Re: solenoids

["John S. Denker" <jsd@MONMOUTH.COM>]

At 09:29 AM 4/3/01 -0400, David E. Clark wrote:
> a number of  students in our intro physics
> classes come to the conclusion that the current enters a solenoid on
> the end with the south pole and leaves on the end with a north pole.  Has
> anyone else seen this observation on the part of the students?


Here's a closely-related observation:

I know of at least one other student who was bothered by the funny
asymmetries of electromagnets.  In particular, he was really bothered by
the apparatus shown here:


              _______/ ________
             |                 |
             |                 |   N
            ---                |   .
           -----               |   .
            ---                |   .
           -----               |   S
             |                 |
             |_________________|


Where N...S represents a compass needle.  Initially everything lies in the
plane of the page.  Everything is completely reflection-symmetric and
non-chiral.

Then close the switch!  A magnetic field is created in the neighborhood of
the wire.  The compass needle rotates out of the plane of the page.  The
apparatus suddenly becomes chiral.  This kid, Pierre, could not understand
how something that was initially non-chiral suddenly became chiral.

Fortunately, he didn't let himself be derailed by this frustration.  He
filed the problem away in his brain, and a few years later he was able to
figure it out.

Along the way he learned a few interesting things about symmetries, and
about the inner workings of bar magnets.  They're not as symmetric as they
look.

  http://www.nobel.se/physics/laureates/1903/pierre-curie-bio.html