Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

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





-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of John Denker
Sent: Monday, March 31, 2014 5:12 PM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] Lenz's law and conservation of energy

On 03/31/2014 09:23 AM, Savinainen Antti wrote:
I hope that questions in fortcoming exams will be of higher standard.

Agreed!

I think that the intention of the examinors 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".

This raises a terminology question. 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. Lenz's law has to do with changing magnetic fields. If a conducting material is in an external magnetic field that is increasing, eddy currents will develop in the conducting material that will produce a magnetic field in the direction opposite that of the external magnetic field, but if the external magnetic field is decreasing, the eddy currents will produce a magnetic field in the same direction as that of the external magnetic field. I don't think it is appropriate to use the terms diamagnetism and paramagnetism respectively for this phenomena. Is it?


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.
_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l