Coulomb's Law 

The electrostatic force between two point charges is
directly proportional to the product of their magnitudes and
inversely proportional to the square of the distance between
their centers. 

This force will be repulsive or attractive depending upon whether the charges are alike or unlike charges. The force always acts along line joining the centers of the two charges. 

Consider two point charges q_{1} and q_{2} held r distance apart in vacuum. According to Coulomb's law, the magnitude of electrostatic force between the charges is given by:


F ∞ q_{1} x q_{2} / r^{2}


or F = k x ( q_{1} x q_{2} / r^{2} )


Where k is a constant whose magnitude depend upon the system of units employed and the nature of medium in which charges are placed in vacuum, 

k = 1 / ( 4π x ε_{0} )
= 1 / ( 4π x 8.854 x 10^{12} )
= 9 x 10^{9}


where ε_{0} = Absolute permittivity of free space
= 8.854 x 10^{12} C^{2} N^{1} m^{2}


F = ( 1 / 4π x ε_{0} ) x ( q_{1} x q_{2} / r^{2} )


or F = 9 x 10^{9} x ( q_{1} x q_{2} / r^{2} )
