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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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