increasing time. The second law is therefore
not valid in the "wrong" direction of time.
In a 4-dimensional formulation of electrodynamics Lenz's law is
not a separate law but is connected with ampere's law as my argument above
suggests. The validity of Lenz's law is therefore guaranteed by the
4-dimensionality of our world, or, if you like, by the fact that we live
in Riemannian rather than Euclidean 4-space.
Regards,
Jack

On Sat, 10 Mar 2001, Doug Craigen wrote:

Peter Schoch wrote:

I have just had a student ask an intriguing question (at least to me).
Unfortuantely, I have no idea how to answer him, so I'm asking for help.

His question, cleaned up a bit, had to do with Lenz's Law for
Induction. He wanted to know if somehow it was tied to the idea in
thermodynamics that no "engine" could be 100% efficient. His reasoning
was that if The induced current wasn't in opposition to the inducing
flux, we could create a perpetual motion machine which would violate
entropy.

Now, I have never linked these two concepts together before. At first
blush it seems reasonable, but I don't feel comfortable enough with
either concept to say it is "definitively" so.

Any comments would be appreciated.

The student is proposing that an inverted Lenz's Law would enable
perpetual motion machines of the "2nd kind". Off the top of my head I
don't see how this works except that it would certainly enable machines
of the "first kind" (violate first law of thermodynamics) which could
then enable second kind machines as a subset.

Actually I've used this argument whenever I present Lenz's Law as a
means of making the dynamics more obvious - "what if" if worked the
other way and showing why the law must be so if we accept the first
law... imagine a magnet sitting beside a wire loop. Now give the magnet
a tiny bump toward the loop. More flux goes through the loop, so if the
result was to generate more flux in the same direction - draw those
lines on and you will see that they will further accelerate the magnet,
causing more change in the flux, causing more acceleration etc. It
would be not only inherently unstable for magnets to be near conductors,
but in this case a tiny bump could used to produce a great means of
shooting magnets through loops (in clear violation of the first law).
Perhaps somebody here could let me know if there are any flaws in the
argument (its more easily explained with a chalkboard for drawing the
various stages).

\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\

Doug Craigen
http://www.dctech.com/physics/about_dc.html


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