F = (1/4piepsilon_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(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 = 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].