a
(a) \({W_{ext}} = \frac{1}{2}C'V{'^2} - \frac{1}{2}C{V^2}\)
\( = \left( {\frac{1}{2}} \right)\,\left( {\frac{C}{2}} \right)\,{(2V)^2} - \frac{1}{2}C{V^2} = \frac{1}{2}C{V^2}\)
\({W_{ext}} = \frac{1}{2} \times 50 \times {10^{ - 6}} \times {(100)^2} = 25 \times {10^{ - 2}}\,J\)