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

[andre adler <andre.adler@gmail.com>]

The thing I love most about this listserv is the chance for me to learn, so
I appreciate this thread.

What I wanted to write about is the following I found in University
Physics, 13th edition, by Young and Freedman, regarding Lenz's Law, section
29.3 on page 967.  This may enlighten on the origin of the question, and
raise further physics questions on E&M theory.  (I am using this book now
in a class I am in the instructor for; the class has about 480 students,
mostly pre-meds, and no physics majors - they take a separate introductory
sequence).

The statements I want to quote are "Lenz's law also helps us gain intuitive
understanding of various induction effects and of the role of energy
conservation."

and on the same page

"Lenz's law is also directly related to energy conservation.If the induced
current in Example 29.6 were in the direction opposite to that given by
Lenz's law, the magnetic force on the rod would accelerate it to
ever-increasing speed with no external energy source, even though electric
energy is being dissipated in the circuit. This would be a clear violation
of energy conservation and doesn't happen in nature."

Since we have a situation where assuming Lenz's law is false leads to a
violation of energy conservation, it is being singled out over other
conservation laws as being particularly significant; students who use this
book may select energy conservation in that question if confronted with it.

Also, while statements about paramagnetism and diamagnetism are found in
the prior chapter, they are towards the end of the chapter, and very brief.

Putting aside the issue of the question that started this thread, are the
statement's in the text false or misleading?

I take it from what was written earlier, there is no proof of energy
consveration that starts with Lenz's law, but is there a general proof (not
a specific example as I quote above) showing that energy would not be
conserved if Lenz's law were false?

Thanks,

Andre Adler
NYU, Deparment of Physics



On Tue, Apr 1, 2014 at 10:33 AM, brian whatcott <betwys1@sbcglobal.net>wrote:

>  * /From/: John Denker <jsd@av8n.com <mailto:jsd%40av8n.com>>
>  * /Date/: Mon, 31 Mar 2014 14:11:55 -0700
>
> ------------------------------------------------------------------------
>
>
> On 03/31/2014 09:23 AM, Savinainen Antti wrote:
>
> > I hope that questions in forthcoming exams will be of higher
> > standard.
>
> Agreed!
>
>  I think that the intention of the examiners was the following:
> >
>
> I call this an "ESP exam" because it mainly tests the students'
> mind-reading skills, not physics skills.  It requires them to
> divine what the examiners had in mind.
>
>
> > When there is a changing magnetic flux through a conducting loop, an
> > emf is induced and in this case a current as well. The current has an
> > associated magnetic field which must oppose the change in the
> > magnetic flux. If it didn't, it would contribute to the change in
> > magnetic flux and subsequently create a greater induced current.
> > This would violate the law of conservation of energy.
>
> That is a cute way of remembering the sign of Lenz's law
> but AFAICT it is fundamentally wrong physics.  As the
> saying goes, it's bad luck to prove things that aren't
> true.
>
> In accordance with Lenz's law, any conducting object will
> be /diamagnetic/.  If the sign were reversed in Lenz's
> law the object would be paramagnetic.  The argument
> quoted above proves that paramagnetism is impossible,
> because it would "violate conservation of energy".
>
> The argument cannot possibly be right, because there are
> lots of paramagnetic things in the world.  Indeed, there's
> vastly more paramagnetic stuff than diamagnetic stuff.
>
> Note the contrast:
>   *) Rote learning teaches the desired answer with no
>    physics, with no understanding.
>   *) The only thing worse than that is to teach the desired
>    answer with fundamentally /wrong/ physics, which is what
>    we have here.
>
> Here's a hint about the real physics:  When there is
> diamagnetism or paramagnetism involved, the energy isn't
> just a function of the strength of the applied field alone.
> There are other factors involved.
>
>
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
>
> I found this response amusing. I do have a problem with it:
>  it was sent about 10 hours early as far as I can tell.
> Dragging diamagnetism/paramagnetism into the discussion
>  of the effect of a current induced in a conducting ring
> was (forgive my directness) gratuitous. As always, perhaps
> there is something I'm missing?   <grin>
>
> Brian Whatcott    Altus OK
>
>
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