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



  On Apr 4, 2014, at 9:59 AM, John Denker wrote:

"My favorite demonstration of Lenz's law is the "jumping
ring" demo.
  http://www.rpi.edu/dept/phys/Dept2/physlabsite/demoFaradayjumpingring.htm

Advantages include
-- There is no doubt that it demonstrates Lenz's law,
  namely it verifies the /sign/ of the induced current.
  The sign is such that the ring moves /away/ from the
  high-field region.
-- All the important parts are out where you can see
  them."

  The
"jumping ring" demo is only one half of the story. The second half is
the case when the external field through the ring is decreasing. In this
case, the ring will be attracted to the strong field region. In terms of the described experiment, the
setup must be rearranged so that the ring is initially placed on a
support below the vertically oriented solenoid with current on. When the current is turned off, the ring will jump up as in the previous case,
but now it will be a jump toward solenoid. In terms of the ring moving across the magnetic field region, the
repelling force during the entering stage will convert into attracting
force during the exit. In both parts of the story, work needs to be done when pushing the ring into the strong field region and when pulling it
out. 

  And the most interesting aspect of the story is that changing the sign in Lenz's law would indeed change the sign in the induced currents, but the ring would still jump away from the field in the first arrangement and toward the field in the second arrangement. 

 

  Moses Fayngold,

   NJIT


    




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