SUPER CONDUCTING MATERIALS
1. Define of super conductivity and super conductors.
The phenomenon of losing the resistivity absolutely to zero, when cooled to sufficiently low temperature ie., below critical temperature (Tc) is called superconductivity.
The materials which exhibit superconductivity phenomena are called superconductors or superconducting materials.
2. What is transition temperature?
The temperature at which a normal material changes into a superconductor is called transition temperature (or) critical temperature (TC).
3. Mention any four property charges that occur in super conductor (or) what are the
properties of a superconductor.
(i) They have zero resistivity.
(ii) They exhibit perfect diamagnetism.
(iii) The super conducting property can be destroyed due to the application of magnetic and electric fields.
(iv) The transition temperature varies due to the presence of isotopes.
The entropy and specific heat decreases at transition temperature.
(vi) The elastic properties, crystal structure and thermal expansion remains constant.
4. What is Meissner effect?
When a material is cooled below its transition temperature i.e., T ≤TC, the material becomes a perfect diamagnetic. The magnetic flux originally present in the material gets ejected out of a superconductor. This effect is known as Meissner effect.
5. Explain the term critical magnetic field in superconductor.
At any temperature below the critical temperature, minimum magnetic field is required to destroy the superconducting property. This magnetic field is known as critical magnetic field (HC).
It is given by the relation, Hc = Ho [1- (T / Tc)2]
HC → Critical magnetic field at any temperature HO → Critical magnetic field at absolute zero
temperature TC → Transitiontemperature of the material
6. What is isotope effect in superconductivity?
In a superconducting material, transition temperature varies with the average isotopic mass M of its constituents.
Tc α[1 / Mα]
Where αis called isotope effect coefficient.
7. What are high TC superconductors? Give an example.
Any superconductor, if transition temperature is above 10 K is called high TC superconductor.
TC = 92K La1.85Ba0.15CuO4
TC = 36K
8. What are the properties of High TC superconductors?
They have high transition temperature.
They have modified pervoskite structure.
Formation of superconducting state in high TC superconductors is direction dependent.
They are oxides of copper in combination with other elements.
9. What are the applications of superconductors?
Superconductors are used for the production of high magnetic field magnets.
By using superconducting materials, it is possible to manufacture electrical generators and transformers in exceptionally small sizes having effiecncy of 99.90% Superconducting materials are used in the construction of very sensitive electrical measuring instruments such as galvanometers.
Superconducting materials if used for power cables will enable transmission of power over very long distances without any significant power loss or drop in voltage.
10. What is magnetic levitation?
The magnetic levitation is based on diamagnetic property of a superconductor which is the rejection of magnetic flux lines. A superconductor can be suspended in air against the repulsive force from a permanent magnet. This magnetic levitation effect can be used for high speed transportation without frictional loss.
11. Distinguish between type –I and II superconductors.
S.No Type –I Superconductors
1. The material loses magnetization suddenly.
2. They exhibit complete Meissner effecti.e., they are completely diamagnetic.
3. There is only one critical magnetic field (HC).
4. No mixed state exists.
Type –II Superconductors
The material loses magnetization gradually.
They do not exhibit complete Meissner effect.
There are two critical magnetic fields i.e., lower critical field (HC1) and upper critical field (HC2).
Mixed state is present.
12. What is SQUID?
SQUID is the acronym for Superconducting Quantum Interference Device. It is a double junction quantum interferometer. Two Josephson junctions mounted on a superconducting ring forms this interferometer. Squids are based on the flux quantization in a superconducting ring. The total magnetic flux passing through the ring is quantized.
It is an ultra-sensitive measuring instrument used for detection of very weak magnetic field in the order of 10-14 tesla.
13. What is meant by persistent current?
When a d.c current of large magnitude is once induced in a super conducting ring, then due to the diamagnetic property of the super conductor, the magnetic flux is trapped inside the ring and hence the current persists in the ring for a longer time. This current is called as persistent current.
14. Define cooper pairs?
The pair of electrons formed due to the electron-lattice-electron interaction, with equal and opposite momentum and spins having the wave vector k-q and k -q are called Cooper pairs.
15. Define coherent length.
It is defined as the distance over which two electrons combine to form a cooper pair.
16. What is cryotron?
Cryotron is a type of switching element made by two different super conductors A and B as shown in fig(Refer cryotron fig in your book), with critical fields HcB >HcA. Here the super conducting property vanishes for the material ‘A’due to the magnetic field produced by material B and hence it can be used as relay (or) switching elements.
17.Distinguish between A.C and D.C Josephson Effect.
S.No D.C Josephson effect
When two super conducting materials are separated by an insulator of very few thickness and are connected by a wire, a d.c. current flows in the external circuit and is called d.c. Josephson effect.
The current persists for a longer time
A.C Josephson effect.
When two super conducting materials are separated by an insulator of very few thickness and are connected to a d.c .power, thane an a.c. microwaves are produced at the junction and this effect is called a.c. josephson effect
The current persists only for a short time.
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