Two positive point charges of 12 and 8 microcoulombs respectively are placed 10 cm apart in air. The work done to bring them 4 cm closer is

Electrostatic potential energy when the charges are 10 cm = 0.1 m apart is 

                 $$\begin{gathered} U_1=\frac{q_1{q}_2}{4{\mathrm{\pi \epsilon }}_0\mathrm{r}}=\frac{12\times {10}^{-6}\times 8\times {10}^{-6}}{4{\mathrm{\pi \epsilon }}_0\times 0.1} \\ =\frac{9.6\times {10}^{-10}}{4{\mathrm{\pi \epsilon }}_0} \end{gathered}$$

Potential energy when the charges are brought 4 cm closer, i.e., when they are 6 cm = 0.06 m apart is 

              $U_2=\frac{12\times {10}^{-6}\times 8\times {10}^{-6}}{4{\mathrm{\pi \epsilon }}_0\times 0.06}=\frac{16\times {10}^{-10}}{4{\mathrm{\pi \epsilon }}_0}$

$$\begin{gathered} \therefore Work done=U_2-U_1=\frac{{10}^{-10}}{4{\mathrm{\pi \epsilon }}_0}\left(16-9.6\right) \\ =9\times {10}^9\times {10}^{-10}\times 6.4=5.76 J=5.8 J \end{gathered}$$