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SUPERCONDUCTIVITY - CHARACTERISTICS AND APPLICATIONS

Superconductivity:


Definition:

The phenomenon of obtaining the property of zero resistance and infinity of conductivity is defined as superconductivity. The substances which exhibit the property of superconductivity are called Superconductors.
Explanation:
superconductivity was discovered by Kamerlingh in 1911. He observed that when a substance is cooled to very low temperature, the resistance of the substance vanishes and the current flows
through it without any abstraction and variation.
The experiment was done with Helium and found that Helium will be a super conductor at the temperature 4.2 K.

Critical temperature ‘Tc’ (or transition temperature):

The temperature where an ordinary substance becomes a superconductor is defined as critical temperature. 
It is denoted by Tc. 
examples : For Mercury Tc=4.15 K and for Cd, Tc =0.56 K

Tungsten     0.01K
Aluminum  1.19 K
NiBi            4.25 K

ZrAl2          0.30 K

Characteristics of superconductors:

1. At room temperature superconductors have large resistance.
2. They act as superconductor only below the critical temperature.
3. The value of critical temperature varies with the substance.
4. The critical temperature depends upon isotopic number. It decreases with increase of atomic weight of isotope.
5. Superconductivity is not effected by adding impurities to the substance.
6. Superconductors are perfectly dia magnetic substances below critical temperature.
7. The elastic properties do not change in this phenomenon.
8. The thermal properties are changed rapidly as the temperature of superconductor is decreased.
9. If the super conductor is placed in an external magnetic field and the field is increased, at a particular value of the field the substance loses the superconductivity. This point of the field is known as critical or transition field ‘Hc’
Effect of external field on superconductors.
If the super conductor is placed in an external magnetic field and the field is increased, at minimum value of applied field the substance loses the superconductivity. This minimum value of the field is known as critical or transition field ‘Hc’
Let Tc is the critical temperature where the substance exhibits superconductivity .If Hc(0) is the value of critical field at the absolute zero temperature and Hc(T) is the value of critical field at any temperature T , then Hc(T) can be expressed as the function of Hc(0).


 Hc(T) =  Hc(0). [ 1- (T^2 / Tc^2) ]

Applications of superconductors.

1. Superconductors are used in power transmission.
2. They are used to prepare strong electro magnets.
3. They are used in electrical appliances like cryotron
4. They are used in maglev vehicles like trains.
5. Used to produce a very strong magnetic field.
6. They are sued in superconducting quantum interference devices (SQID).
They are the storage devices of magnetic flux.

7. In the progress of computer technology superconductors are used.


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