Physics Photo of the Week
February
20, 2009
Levitating magnet
Discussion
by Jenifer Donovan
Generally, a material’s resistivity - its opposition to the flow of
electric current - decreases with temperature. Most common materials
are unable to reach “zero” resistance even when pure. However, a few
materials may reach superconductivity when cooled below a critical
temperature. These materials are called superconductors. Below the
critical temperature, which is unique to each material, superconductors
exhibit “zero electrical resistance” and exclude interior magnetic
fields. The latter characteristic is known as the Meissner effect.
The video picture above shows the black disk consisting of the high
temperature superconductor - a ceramic consisting of YBa2Cu3O7.
This black disk is chilled by liquid nitrogen that reaches the bottom
surface of the disk. Liquid Nitrogen is below the critical
temperature for this material. The small object levitating above
the disk is a tiny magnet. The superconductor levitates the small
magnet as explained below.
In a weak applied magnetic field, below the critical temperature,
superconductors produce spontaneous “surface currents” called
persistent currents. These currents, which do not decay over time, form
a magnetic field that opposes the magnet's magnetic field. Combined,
the two opposing fields yield a “net zero” magnetic field. The
interactions of the two fields can repel nearby magnetic fields. For
example, if a magnet was place atop a superconductor that had been
cooled below its critical temperature, the magnet would float in thin
air. The levitation is caused by the repulsion of the magnet’s field by
the magnetic field of the supercurrents in the superconductor.
Repulsion from the superconductor pushes the magnet “up” while Earth’s
gravity pulls the magnet down so that it does not fly out into
space. If the magnet is lowered onto a normal metal, the induced
currents rapidly die out due to electrical resistance and the magnet
sinks to rest on the metal, in stark contrast to the levitation above
the superconductor.
Interestingly, the levitating magnet may “spin without friction” while
maintaining its position above the superconductor. Flux pinning keeps
the magnet from sliding out of position atop the superconductor. This
effect is due to the magnetic forces of the material becoming entangled
in
microscopic voids in the superconducting ceramic.
Our thanks to Sara Bacon for
help in image processing to obtain the video clip.
Physics
Photo of the
Week is
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
Department. These photos feature interesting phenomena in
the world around us. Students, faculty, and others are invited to
submit digital (or film) photographs for publication and
explanation. Atmospheric phenomena are especially welcome.
Please send any photos to dcollins@warren-wilson.edu.
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