Electric Force

$$\begin{array}{rl}& {\mathbf{f}}_{12}=k\frac{q_1{q}_2}{{\Vert \left({\mathbf{x}}_1-{\mathbf{x}}_2\right)\Vert }^2}\frac{{\mathbf{x}}_1-{\mathbf{x}}_2}{\Vert {\mathbf{x}}_1-{\mathbf{x}}_2)\Vert }\\ & {\mathbf{f}}_{21}=-{\mathbf{f}}_{12}\end{array}$$

• $q_1$ charge of particle $1$
• $q_2$ charge of particle $2$