Effect of temperature on materials

Effect of temperature on materials
The resistance of all pure metallic conductors increases with increase in temperature but the resistance of the insulators and non-metallic materials generally decreases with increase in temperature.

The resistance of conductors increases with the increase in temperature , on high temperatures good conductors become bad conductors even.

The insulators lose their insulating power with the increase in temperature and some may even become good conductors if they are strongly heated.

Insulation Resistance of a Single Core Cable

Insulation Resistance of a Single Core Cable
The cable conductor is provided with an insulation of suitable thickness so as to avoid leakage of current. The path for leakage current is radial , through the insulation material. the opposition offered by the insulation to the leakage current is called the insulation resistance of the cable.

Semi-conductors

Semi-conductors
Materials lying between conductors and insulators are known as semi-conductors.
Example:- Germanium, silicon, sulphur, carbon 

Insulators

Insulators
The materials which offer high resistance to the flow of current.

Conductors

Conductors
The materials which allow least resistance to the flow of electric current pass through it.
or
The materials which allow the free flow of electric current to pass through it.

Types of supply

Types of supply
AC:-A current (or voltage) is called alternating if it periodically changes its direction and magnitude
DC:- A direct current( or voltage) is steady and in one direction. 

Current Density

Current Density
Current Density is the product of electrical conductivity or material and applied electric field.

Conductivity

Conductivity or specific conductance
It is defined as th conductance between the two opposite faces of a unit cube. Also the reciprocal of resistivity or specific resistance.

Unit:- Siemens/metre

Conductance

Conductance 
The reciprocal of resistance. It is defined as the inducement offered by the conductor to the flow of current .

Unit:- mho

Resistivity or Specific Resistance

Resistivity or Specific Resistance
The resistance of the material of unit length and unit cross-sectional area.Also defined as the resistance between opposite faces of a unit cube of that material.

Unit- Ohm per metre cube