Mathematically, ,where k is the proportionality constant.

In SI units, Q₁ and Q₂ are expressed in Coulombs(C) and R is in meters.

Force F is in Newtons (N) and , is called the permittivity of free space.

(We are assuming the charges are in free space. If the charges are any other dielectric medium, we will use instead where is called the relative permittivity or the dielectric constant of the medium).

Therefore .......................(2.1)

As shown in the Figure 2.1 let the position vectors of the point charges Q₁and Q₂ are given by and . Let represent the force on Q₁ due to charge Q₂.