# The work done in placing a charge of $\;8\times10^{-18}\;$ coulomb on a condenser of capacity 100 micro-farad is

$(a)\;16\times10^{-32}\;J\qquad(b)\;31\times10^{-26}\;J\qquad(c)\;4\times10^{-10}\;J\qquad(d)\;32\times10^{-32}\;J$

Answer : (d) $\;32\times10^{-32}\;J$
Explanation : The work done is stored as the potential energy . The potential energy stored in a capacitor is given by
$W=\large\frac{1}{2}\;\large\frac{Q^2}{c}=\large\frac{1}{2}\;\large\frac{(8\times10^{-18})^2}{100\times10^{-6}}$
$=32\times10^{-32}\;J\;.$
edited Apr 2, 2014