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Re: [Phys-L] Lenz's law and conservation of energy



On 04/01/2014 07:31 AM, Jeffrey Schnick wrote:
This raises a terminology question.

Calling it a "question" is a polite way of pointing out
that I got the terminology wrong in my previous message.

I think of a diamagnetic
material as one that, when in a static external magnetic field, is
making a contribution (to the total magnetic field) that is in the
direction opposite to that of the external magnetic field.

Agreed.

Lenz's law has to do with changing magnetic fields.

Agreed. Flux is not the same as Δ(flux).

I blurred the distinction in my previous message.

I don't want to make excuses, but let me explain why
what I said, while wrong, is fixable, not completely
crazy.

There are some things -- such as superconductors and
the circulating currents with atoms -- where the current
flows with no dissipation whatsoever. For these systems,
there is a one-to-one relationship between flux and
Δ(flux), except for a constant of integration, and in
a subset(*) of these systems the constant is zero. In
this /subset/ of situations, what I said makes sense.

This subset suffices to prove the higher-level point
I was making, namely that the "energy" argument for
"proving" Lenz's law is bogus.

On the other hand, at a yet-higher level, it means
I was guilty of something I was complaining about,
namely using a bogus derivation to obtain the right
answer. Sorry.

At least my argument is fixable.

=====================

(*) The other subset consists of situations were the
constant of integration is not zero. Ferromagnets
are a spectacularly interesting example.