Physics Photo of the Week
February
18,
2011
Meissner Effect - Photo and video by Kyle Brown
Discussion by Owen Kroening and Kyle Brown
Superconductivity,
a
property shared by most metals, occurs when the temperature of a
metal is depressed to a point at which the metal generates no
electrical resistance.
While this phenomenon was first observed at liquid helium temperatures
(boiling point: 4 K) using mercury metal; it has since been observed at
liquid hydrogen temperatures (boiling point: 20 K) using other metals
and some alloys. Much more recently a new class of
superconducting materials was discovered consisting of a
crystalline-structured, ceramic composite (YBa2Cu3O7).
This
material will become superconducting at the temperature of much
cheaper liquid nitrogen (boiling point: 77 K).
In these photos, a magnet can be seen floating 
above a disk of YBa2Cu3O7
material, which has been cooled using liquid nitrogen. This phenomenon,
known as the Meissner effect, discovered in 1933 by Walther Meissner
and Robert Ochsenfeld, displays a unique property inherent in
superconductors where external magnetic fields are expelled in the
superconductive state. The reason for this expulsion is due to the
superconductor’s tendency to create electric currents at its surface,
which effectively cancels any applied field within the superconductor.
Because this expulsion does not change over time, the observed effect
remains constant, as the produced currents do not decay, so long as the
superconductor remains below critical temperature. The name
‘superconductor’ appropriately refers to this ever-sustaining
conductive ability.
In a normal state (above critical temperature), a superconductor will
not exhibit the effective expulsion of weak magnetic fields, but allow
the fields to penetrate as is normal with other conductors. As
seen in the video here, the magnet rests on the black superconducting
disk. The temperature is above the transition temperature and the
superconductor is in its normal state. Once the temperature of
the superconductor cools below the transition temperature, the
repulsion of the external field created by the small magnet becomes
observable and the magnet levitates.
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.
All photos and discussions are copyright by Donald
Collins or by the person credited for the photo and/or
discussion. These photos and discussions may be used for private
individual use or educational use. Any commercial use without
written permission of the photoprovider is forbidden.
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