Four condensers each of capacity 4 $\mu$F are connected as shown in figure, if  $V_P-V_Q=15 Volts$ The energy stored in the system is    

 

equivalent capacitance of the diagram is two are in parallel and combination in series.

$$\begin{gathered} \frac{1}{C}=\frac{1}{4}+\frac{1}{4}+\frac{1}{8}=\frac{1}{2}+\frac{1}{8}=\frac{5}{8} \\ C=\frac{8}{5}\mu F \end{gathered}$$

energy stored in the system is $\frac{1}{2}C{V}^2$=$\frac{1}{2}\frac{8}{5}\times {10}^{-6}\times 15\times 15=180\times {10}^{-6}J$

or $1800\times {10}^{-7}J=1800 ergs$