Re: Lenz's Law

[Jack Uretsky <jlu@HEP.ANL.GOV>]

Hi all-
         I agree with John that there is little analogy between
"Lenz's Rule" (Feynman's term) and Newton's Third Law.  I disagree that
that the importance of the Third Law is that it leads to conservation of
momentum.  It is true that the conservation law can be argued from the
third law.  But Feynman gives an alternative argument from Galilean
relativity (the second law).  See the Lectures I-10-2.
        I think that the real importance of the third law lies in its use
for solving problems by isolating small parts of a complicated system and
analyzing each part separately.  So we say, in our shorthand lingo, that
we can make a "free-body diagram" for each part of the system and apply
Newton's second law to that part.  We then use our knowledge of the
relationship of the parts to discover how the system functions.
                                                Regards,
                                                        Jack

On Thu, 7 Jun 2001, John S. Denker wrote:

> At 02:37 PM 6/7/01 +1000, Peter Craft wrote:
> >  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.
>
> I'm never quite happy calling it "Lenz's Law".  I'd rather call it "Lenz's
> tendency", because it is such a limited and inexact rule.  Although there's
> nothing wrong with inexact laws (most of our laws are inexact if you look
> closely enough) my point is that Lenz's "law" is a lot lower on the totem
> pole than Newton's third law.
>
> Newton's third law is related to conservation of momentum.  This is one of
> the most profound and quantitative laws of physics.  It applies to all
> three components of momentum, for all objects big or small, red or green or
> purple.
>
> Lenz's "law" is related to the conservation of flux.  It constrains only
> one component of the magnetic field;  the two components parallel to the
> surface are unconstrained.  There is real conservation only for type-I
> superconductors, not for type-II superconductors and certainly not for
> ordinary non-super conductors.  There is no conservation for
> non-conductors.  There is no practical conservation even for conducting
> materials, if the material is sliced in to thin mutually-insulated sheets.
>
> As applied to an ordinary chunk of metal, Lenz's tendency gives a rule of
> thumb as to the sign of the initial (short-term) effect.  For times greater
> than t=0, there is no quantitative prediction.  In contrast, Newton's law
> makes a highly quantitative prediction for all time.
>
> Conservation of momentum is the most convenient and reliable way to
> remember a powerful concept.  In contrast, I find Lenz's tendency to be
> much less convenient and much less reliable than a direct appeal to the
> Maxwell equation (V = phi dot) and Ohm's law.
>
> A "law" that gives me only one bit (the _sign_ of the initial effect) is
> pretty small potatoes compared to the power and grandeur and precision of
> conservation of momentum.
>
> I would say that the concepts are
>   -- in some ways analogous, but
>   -- in many ways not analogous.
>
> It seems misleading to say "an analogy could and should be made" without
> detailing how severely limited the analogy is.

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