The resistivity of a material is the characteristic of that particular material. The materials can be broadly classified into conductors and insulators.
Ordinary conductors of electricity become better conductors at lower temperatures. The ability of certain metals, their compounds and alloys to conduct electricity with zero resistance at very low temperatures is called superconductivity. The materials which exhibit this property are called superconductors.
The phenomenon of superconductivity was first observed by Kammerlingh Onnes in 1911. He found that mercury suddenly showed zero resistance at 4.2 K (Fig 2.3). The first theoretical explanation of superconductivity was given by Bardeen, Cooper and Schrieffer in 1957 and it is called the BCS theory.
The temperature at which electrical resistivity of the material suddenly drops to zero and the material changes from normal conductor to a superconductor is called the transition temperature or critical temperature TC. At the transition temperature the following changes are observed :
1. The electrical resistivity drops to zero.
2. The conductivity becomes infinity
3. The magnetic flux lines are excluded from the material.
Applications of superconductors 1. Superconductors form the basis of energy saving power systems, namely the superconducting generators, which are smaller in size and weight, in comparison with conventional generators. 2. Superconducting magnets have been used to levitate trains above its rails. They can be driven at high speed with minimal expenditure of energy. 3. Superconducting magnetic propulsion systems may be used to launch satellites into orbits directly from the earth without the use of rockets. 4. High efficiency ore�'separating machines may be built using superconducting magnets which can be used to separate tumor cells from healthy cells by high gradient magnetic separation method. 5. Since the current in a superconducting wire can flow without any change in magnitude, it can be used for transmission lines.
6. Superconductors can be used as memory or storage elements in computers.
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