Question

A 5.00-μf parallel-plate capacitor has charges of 30.0 μc on its plates. how much potential energy is stored in this capacitor?

Answer 1

The potential energy stored by a capacitor is given by:
$U= \frac{1}{2} \frac{Q^2}{C}$
where Q is the charge stored in the capacitor and C is the capacitance.

In this problem, 
$Q=30.0 \mu C=30.0 \cdot 10^{-6} C$
$C=5.0 \mu F=5.0 \cdot 10^{-6} F$
Therefore, the potential energy stored in the capacitor is
$U= \frac{1}{2} \frac{(30.0 \cdot 10^{-6}C)^2}{5.0 \cdot 10^{-6}F}=9.0 \cdot 10^{-5} J$