Three isolated metal spheres A, B, C have radius R, 2R, 3R respectively, and same charge Q. UA, UB and UC be the energy density just outside the surface of the spheres. The relation between UA, UB and UC is
Two charges, \( q_1 = +3 \, \mu C \) and \( q_2 = -4 \, \mu C \), are placed 20 cm apart. Calculate the force between the charges.