How do we know the resistance of a superconductor is zero?
What is a Critical Temperature?
discovered these phenomena?
mercury is cooled below 4.2 K, it loses all electric resistance -
it becomes a superconductor..
was made by H. Kammerlingh Onnes in 1911. Observations were then made that
other metals also exhibit zero resistivity below a certain critical temperature.
do we know the resistance of a superconductor is zero?
fact that the resistance is zero has been shown by the observation that
currents in superconducting lead rings remains constant for many years with
no measurable reduction in value. Resistance to a current is rather like the
friction experienced by a moving object. If there was no friction and you set
an object moving it would theoretically continue at the same speed for ever.
The same is true with current. Once you set the charge moving it would
continue to move at a sateady rate ( constant current). This has been
observed in supercooled metals - they become super conductors.
induced current in an ordinary metal ring would decay rapidly from the
dissipation of heat energy resulting from ordinary resistance, but
superconducting rings have exhibited a 'decay constant' for the current of
over a billion years!
The critical temperature for a superconductor is
the temperature at which the electrical resistivity of the substance drops to
transition is so sudden and so complete that it appears to be a transition to
a different phase of matter.. Several materials exhibit superconducting phase
transitions at low temperatures.
highest critical temperature was about 23 K until the discovery in 1986 of
some high temperature superconductors. The 'high' temperatures are still
low (125K is considered 'very high'!). These materials with critical
temperatures in the range 120 K have received a great deal of attention
because they can be maintained in the superconducting state with liquid
nitrogen (77 K).They can therefore have easier practical applications. The
surprising thing about them is that they are not metals - but ceramics!
electromagnet is an electromagnet that is built using
coils of superconducting wire. They must be cooled to cryogenic
temperatures during operation. Their advantages are that they can produce
stronger magnetic fields than ordinary iron-core electromagnets, and
can be cheaper to operate, since no enegy is lost as heat because of ohmic
resistance of the windings. During operation, the magnet windings must be
cooled below their critical temperature; the temperature at
which the winding material changes from the normal resistive state and becomes
a superconductor. Liquid helium is used as a coolant for
most superconductive coils.
magnets are widely used inMRImachines,NMRequipment, mass
spectrometers, magnetic separation processes,
and particle accelerators.
are preferred to ordinary electromagnets because:
·They can achieve field that is 10x stronger than
ordinary ferromagnetic-core electromagnets.
·The field is generally more stable, resulting in less
·They can be smaller, and the area at the center
of the magnet is empty rather than being occupied by an iron core.
·Most importantly, for large magnets they can consume much
less power. (Once set up and stable the only power the magnet consumes
is that needed for any refrigeration equipment to preserve the cryogenic
Superconducting power cables
High-temperature superconductors promise to revolutionize
power distribution by providing lossless transmission of electrical
development of superconductors with transition temperatures higher than the
boiling point of liquid nitrogen has made the concept of
superconducting power lines commercially feasible, at least for high-load applications. It
has been estimated that the waste would be halved using this method, since the
necessary refrigeration equipment would consume about half the power saved by
the elimination of the majority of resistive losses.
hypothetical future system called a Super Grid, the cost of cooling would
be eliminated by coupling the transmission line with a liquid hydrogen
Superconducting cables are
particularly suited to high load density areas such as the business district of
large cities, where purchase of an easement for cables (permission to put
cables in has to be bought!) would be very costly.
TASK 3: A MAGLEV TRAIN.
What is a maglev
How does it works?
Maglev-short for magnetic levitation trains can trace their
roots to technology pioneered at Brookhaven National Laboratory. James Powell
and Gordon Danby of Brookhaven received the first patent for a magnetically
levitated train design in the late 1960s. The idea came to Powell as he sat in
a traffic jam, thinking that there must be a better way to travel on land than
cars or traditional trains. He dreamed up the idea of using superconducting
magnets to levitate a train car. Superconducting magnets are electromagnets
that are cooled to extreme temperatures during use, which dramatically
increases the power of the magnetic
commercially operated high-speed superconducting maglev train opened in
Shanghai in 2004, while others are in operation in Japan and South Korea. In
the United States, a number of routes are being explored to connect cities such
as Baltimore and Washington, D.C.
In maglev, superconducting magnets suspend
a train car above a U-shaped concrete guideway. Like ordinary magnets, these
magnets repel one another when matching poles face each other.
Credit: Carly Wilkins
maglev train car is just a box with magnets on the four corners," says
Jesse Powell, the son of the maglev inventor, who now works with his father.
It's a bit more complex than that, but the concept is simple. The magnets
employed are superconducting, which means that when they are cooled to less
than 450 degrees Fahrenheit below zero, they can generate magnetic fields up to
10 times stronger than ordinary electromagnets, enough to suspend and propel a
These magnetic fields
interact with simple metallic loops set into the concrete walls of the maglev
guideway. The loops are made of conductive materials, like aluminum, and when a
magnetic field moves past, it creates an electric current that generates
another magnetic field.
Three types of loops
are set into the guideway at specific intervals to do three important tasks:
one creates a field that makes the train hover about 5 inches above the
guideway; a second keeps the train stable horizontally. Both loops use magnetic
repulsion to keep the train car in the optimal spot; the further it gets from
the center of the guideway or the closer to the bottom, the more magnetic
resistance pushes it back on track.
The third set of
loops is a propulsion system run by alternating current power. Here, both
magnetic attraction and repulsion are used to move the train car along the
guideway. Imagine the box with four magnets—one on each corner. The front
corners have magnets with north poles facing out, and the back corners have
magnets with south poles outward. Electrifying the propulsion loops generates
magnetic fields that both pull the train forward from the front and push it
forward from behind.
This floating magnet
design creates a smooth trip. Even though the train can travel up to 375 miles
per hour, a rider experiences less turbulence than on traditional steel wheel
trains because the only source of friction is air.
Another big benefit
is safety. Maglev trains are "driven" by the powered guideway. Any
two trains traveling the same route cannot catch up and crash into one another
because they're all being powered to move at the same speed. Similarly, traditional
train derailments that occur because of cornering too quickly can't happen with
maglev. The further a maglev train gets from its normal position between the
guideway walls, the stronger the magnetic force pushing it back into place
This core feature is
what's most exciting to Jesse Powell. "With maglev, there is no driver.
The vehicles have to move where the network sends them. That's basic physics.
So now that we have computer algorithms for routing things very efficiently, we
could change the scheduling of the entire network on the fly. It leads to a
much more flexible transportation system in the future," he said.
While this exciting
technology isn't deployed in the United States today, if Powell and his team
get their way, you could someday be floating your way to your next destination.
Multiple Choice questions: Superconductivity
1.The current in a
voltage drop across it
small voltage drop across it
large voltage drop across it
electric field around it
first observed by
3.Super conductivity is
hydrogen at 4.2 K
mercury at 4.0 K
mercury at 4.2 K
potassium at 4.2 K
4.The first successful
theory on superconductivity was due to
Ampere and Schrieffer
Bardeen Cooper and
5.At the critical
temperature, the resistance of a super conductor
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READING TASK: TASK 1: SUPERCONDUCTIVITY
Questions: What is a superconducting magnet?
The first commercially operated high-speed superconducting maglev train opened in
It leads to a much more flexible transportation system in the future," he said