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Q)

A $900 \;pF$ capacitor is charged by $100\; V$ battery. How much electrostatic energy is stored by the capacitor?

$\begin{array}{1 1} 4.5 \times 10^{-6}\;J \\ 4.5 \times 10^{-4}\;J \\ 3.5 \times 10^{-6}\;J \\ 4.5 \times 10^{-4}\;J \end{array}$

$Q= CV=900 \times 10^{-12}F \times 100\;V$
$\quad = 9 \times 10^{-8}\;C$
$\qquad= \large\frac{1}{2}$$CV^2 =(1/2)QV$
$\qquad= (1/2) \times 9 \times 10^{-8}\;C \times 100\;V$
$\qquad= 4.5 \times 10^{-6}\;J$