A conductor is a material which facilitates the flow of electrical charge, known as current. Typically, these charges are negative, in the form of electrons, however in some circumstances, positively charged ions can be used as a mobile charge.

In traditional conductors, electrical resistance, a quantity comparable to friction, acts to impede an electrical current, through converting electrical energy into other forms, such as heat. Resistance is commonly defined in terms of Ohm's Law, a mathematical relationship given by:

Where I corresponds to current, V to potential difference, and R to resistance. In the case of classical physics, this means that a material can never have an electrical resistance of zero: as Ohm's Law shows, this would result in an infinitely large current. In the realm of quantum mechanics however, such a phenomenon is possible, with materials, known as superconductors, having electrical resistances equal to zero.

History

Superconductors were first invented in 1911 by the Dutch physicist Heike Kamerlingh-Onnes of Leiden University in Holland. He discovered that when Mercury (Hg) is cooled to 4 Kelvin or -269.15°C that it acted as a conductor with no resistance. Kamerlingh-Onnes also experimented with Tin (Sn) and Lead (Pb) and discovered that they all had no resistance when cooled to extremely low temperatures. Since these elements could conduct with no resistance, they were called superconductors. Kamerlingh-Onnes won  the Nobel Prize for his work in 1913.

Heike Kamerlingh-Onnes                               Walther Meissner                                  Robert Ochsenfeld

In 1933, German researchers Walther Meissner and Robert Ochsenfeld made the next great discovery in the science of superconductors. They discovered that “a superconducting material will repel a magnetic field.” This is known as strong diamagnetism or the Meissner Effect. The Meissner Effect causes a magnet to levitate above a superconductive metal.

In 1957, the American physicists John Bardeen, Leon Cooper and Robert Schrieffer from the University of Illinois came up with the BCS theory (named after the first initial of their last names) of superconductivity. The theory helps to explain the behaviour of superconductive materials. For a more detailed description of the BCS theory see: http://www.britannica.com/EBchecked/topic/57052/BCS-theory. In 1972, the three physicists won the Nobel Prize for their work.

                                                 John Bardeen      Leon Cooper    Robert Schrieffer

Another extremely important discovery was made in the field of superconductors in 1986. Researchers Georg Bednorz and Alex Mueller at the IBM laboratory in Switzerland discovered that a class of ceramics called perovskites became superconductive at only 35 Kelvin. They won the Nobel Prize in 1987. That same year, a perovskite ceramic material was found to be a superconductor at 90 Kelvin. Due to this discovery, it was possible to use liquid nitrogen as the refrigerant for superconductors. Materials that displayed these properties are called High Temperature Superconductors because they are superconductive at higher temperatures than most materials. The discovery of High Temperature Superconductors led to dozens of other superconductors being discovered and to the eventual discovery of materials that are superconductive at 135 Kelvin.

                                                 Georg Bednorz                                Alex Mueller

Physics

Superconductivity is a state, much like the state of being a solid, or being gaseous: just as water begins to turn to ice as the temperature decreases, so too does the resistance of a metal begin to decrease as it is cooled. At some critical temperature, a metal begins to exhibit new conductive properties, zero resistance being a common example, and it is at this temperature that a conductor becomes a superconductor.

A conductor in such a state begins to exhibit behaviour known as persistent currents, in which an electrical current flowing through a loop of superconducting wire is able to flow near-perpetually, with little to no energy loss. At the moment, this phenomenon is the closest example of perpetual motion ever observed, with the life-span of some persistent currents being experimentally estimated at over one-hundred-thousand years, and theoretical estimations outliving the lifetime of the universe.

Beyond simple having an electrical resistance of zero, superconductors also exhibit a strange phenomenon known as the Meissner Effect, in which a superconductor is able prevent an external magnetic field from penetrating its interior. This results from the superconductor mirroring any magnetic fields which come in contact with it, resulting in the two fields canceling each other out. This effect was among the first to display just how different superconductors are from theoretical, zero-resistance conductors, and is often demonstrated through using a superconductor to levitate a magnet above it. The mathematics behind proving this phenomenon are beyond the scope of this article, however the formulae used when dealing with the Meissner effect are known as the London equations:

 and 

While superconductivity is not yet understood in its entirety, certain theories, such as BCS theory, do serve to explain many aspects of the phenomenon. BCS theory, named after the three physicists responsible for developing it: Bardeen, Cooper, and Schrieffer, is based upon the idea of Cooper pairs. A Cooper pair is a pair of fermions (particle with ½ spins such as electrons) which bind together at low temperatures, to form a composite boson, that is a particle with an integer spin. These false-bosons have a lesser energy level than their individual electrons, resulting in a slight energy gap at approximately 0.001 eV. This restriction placed on the electron's position, combined with the fact that many more electrons find themselves with the same restriction, result in there being a large number of bosons occupying the same energy state, forming a Bose-Einstein condensate. With a condensate formed, separating the Cooper pairs now requires there there exist enough energy to separate all Cooper pairs, and so at low temperatures, the electrons are able to resist all vibrations, and thus, exhibit no resistance of their own.

Applications

Currently, superconductors are used in many fields, such as in electronics, electricity, medical applications, and even trains.  Superconductors are used in particle accelerators,ultra-sensitivemagnetic detectors called SQUIDS, and in superconducting coil magnets.  

The use of superconductors in our everyday lives is limited by the fact that superconductors must be cooled to very low temperatures in order to be superconductive.  But, laboratories are working on making the cooling process less expensive and better, and looking for materials that can superconduct without cooling.  If these labs succeed, we can expect a revolution in energy production, transportation, computers, science, and many other areas, all thanks to superconductors.

Electronic Filters

The exceptional properties of superconductors enable very high-performance electronic circuits to be made.  Electronic filters are an electronic circuit that uses some select frequencies and blocks all of the others, like a radio plays a signal from one station and blocks all of the others.  Filters are important in electronic circuits, but they are not flawless; filters cannot block all of the other signals, especially if they are close to the required signal.  

Superconductors are used to overcome this obstacle.  Filters made of superconducting materials are more efficient that regular filters.  Superconducting filters allow specific frequencies to be allowed and others to be blocked with almost no interference or unwanted frequencies.  These filters allow antennas for cell phones to pick up signals from further away, and are used in telecommunication satellites.  

Power Cables

Power cables can only carry limited amounts of current, or they would heat up and melt.  Since superconducting power cables because they allow about 1000 times more electric current to flow through them, so smaller cable can transport more current.  Superconducting cables would be unprofitable with current technology, because the cables would have to be cooled to be superconductors, but, prototypes of networks of superconducting cables have been built in small scales, using liquid nitrogen to cool the system.

MRIs: Magnetic Resonance Imaging  

MRI machines use a powerful magnetic force to align the magnetization of some atoms in the body, producing detailed images of the inside of one’s body.  Basically, an MRI works by putting a person inside a huge, strong magnetic field.  This field changes the spin on some protons in the body as they gain energy.  When the filed is stopped, the protons in the body go back to normal, and they release the energy in the form of a photon.  The photons that are released from different types of atoms in the body are what create the images from an MRI.

For an MRI to work, it needs a very wide electric field.  To produce this field, a strong electric current flows through thousands of coils of wire loops.  Metal wires would heat up too much, so the wires are made out of, you guessed it, superconductors.  The wires are plunged in a very cold liquid, such as helium or nitrogen.  Since there is no electric resistance, the current keeps flowing and creating a magnetic field even when the power source is cut off. 

High Speed Trains

The record for the highest train speed was set by a Maglev train in 2003, with a speed of 581 km/h, or 161 m/s.  These trains are levitated and propelled by a magnetic field that is produced by superconductors.  Another group did their project on Maglev Trains. 

See Also

SuperConductors.pdf

Superconductors-CriticalTemperatures.pdf

Maglev Trains

Image Citations 

http://nobelprize.org/nobel_prizes/physics/laure

ates/1987/muller-autobio.html

http://nobelprize.org/nobel_prizes/physics/laureates/1987/bednorz-autobio.html

http://en.wikipedia.org/wiki/File:Kamerlingh_portret.jpg

http://www.ptb.de/cms/en/ib/geschichte-ib.html

http://www.enotes.com/topic/Walther_Meissner

http://www.metox.biz/super_history

http://www.magnet.fsu.edu/education/tutorials/magnetacademy/superconductivity101/fullarticle.html

Notes

• (http://wsabstract.com), Website Abstraction. "Type 1 Superconductors." Superconductors. N.p., n.d. Web. 17 Apr. 2011. <http://www.superconductors.org/Type1.htm>.
• (http://wsabstract.com), Website Abstraction. "Superconductor History." Superconductors. N.p., n.d. Web. 17 Apr. 2011. <http://www.superconductors.org/History.htm>.
• (http://wsabstract.com), Website Abstraction. "Type 2 Superconductors." Superconductors. N.p., n.d. Web. 17 Apr. 2011. <http://www.superconductors.org/Type2.htm>.
• "BCS theory (physics) -- Britannica Online Encyclopedia." Encyclopedia - Britannica Online Encyclopedia. N.p., n.d. Web. 17 Apr. 2011. <http://www.britannica.com/EBchecked/topic/57052/BCS-theory>.
• Bellis, Mary. "Superconductors - Georg Bednorz and Karl Alex ." Inventors. N.p., n.d. Web. 17 Apr. 2011. <http://inventors.about.com/od/sstartinventions/a/superconductors.htm>.
• "Characteristic Lengths in Superconductors." Characteristic Lengths in Superconductors. N.p., n.d. Web. 18 Apr. 2011. <http://hyperphysics.phy-astr.gsu.edu/hbase/solids/chrlen.html>.
• "History of Superconductivity." Qwest.net. N.p., n.d. Web. 17 Apr. 2011. <http://www.users.qwest.net/~csconductor/Experiment_Guide/History%20of%20Superconductivity.htm>.
• "La supraconductivité dans tous ses états." La supraconductivité dans tous ses états. N.p., n.d. Web. 16 Apr. 2011. <http://www.supraconductivite.fr/en/index.php>.
• Vasiliev, B. "About the London penetration depth." arXiv 3 (2011): n. pag. arXiv.org. Web. 16 Apr. 2011.

External Links

An awesome MRI of a beating heart.

 http://www.supraconductivite.fr/en/index.php?p=applications-medical-irm 

The Meissner effect in action:

A concise explanation of the Meissner effect.