Derive the expression for the resistivity of a substance and give the meaning of each symbol which occurs in it.

The resistance of a given conductor is directly proportional to its length, i.e.,
R ∝ l ----(1)
The resistance of a given conductor is inversely proportional to its area of cross-section, i.e.,
R ∝ ----(2)
From equation (1) and (2), we get
R ∝
R = ----(3)
where p is the constant known as resistivity of the material of the conductor.
Rearranging equation (3), we get
p =
where
R = Resistance of the conductor
A = Area of cross-section of the conductor
l = length of the conductor