[Phys-L] Re: Lenz's law

[John Denker <jsd@AV8N.COM>]

On 04/11/05 17:49, Mangala Joshua wrote:
> I have a question about a demonstration showing  Lenz's Law. While a
> ring is hanging  if we bring a fairly strong magnet towards it the ring
> repels and  when the  magnet is moved out  the ring attracts. We usually
> don't see this effect if we have a ring that has a slit in it. However
> today I used a very strong magnet and I was able to move the ring with
> the slit.

I hypothesize that the material of the split ring
has nontrivial cross-sectional area.  So ... consider
the following path, where there is a high current
density just inside the surface of the material.
      _________________
     |                |
     |\/   ______   /\|
     |    |      |    |
     |    |      |    |
     |    |      |____|
     |  /\|       ____
     |    |      |    |
     |    |      |    |
     |    |______|    |
     |\/            /\|
     |________________|


This is the celebrated "skin effect" discussed in
E&M books (e.g. Jackson).

The total force depends on the area from which the
field is exluded by the screeing currents.  For
the split loop, the area will just be the projected
area of the material.  For the unsplit loop, the
area will be larger, namely the whole area
encompassed by the loop, including the hole.
_______________________________________________
Phys-L mailing list
Phys-L@electron.physics.buffalo.edu
https://www.physics.buffalo.edu/mailman/listinfo/phys-l