Coulomb’s Law: In presence of two or more charges there is a force of attraction/repulsion which is directly proportional to strength of charges and inversely proportional to the square of distance between them.

$F ∝Q_1.Q_2 \\ \dfrac{F ∝1}{R^2} \\ F ∝ \dfrac{Q_1.Q_2}{R^2} \\ F=k.\dfrac{Q_1.Q_2}{R^2} \\ F=\dfrac{1}{4πε_o ε_r} . \dfrac{Q_1.Q_2}{R^2} N \\ \overline{F}= \dfrac{1}{4πε_o ε_r} . \dfrac{Q_1.Q_2}{R^2} \overline{a_R}$