${\displaystyle F_{12}=k\cdot \frac{Q_1{Q}_2}{r^2}\cdot \frac{r_{21}}{r}=k\cdot \frac{Q_1{Q}_2}{r^2}\cdot e_{21}}$

where F₁₂ is the force exerted on the particle with electric charge Q₁ by the particle with electric charge Q₂ , k a positive constant, r₂₁ the vector from the particle with electric charge Q₂ to the particle with electric charge Q₁, r = |r₂₁| the distance between particles and e₂₁ the unit vector r₂₁/r

NOTE – In vacuum the constant k is equal to 1/4πε₀ where ε₀ is the electric constant.