F = $\frac{1}{4*pi*epsilon_0}$ * |q1q2|/$r^2$ = k|q1q2|/$r^2$ where k = 9 x 10^9 N $m^2$ C^(-2). The force acts along the line joining the two charges. In vector form: $F_{12}$ = kq1q2$\frac{r_12_hat}{r}$^2 where $r_{12}$ is the unit vector from q1 to q2. In a medium with relative permittivity (dielectric constant) K: $F_{medium}$ = $\frac{F_vacuum}{K}$. Important: Coulomb's law applies strictly to point charges. For extended bodies, integration over the charge distribution is required. The constant $epsilon_0$ = 8.854 x 10^(-12) $C^2$ N^(-1) m^(-2) with dimensional formula [M^(-1) L^(-3) $T^4$ $A^2$].