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

[Philip Keller <pkeller@holmdelschools.org>]

Thank you both (John and Moses) for the helpful responses.  I'm convinced.
 And I want to add two notes based on your responses:

1. MF's response reminds me that you can't just break ONE law of physics.
 The threads are woven together.  I wanted to reverse the direction of the
induced current without changing anything else.

2. As for JD's suggestion about having a student hold the ring down in a
ring-toss demo, I just tried it myself for the first time.  You might
expect a subtle effect and wonder "will I notice the heating?" I can report
that it isn't subtle.  Holy cow...looking forward to springing this on my
classes.

Thanks,
Phil


On Fri, Apr 4, 2014 at 10:57 AM, Moses Fayngold <moshfarlan@yahoo.com>wrote:

> On Thursday, April 3, 2014 11:29 AM, Philip Keller <
> pkeller@holmdelschools.org> wrote:
>
>
> "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?"
>
>   My answer to the last question is: "No, you could not." And my argument
> for it is as following:
> 1)  The actual direction of the current in the bar is predicted by the
> Lenz law. But you could predict the same direction just from the Lorentz
> force law  F = q V X B, without any other references. This means that at
> least in the textbook example when the change of flux is due to motion of
> the bar in a field B, the Lenz law is merely a corollary of the Lorentz
> force law.
> 2)  In this case, the change of sign in the Lenz law would be equivalent
> to changing sign in the Lorentz force law
> 3)  That would indeed, change the direction of induced current; but by the
> same token, the inverted current would now be subjected to the Lorentz
> force law with the opposite sign. And the product (inverted current times
> the inverted Lorentz force) would produce exactly the same outcome as
> before - you would need to push the bar with the same force.
>  Conclusion:  The necessary force applied to the bar and thereby the
> energy input is insensitive to change of sign in the Lenz law.
>   One could raise another question: change of sign in the Lenz law would
> increase the magnetic field B within the loop. Will this not increase the
> magnetic energy of the system? The answer is "No" because by the same token
> it would decrease B outside the loop. Again, at least in the framework of
> considered example, it is evident that positive and negative contributions
> cancel each other, so that change of sign in the Lenz law would not affect
> the net magnetic energy of the system, either.
>   So my conclusion remains the same: the minus sign in the Lenz law has
> nothing to do with energy conservation.
>
> Moses Fayngold,
> NJIT
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