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



I think the arguments that have been put forth so far are:
1) This is instability, not a violation of the conservation of energy.
2) This would be a violation of the second law of thermodynamics, not a violation of conservation of energy.
3) If this were a violation of conservation of energy, there would be no such thing as paramagnetism.
4) Flux only penetrates superconducting materials to a certain skin depth, hence this would not be a conservation of energy.
5) Lenz's law comes from Maxwell's equations, not the other way round.
6) Lenz's law has nothing to do with conservation of energy.

I don't buy any of these arguments. Lenz's law distinguishes between two possibilities. I think the other possibility would lead to a violation of conservation of energy. The fact that it would lead to instabilities does not mean that it would not lead to a violation of conservation of energy. If it would also lead to a violation of the second law of thermodynamics, that would also not mean that that it would not lead to a violation of conservation of energy. It being a violation of conservation of energy would not rule out paramagnetism. Paramagnetism has to do with static fields, Lenz's law has to do with the time rate of change of a dot product of a field and an area vector. Argument 5 might be a good academic argument that, the fact that the other possibility would lead to a violation of Lenz's law does not mean that "Lenz's law was derived from the principal of conservation of energy." Argument 6 is a bold assertion. Argument 4 has not been fleshed out enough for me to be able to comment on it.

-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of Philip Keller
Sent: Thursday, April 03, 2014 11:14 AM
To: Phys-L@phys-l.org
Subject: Re: [Phys-L] Lenz's law and conservation of energy

Once again, I come to the list to see what I have been teaching incorrectly for
the past decades. I would guess that a pretty large fraction of what I "know"
about physics comes from Sears, Zemansky, Young and also Halliday and
Resnick. So here goes:

A u-shaped circuit is closed by a bar that can slide across the rails.
There is a magnetic field directed down into the plane of the rails. I apply a
constant force to drag the bar to the right. There are a number of ways to
predict the direction of the resulting current. One of them is to say that the
increase in the enclosed flux due to the increased area of the loop must be
opposed by the outward field caused by the resulting counter-clockwise
current.

Is that not an example of Lenz's law?

And if the current were to flow in the direction opposite to that predicted by
Lenz's law, would I not get a current that would help me to drag the bar?
Couldn't I then let go of the bar and let that induced current continue to
accelerate the bar for me, thus producing free energy?

I am teaching this next week. If I am about to teach it wrong (again), I'd like
to know! (Is this what they mean by "just in time teaching"?)




On Wed, Apr 2, 2014 at 8:48 PM, andre adler <andre.adler@gmail.com>
wrote:

So the quote I posted, earlier in this thread, from the 13th edition
of University Physics connecting Lenz's Law to conservation of energy
is in error - perhaps a more serious concern as presumably far more
students use that book in a course then might come across that question
on a test.


On Wed, Apr 2, 2014 at 8:11 PM, Moses Fayngold
<moshfarlan@yahoo.com>
wrote:

Lenz's law has nothing to do with conservation of energy. It can
be restated as a direct consequence of the Lorentz force law,
without any references to magnetic flux. But formulation in terms of
flux is also
fine
- it emphasizes another aspect of EM interactions: a tendency to
conserve flux. As J. Griffiths put it in his textbook: "Nature
abhors changes in flux". This is manifest in full sway in
superconducting circuits, where
the
magnetic flux just conserves. But precisely this aspect has not been
mentioned among the options a) - d).
It is easy to show on a simple example of a conducting loop
passing through an external magnetic field, that the change of sign
in Lenz's law would affect the direction of the induced current, but
would not affect conservation of energy. As to conservation of
current, there is no such thing at all. There is conservation of
charge, but the Lenz law has
nothing
to do with it, either. So I agree that we have here an ill-posed problem.

Moses Fayngold,
NJIT


On Wednesday, April 2, 2014 7:24 AM, "Folkerts, Timothy J" <
FolkertsT@bartonccc.edu> wrote:

You can heat materials using induced currents.
You can levitate materials using induced currents.

With a little cleverness, you can do both at once, thereby melting
metals without any container!
https://www.youtube.com/watch?v=VydPQuLyEns


________________________________________
From: Phys-l [phys-l-bounces@phys-l.org] on behalf of Bernard Cleyet
[ bernard@cleyet.org]
Sent: Tuesday, April 01, 2014 8:35 PM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] Lenz's law and conservation of energy

On 2014, Apr 01, , at 15:24, Anthony Lapinski
<Anthony_Lapinski@pds.org>
wrote:

I've wondered about induction cooking ever since a student asked
me
about
it in class a few weeks ago. So the pans are made of special
materials that heat up, but the stovetop does not?


The stove top is ceramic w/ the RF coil imbedded. Yes?


Ohms / inch ?!

Give relative efficacy of various metals and frequency, etc.:

Induction cooking - Wikipedia, the free encyclopedia

http://en.wikipedia.org/wiki/Induction_cooking

bc

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