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# Keeping the voltage of the charging source constant, what would be the percentage change in the energy stored in a parallel plate capacitor, if the separation between its plates were to be decreased by $10 \%$?

Energy stored in a parallel plate capacitor is $U=\large\frac{1}{2}$$CV^2 \qquad =\large\frac{1}{2} \frac{\in_0 A}{d}$$V^2$------(i)
When separation between the plates is decreased by 10%, $d'=d \large\frac{10}{100}$$d=0.9 d Change in energy (U' - U)=\large\frac{1}{2} \in_0$$AV^2 \bigg[ \large\frac{1}{0.9} -\frac{1}{1}\bigg]$
% age change in energy= $\large\frac{(U'-U)}{U} $$\times 100 =\large\frac{100}{9}$$=11.1 \%$