Superconductivity
When conductors lose all of their electrical
resistance when cooled to super-low temperatures (near
absolute zero, about -273o Celsius). It must be
understood that superconductivity is not merely an
extrapolation of most conductors' tendency to gradually lose
resistance with decreases in temperature; rather, it is a
sudden, quantum leap in resistivity from finite to nothing.
A superconducting material has absolutely zero electrical
resistance, not just some small amount.
Superconductivity was first discovered by H.
Kamerlingh Onnes at the University of Leiden, Netherlands in
1911. Just three years earlier, in 1908, Onnes had developed
a method of liquefying helium gas, which provided a medium
for which to supercool experimental objects to just a few
degrees above absolute zero. Deciding to investigate changes
in electrical resistance of mercury when cooled to this low
of a temperature, he discovered that its resistance dropped
to nothing just below the boiling point of helium.
There is some debate over exactly how and
why superconducting materials superconduct. One theory holds
that electrons group together and travel in pairs (called
Cooper pairs) within a superconductor rather than travel
independently, and that has something to do with their
frictionless flow. Interestingly enough, another phenomenon
of super-cold temperatures, superfluidity, happens
with certain liquids (especially liquid helium), resulting
in frictionless flow of molecules.
Superconductivity promises extraordinary
capabilities for electric circuits. If conductor resistance
could be eliminated entirely, there would be no power losses
or inefficiencies in electric power systems due to stray
resistances. Electric motors could be made almost perfectly
(100%) efficient. Components such as capacitors and
inductors, whose ideal characteristics are normally spoiled
by inherent wire resistances, could be made ideal in a
practical sense. Already, some practical superconducting
conductors, motors, and capacitors have been developed, but
their use at this present time is limited due to the
practical problems intrinsic to maintaining super-cold
temperatures.
The threshold temperature for a
superconductor to switch from normal conduction to
superconductivity is called the transition temperature.
Transition temperatures for "classic" superconductors are in
the cryogenic range (near absolute zero), but much progress
has been made in developing "high-temperature"
superconductors which superconduct at warmer temperatures.
One type is a ceramic mixture of yttrium, barium, copper,
and oxygen which transitions at a relatively balmy -160o
Celsius. Ideally, a superconductor should be able to operate
within the range of ambient temperatures, or at least within
the range of inexpensive refrigeration equipment.
The critical temperatures for a few common
substances are shown here in this table. Temperatures are
given in degrees Kelvin, which has the same incremental span
as degrees Celsius (an increase or decrease of 1o
Kelvin is the same amount of temperature change as 1o
Celsius), only offset so that 0o K is absolute
zero. This way, we don't have to deal with a lot of negative
figures.
Material Element/Alloy Critical temp. (degrees K)
==========================================================
Aluminum -------- Element --------------- 1.20
Cadmium --------- Element --------------- 0.56
Lead ------------ Element --------------- 7.2
Mercury --------- Element --------------- 4.16
Niobium --------- Element --------------- 8.70
Thorium --------- Element --------------- 1.37
Tin ------------- Element --------------- 3.72
Titanium -------- Element --------------- 0.39
Uranium --------- Element --------------- 1.0
Zinc ------------ Element --------------- 0.91
Niobium/Tin ------ Alloy ---------------- 18.1
Cupric sulphide - Compound -------------- 1.
Superconducting materials also interact in
interesting ways with magnetic fields. While in the
superconducting state, a superconducting material will tend
to exclude all magnetic fields, a phenomenon known as the
Meissner effect. However, if the magnetic field strength
intensifies beyond a critical level, the superconducting
material will be rendered non-superconductive. In other
words, superconducting materials will lose their
superconductivity (no matter how cold you make them) if
exposed to too strong of a magnetic field. In fact, the
presence of any magnetic field tends to lower the
critical temperature of any superconducting material: the
more magnetic field present, the colder you have to make the
material before it will superconduct.
This is another practical limitation to
superconductors in circuit design, since electric current
through any conductor produces a magnetic field. Even though
a superconducting wire would have zero resistance to oppose
current, there will still be a limit of how much
current could practically go through that wire due to its
critical magnetic field limit.
There are already a few industrial
applications of superconductors, especially since the recent
(1987) advent of the yttrium-barium-copper-oxygen ceramic,
which only requires liquid nitrogen to cool, as opposed to
liquid helium. It is even possible to order
superconductivity kits from educational suppliers which can
be operated in high school labs (liquid nitrogen not
included). Typically, these kits exhibit superconductivity
by the Meissner effect, suspending a tiny magnet in mid-air
over a superconducting disk cooled by a bath of liquid
nitrogen.
The zero resistance offered by
superconducting circuits leads to unique consequences. In a
superconducting short-circuit, it is possible to maintain
large currents indefinitely with zero applied voltage!
Rings of superconducting material have been
experimentally proven to sustain continuous current for
years with no applied voltage. So far as anyone knows, there
is no theoretical time limit to how long an unaided current
could be sustained in a superconducting circuit. If you're
thinking this appears to be a form of perpetual motion,
you're correct! Contrary to popular belief, there is no law
of physics prohibiting perpetual motion; rather, the
prohibition stands against any machine or system generating
more energy than it consumes (what would be referred to as
an over-unity device). At best, all a perpetual
motion machine (like the superconducting ring) would be good
for is to store energy, not generate it
freely!
Superconductors also offer some strange
possibilities having nothing to do with Ohm's Law. One such
possibility is the construction of a device called a
Josephson Junction, which acts as a relay of sorts,
controlling one current with another current (with no moving
parts, of course). The small size and fast switching time of
Josephson Junctions may lead to new computer circuit
designs: an alternative to using semiconductor transistors.
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REVIEW:
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Superconductors are materials which have
absolutely zero electrical resistance.
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All presently known superconductive
materials need to be cooled far below ambient temperature
to superconduct. The maximum temperature at which they do
so is called the transition temperature.
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