Certain Metals when cooled, their
electrical resistance decreases in usual way as in normal conductors, but on
reaching a temperature a few degrees above absolute zero they suddenly lose all
trace of electrical resistance. Then they are said to have passed into
superconducting state.
Kamarlingh Onne was the first to
observe this peculiar property in case of Mercury in 1911. In the course of his
investigation of electrical resistance of a sample of Mercury dropped from 0.08Ω
at about 4K to less than 3x10-6Ω over a temperature interval of 0.01K.
Super Conductors are extra ordinary
because they take no energy at all to make current flow in a conductor and no
energy is lost to friction to sustain the current either. Its electrical
resistance is precisely zero.
Critical or Transition Temperature
One of the important characteristic
property of super conductors is that their electrical resistance, for all
practical purposes is zero below a well defined temperature Tc,
called Critical or Transition Temperature. For instance, semiconducting mixed
oxides of Barium, Lead and Bismuth.
Important Properties of Super
Conductors
1. The current in super conductors
persists for a very long time.
2.
The magnetic field does not penetrate into the body of super conductor. This
property known as Meissner Effect is fundamental characterization of super
conductivity.
3.
When the applied magnetic field ‘B’ is greater than Critical value Bc(T),
the super conductor becomes a normal conductor. Bc(T) is zero at T=Tc and has the
largest value at T=0.
4.
When the current through Super Conductor is increased beyond a Critical value Ic(T),
Super Conductor becomes a normal conductor.
5.
Specific heat of the Super Conducting materials shows an abrupt change at T=Tc
jumping to a larger value for T<Tc.
Effect of Magnetic Field on Super Conductors
The super conducting state of
metal exists only in a particular range of temperature and field strength.
Super conductivity state will disappear if the temperature of specimen is
raised above its critical temperature Tc or if a sufficient strong magnetic field
is employed.
The critical field for which super conducting property loses is a
function of temperature.
Hc =H0[1-(T/Tc)^2]
Hc is Maximum Critical Field
Strength at temperature T
H0 is Maximum Critical Field
Strength at temperature absolute zero.
Tc is Critical Temperature,
the highest temperature for super conductivity
The Meissner Effect
The Meissner Effect
Superconductors which are resistance less materials have an additional property of exclusion of applied magnetic field on it i.e. inside a superconducting material, we always have B=0.
The property of perfect diamagnetism arises in super conductor because when a magnetic field ‘Ba’ is applied surface screening currents circulate so as to produce a flux density ‘Bi’ which every where inside the metal exactly cancels the flux density due to applied field Bi=-Ba.
The property of perfect diamagnetism arises in super conductor because when a magnetic field ‘Ba’ is applied surface screening currents circulate so as to produce a flux density ‘Bi’ which every where inside the metal exactly cancels the flux density due to applied field Bi=-Ba.
For a Super Conductor μr=0;
i.e. B=μrBa=0
This property of exhibiting perfect diamagnetism by
super conductor is known as Meissner Effect.
