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Coulomb’s Laws

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

     Force Between Two Electrostatic Charges

or              F = K Q1Q2

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  

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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