Electric Force f12=kq1q2∥(x1−x2)∥2x1−x2∥x1−x2)∥f21=−f12\begin{aligned} &\mathbf{f}_{12}=k \frac{q_1 q_2}{\left\|\left(\mathbf{x}_1-\mathbf{x}_2\right)\right\|^2} \frac{\mathbf{x}_1-\mathbf{x}_2}{\left.\| \mathbf{x}_1-\mathbf{x}_2\right) \|} \\ &\mathbf{f}_{21}=-\mathbf{f}_{12} \end{aligned}f12=k∥(x1−x2)∥2q1q2∥x1−x2)∥x1−x2f21=−f12 q1q_1q1 charge of particle 111 q2q_2q2 charge of particle 222