Resistivity
Resistivity is known as specific electrical resistance or volume resistivity. It can be defined as the intrinsic property of a given material that shows how it opposes the flow of current. It can also be defined as the resistance offered by a conductor having unit length and unit area of cross section. So it does not depend upon the length and area of cross section of a material. But the resistance of a material depends upon the length and area of cross section of the material. The resistivity is expressed as ρ = R A/L, where R is the resistance in ohms, A is the area of cross section in square meters and L is the length in meters. The unit of resistivity is ohm meter.
Temperature Dependence of Resistivity
The resistivity of materials depend on the temperature. ρt = ρ0 [1 + α (T – T0) is the equation that shows the relation between the temperature and the resistivity of a material. In the equation ρ0 is the resistivity at a standard temperature, ρt is the resistivity at t0 C, T0 is the reference temperature and α is the temperature co – efficient of resistivity.
Temperature dependence of resistivity in superconducting materials
We know that when an electric current is passed through a conductors some energy is lost in the form of heat. The amount of energy loss varies depending on the resistance of the material. In 1911 some scientists cooled a sample of mercury down to 4.2° above absolute zero. Thus the resistance of the material changed to zero. Thus the first super conductor was discovered. Thus the scientists found that in some circumstances some materials do not show any resistance. The materials with zero resistance are called super conductors. At zero resistance the materials conducts current without any loss of energy. When the temperature of such materials are decreased, the free electrons stop colliding with the positive ions and thus it offers zero resistance. The temperature at which the resistance falls to zero is called Critical Temperature.
When the superconductor is placed in the magnetic field, the magnetic field bends around the material as it does not allow the magnetic field to pass through them. When the intensity of the magnetic field is increased, at a certain point the field is able to penetrate through the super conductor and thus its behavior is destroyed.
Consider an electric current is passed through the superconductor. Suppose the density of the current is increased, at a particular value of current density, it loses its superconductivity and finally behaves like a normal material. The current density above which the material loses its superconductivity is called the critical current density. High temperature, high magnetic field and high current density will destroy the behavior of superconductivity of a material. Now a days these materials are used in MRI machines.
