We have seen that when two charged bodies are placed near each other mechanical force is experienced on them. According to Coulomb’s first law charges of same nature repel one another and that of opposite nature attract i.e. force experienced between two bodies charged with the charge of same nature will be force of repulsion and force experienced between two bodies charged with charges of opposite nature will be force of attraction.

According to Coulomb’s second law the force between the two charges (i) is directly proportional to the product of the charges (ii) inversely proportional to the square of the distance between them and (iii) depends on the nature of the medium surrounding charges.

Mathematically force F between two point charges of strength Q₁ and  Q₂ placed distance

F∝  Q₁Q₂
         d²


or              F = K Q₁Q₂
                            d²

Where K is any constant. The value of this constant depends upon the surrounding medium and the system of units employed.

In R.M.K.S.A. or S I system the force is measured in newtons, distance in meters charge in coulombs and the force experienced between the two charges is given by an expression

F =  Q₁Q₂  
     4π∈_o∈_rd²

Where ?_o is the permittivity of evacuated space or air and = 8.854 x 10-12 F/m and ∈_r is the relative permittivity of the surrounding medium with respect to evacuated space or air.

Substituting Q₁ = Q₂ = Q (say), d = 1m, ∈_r = 1 F = 9 x 10⁹ newtons in expression

(i)    we get

                  9 X 10⁹ =              Q₂                 
                                4πX 8.854X10^-12X1X(1)²

                                 Q = ± 1 coulomb

Hence one coulomb of charge may be defined as that charge which when placed in air at a distance of one meter for an equal and similar charge, is repelled with a force of 9 x 10⁹ newtons.

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