Calculation of electrostatic energy in $12\mathrm{pF}$ capacitor
Total charge stored in combination
Potential difference across each capacitor
Energy stored, in the capacitor of capacitance 12 $\mathrm{pF}$ ,
$\mathit{U}=\frac{1}{2}{\mathit{C}}_1{\mathit{V}}^2$
$=\frac{1}{2}\times 12\times {10}^{-12}\times 50\times 50\mathrm{J}$
$=1.5\times {10}^{-8}\mathrm{J}$
$\mathrm{C}=$ Equivalent capacitance of $12\mathrm{pF}$ and $6\mathrm{pF}$, in series, is given by


$\therefore$ Charge stored across each capacitor
$\mathit{q}={\mathit{C}}_{\mathit{e}\mathit{q}}\mathit{V}$
$=4\times {10}^{-12}\times 50\mathrm{C}$
$=2\times {10}^{-10}\mathrm{C}$
Charge on each capacitor $12\mathrm{pF}$ as well as $6\mathrm{pF}$
$\therefore$ Potential difference across capacitor ${\mathit{C}}_1$

Potential difference across capacitor ${\mathit{C}}_2$