Re: jumping ring demo

[Rondo JEFFERY <RJEFFERY@WEBER.EDU>]

Herb Gottlieb informs us (incorrectly) that the ring is ALWAYS
repelled in the jumping ring:

Rondo N. Jeffery
Physics Department
Weber State University
Ogden, UT 84408-2508

> > > Herbert H Gottlieb <herbgottlieb@JUNO.COM> 04/12 10:16 AM >>>
> When energized by an alternating current, the coils in the
electromagnet
> set up a changing magnetic field that induces a changing current
the
> aluminum ring. According to Lenz' Law, the direction of this
> instantaneous current in the alumininum ring is such that it creates
a
> magnetic field that opposes the magnetic field of the coils.

> Thus, regardless of the alternating reversing field created by the
coils,
> the induced magnetic field surrounding the aluminum ring *** always
repels  ***
> that of the coils.  [emphasis added]

> Herb Gottlieb from New York City
> (Where the electromagnetic coils and the aluminum ring were
divorced
> because they never had any attraction for each other)

Herb,
   If you go through the four quadrants of the cycle, remembering
that the induced current is in such a direction as to OPPOSE the
CHANGING magnetic flux, you notice that the ring is first repelled,
then attracted, then repelled, then attracted.  The net effect-- if
the ring current and induced emf in the ring are IN PHASE-- is that
there is ZERO NET FORCE averaged over a complete cycle.  Also, the
time it takes the ring to clear the top of the extended iron core is
several cycles at 60 Hz, and hence the argument cannot be made that it
is an impulse on the first quarter cycle.  (Two popular textbooks--
Serway and Griffiths give different wrong explanations for this!)   In
fact, the only way the ring can jump off the core is if there is a
PHASE SHIFT between the ring current and the induced emf.
   I have been working on this interesting effect off and on for
several years, and have presented my results and analysis at our local
(ID-UT) AAPT section meeting.  I'm trying to get it submitted to a
journal, either AJP or TPT.  There are several other interesting
features to this problem besides just the phase shift which I think
people would be interested in seeing.  Several other papers have been
published on the jumping ring, as pointed out by others during this
thread.  Most indicate the importance of the phase shift.  The problem
with submitting another paper, of course, is that is can't be JUST
another paper.  :(

   (BTW, Herb, I love the NYC quips in your postings!  Keep it up.
:-)   )