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

[Jeffrey Schnick <JSchnick@Anselm.Edu>]

It seems to me that you would always have a runaway effect.  If changing flux through a conducting loop were to result in more flux in the same direction as the change, then, including the self-induction, the flux would be changing faster, which would result in more flux in the same direction as the change meaning even faster changing flux, so the magnetic field produced by the induced current would always be increasing and the energy of the magnetic field would thus continually be increasing.  (There could be a period of time while the total magnetic field was decreasing if that which got the ball rolling was a decreasing flux but after it got to be zero it would be continually increasing in the direction opposite that of the original external magnetic field and hence from that time on the energy in the magnetic field would be continually increasing.)

> -----Original Message-----
> From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of andre adler
> Sent: Tuesday, April 01, 2014 11:37 AM
> To: Phys-L@phys-l.org
> Subject: Re: [Phys-L] Lenz's law and conservation of energy
>
> 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