Resistance Basic Theory:

Resistance may be defined as that property of the substance which opposes the flow of electrical current through it.

The resistance of a wire depends upon its length, area of cross-section, type of material, purity and hardness of material of which it is made of any working temperature.

Resistance of the wire is proportional to the length of the wire R α l

Inversely proportional to the area of cross-section i.e, R  α1/a

Combining the above two facts we have R α l/a or R= ρl/a

Where (rho) is a constant and depends upon the nature of the material and is known as specific resistance or resistivity of the material of the wire.

To determine the nature of the constant ( ) imagine a conductor having unit length and unit cross sectional area, for example a cube whose edges are each of length one unit and let the current flow into the cube at right angles to one face and leave the cube in other opposite face. Then by putting l=1 and a=1 in the above expression R = l/a we will get R =rho. Hence resistance of the material of unit length having unit cross-sectional area is defied as the resistance of the material.

Specific Resistance or Resistivity of a material is also defined as the resistance between the opposite faces of a unit cube of the material

The reciprocal of resistance i.e, 1/R is called the conductance and generally is denoted by English Letter G. It is defined as the inducement offered by the conductor to the flow of current and is measured in mho.

Where k = 1/ and is known as conductivity of material. Hence conductivity is the reciprocal of the resistivity and is defined as conductance between the two opposite faces of a cube. Its unit is mho per meter.