Question

compute the electric force between two charges of 5×10^-9 C and -3×10-8 C which are reparated by r=10 cm

Answer 1

The Coulomb's law says

$$F=k\frac{q_1q_2}{r^2}\\ =9*10^9*\frac{5*10^{-9}*3*10^{-8}}{0.1^2}=1.35*10^{-4}\;\rm N$$