Re: Lenz's Law

[Hugh Haskell <hhaskell@MINDSPRING.COM>]

At 14:37 +1000 6/7/01, Peter Craft wrote:

> Anyway, a recent advice document circulated around our schools suggested an
> anology could and should be made between Newton's Third Law of
> Motion and Lenz's
> Law.

Maybe someone else sees a symmetry between these two that I don't but
AFAICS I don't see the analogy at all. Lenz's law is related to he
conservation of energy, and NTL is related to conservation of
momentum. They both use the word "opposite" but beyond that I don't
see much connection. There may be somethng down at the Hamiltonian
level, but I don't see it.

In the case of NTL, as long as the (Newtonian) interaction between
the objects exists, the mutual forces between them are equal and
opposite. In the case of Lenz, the "back EMF" (which is of course not
a force at all) is such as to induce a magnetic field opposite to the
change in the previously existing field, but the fields are not equal
and opposite. . . Nah, The two laws aren't even referring to the same
sorts of interaction, it seems to me. Actually, this is all Faraday's
law and all Lenz really did was insert the negative sign into the
equation. Maybe it's just the late hour.

I am, however, a bit bothered by the absence of a negative sign in
front of the change in electric flux in the Ampere-Maxwell law.
Halliday and Resnick's comment that "that's just the way it is," is
not very satisfying. I guess one day I'll have to break out my copy
of Jackson and see what he says about it.

However, I've been wrong before and there are certainly those on this
list with deeper understanding of the fundamentals than I, so I
eagerly await other opinions.

Hugh
--

Hugh Haskell
<mailto://haskell@ncssm.edu>
<mailto://hhaskell@mindspring.com>

(919) 467-7610

Let's face it. People use a Mac because they want to, Windows because they
have to..
******************************************************