Two  identical rings P and Q of radius 0.1 m are mounted co-axially at a distance 0.5 m apart.  The charges on the two rings are 2 mC and 4 mC respectively.  The work done is transferring a charge 5 mC from centre of P to centre of Q is

0.72J

$ W = Q\Delta V = \frac{Q}{4\pi \epsilon_0}(\frac{q_1}{0.1}+\frac{q_2}{\sqrt (0.1^2+0.5^2)} - \frac{q_2}{0.5}+\frac{q_1}{\sqrt (0.1^2+0.5^2)} = 45\times 10^3 (q_1 - q_2)(10 -2)= 45\times 10^3 (2\times 10^{-6})(10 - 2) = 0.72J$