A 900 pF capacitor is charged by a 100V battery. The electrostatic energy is stored by the capacitor is ______ $\times {10}^{-6} J$ ?

The charge on the capacitor is 
$q=CV=900\times {10}^{-12}F\times 100V=9\times {10}^{-8}C$ 
The energy stored by the capacitor is 
 $$\begin{gathered} U=\frac{1}{2}C{V}^2=\frac{1}{2}qV=\frac{1}{2}\times 9\times {10}^{-8}C\times 100V \\ =4.5\times {10}^{-6}J \end{gathered}$$