c
(c) \(\Delta U = \frac{1}{2}\frac{{{C_1}{C_2}{{({V_2} - {V_1})}^2}}}{{({C_1} + {C_2})}}\)\( = \frac{{(3 \times 5) \times {{10}^{ - 12}} \times {{(500 - 300)}^2}}}{{(3 + 5) \times {{10}^{ - 6}}}}\)
\( = \frac{{15 \times {{10}^{ - 12}} \times 4 \times {{10}^4}}}{{8 \times {{10}^{ - 6}}}} = 0.0375\,J\)