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

$C = 900 pF = 900 \times 10^{-12} F = 9 \times 10^{-10} F V = 100 V$
$Q = CV = 9 \times 10^{–10} F × 100 V = 9 × 10^{–8} C$
$U= \large\frac{1}{2}$$CV^2 \quad= \large\frac{1}{2}$$ \times 9 \times 10^{-8} \times 100 V$
$\qquad= 4.5 \times 10^{-6}J$