MadSci Network: Engineering

Re: At what rate of passing will Al reflect a magnets field back upon itself?

Date: Mon Mar 26 13:55:35 2001
Posted By: Ronald Fisch, Physics, Washington University
Area of science: Engineering
ID: 984033406.Eg

The demonstration you describe is indeed an impressive one.
I once saw a similar, but even more impressive version of essentially the
same effect.  It was done on the last day of our sophomore college physics
course, when we had learned enough to understand what was happening.  This
is a demonstration of electromagnetic induction, which is the effect that
makes electric motors work.

The professor charged up a large capacitor.  He had an aluminum disc with
a hole in the center, which fit around a large induction coil.  The
professor closed a switch, and the capacitor rapidly discharged through
the induction coil, generating a large transient magnetic field around
the coil.  The aluminum disc (which had been cooled with liquid nitrogen
to reduce its electrical resistance) shot up and hit the ceiling of the
lecture hall!  This device is a form of the rail gun.

The ability of the rotating aluminum disc to support a permanent magnet
depends both on the rotational speed of the disc, and also on the ratio
of magnetic field strength to weight of the magnet.  Thus, without
knowing the type of magnet being used, it is not possible to say what
is the minimum rotational speed which is needed by the disc.

The demonstration you saw on NOVA probably used a NdFeB magnet, or
some other type of high performance magnet.  These rare-earth alloy
magnets have been developed over the last 20 years.  If you try to
repeat this demonstration with an ordinary iron alloy magnet, it
probably won't work.  So perhaps you are too old to have seen this done
when you were in grade school.

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