Question

What is the magnitude of the force that a 25μC-charge exerts on a 10μC charge {\rm 8.5\, cm}8.5cm away? (Take k=9×109 N.m2/C2)

Answer 1

The Coulomb's law says

$$F=k\frac{q_1q_2}{r^2}$$

$$F=9*10^9*\frac{25*10^{-9}*10*10^{-9}}{(0.085)^2}\\ =311*10^{-6}\:\rm N$$