Ans. Superconductors
are those materials that have no resistance to flow of electricity.
Based on
magnetic behavior, the superconductors are classified into following two categories:
1. Type
– I superconductors or soft superconductors.
2. Type
– II superconductors.
Type I
Superconductors: -
In type – I superconductor is one in which
the transition from superconducting state to normal state, in presence of
magnetic field occurs sharply at the critical value HC.
Type II
Superconductors: -
The type II
superconductor is characterized by two critical field HC1 and HC2 and
transition to normal state takes place gradually as magnetic field is increased
from HC1 to HC2. The most
important advantages of type II superconductor is the value of critical field HC2 which is many more times (even 100 times) higher than the value of HC1 for type I superconductor. So, the most
important use of type II superconductor is to built up a device which can work
in high magnetic fields such as superconducting magnets.
Q.4 Explain critical magnetic field and its
dependence on temperature.
Ans. Critical Magnetic Field: -
The superconducting material
restores its normal resistance when a strong magnetic field is applied. The
minimum magnetic field which is necessary to regain the normal resistivity is
called critical magnetic field (HC). If the applied magnetic field exceeds the critical value HC, the
superconducting state is destroyed.
As seen from the
figure, the curve is nearly parabolic and can be represented by the following
relation.
The highest value of HC(0) is 2000 Gauss
and it is exhibited by niobium (Nb). The variation of critical field with
temperature for some superconductors are shown in figure.
Q.5 Explain BCS theory in brief.
Ans. BCS Theory: -
The basis of BCS theory is the
interaction of a gas of conduction electrons with elastric waves of critical
lattice. Ordinarily the electrons repel each other by the coulomb force, but in
the special case of a superconductors at sufficiently low temperatures there is
net attraction between two electrons that forms the ‘cooper pairs’. Below
critical temperature TC, the attraction permits the formation of
cooper pairs that are pairs of electrons having opposite momenta and spin. All
cooper pairs move in a single current motion, so a local perturbation, like an
impurity, cannot scatter an individual pair. Once this, collective highly
coordinate state of coherent super electrons is set in motion, its flow is
without any dissipation.
Q.6 A long thin superconducting wire of a metal
produces a magnetic field 105´103 amp/m on its surface due to the current
through it at a certain temperature T. the critical field of the metal is 150 ´ 103 amp/m at T = 0 K. The critical temperature TC of the metal is 9.2 K. What is the value of T?
Ans. As we know that
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