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



* /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