Coulomb's law states that the force F between two point charges $Q_1$ and $Q_2$ is:

1.Along the line joining them

2.Directly proportional to the product $Q_t$ $Q_2$ of the charges

3.Inversely proportional to the square of the distance R between them.'

Expressed mathematically,

$F = \dfrac{kQ_{1}Q_{2}}{{R}^2}$ ....(1)

where k is the proportionality constant. In SI units, charges $Q_1$ and $Q_2$ are in coulombs (C),the distance R is in meters (m), and the force F is in newtons $(N)$ so that $k = \dfrac{1}{4\pi\epsilon_0}$. The constant $\epsilon_0$ is known as the permittivity of free space(in farads per meter) and has the value

$\boxed{\epsilon = 8.854\times 10^{-12} \approx \dfrac{10^{-9}}{36\pi} F/m}$

or $\boxed{k = \dfrac{1}{4\pi\epsilon_0} \approx 9 \times 10^9 m/F }$ ....(2)

Thus eq.(1) becomes

$F = \dfrac{Q_{1}Q_{2}}{4\pi\epsilon_{0}R^2}$