Superconductivity is a phenomenon observed in several metals and ceramic materials. When these materials are cooled to temperatures ranging from near absolute zero (-459 degrees Fahrenheit, 0 degrees Kelvin, -273 degrees Celsius) to liquid nitrogen temperatures (-321 F, 77 K, -196 C), they have no electrical resistance. The temperature at which electrical resistance is zero is called the critical temperature (Tc) and varies with the individual material. For practical purposes, critical temperatures are achieved by cooling materials with either liquid helium or liquid nitrogen. The following table shows the critical temperatures of various superconductors:
Because these materials have no electrical resistance, meaning electrons can travel through them freely, they can carry large amounts of electrical current for long periods of time without losing energy as heat. Superconducting loops of wire have been shown to carry electrical currents for several years with no measurable loss. This property has implications for electrical power transmission, if transmission lines can be made of superconducting ceramics, and for electrical-storage devices.
The classic demonstration of the Meissner Effect. A superconductive disk on the bottom, cooled by liquid nitrogen, causes the magnet above to levitate. The floating magnet induces a current, and therefore a magnetic field, in the superconductor, and the two magnetic fields repel to levitate the magnet.
Another property of a superconductor is that once the transition from the normal state to the superconducting state occurs, external magnetic fields can't penetrate it. This effect is called the Meissner effect and has implications for making high speed, magnetically-levitated trains (see How Maglev Trains Will Work for details). It also has implications for making powerful, small, superconducting magnets for magnetic resonance imaging (MRI).
How do electrons travel through superconductors with no resistance? Lets's look at this more closely.
The atomic structure of most metals is a lattice structure, much like a window screen in which the intersection of each set of perpendicular wires is an atom. Metals hold on to their electrons quite loosely, so these particles can move freely within the lattice -- this is why metals conduct heat and electricity very well. As electrons move through a typical metal in the normal state, they collide with atoms and lose energy in the form of heat. In a superconductor, the electrons travel in pairs and move quickly between the atoms with less energy loss.
As a negatively-charged electron moves through the space between two rows of positively-charged atoms (like the wires in a window screen), it pulls inward on the atoms. This distortion attracts a second electron to move in behind it. This second electron encounters less resistance, much like a passenger car following a truck on the freeway encounters less air resistance. The two electrons form a weak attraction, travel together in a pair and encounter less resistance overall. In a superconductor, electron pairs are constantly forming, breaking and reforming, but the overall effect is that electrons flow with little or no resistance. The low temperature makes it easier for the electrons to pair up (see A Teacher's Guide to Superconductivity for High School Students for details).
One final property of superconductors is that when two of them are joined by a thin, insulating layer, it is easier for the electron pairs to pass from one superconductor to another without resistance (dc Josephson effect). This effect has implications for superfast electrical switches that can be used to make small, high-speed computers.
The future of superconductivity research is to find materials that can become superconductors at room temperature. Once this happens, the whole world of electronics, power and transportation will be revolutionized.
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