\( = {U_t} - {U_i}\)
\( = \frac{{{q^2}}}{{2{C_r}}} - \frac{{{q^2}}}{{2{C_i}}}\)
\( = \frac{{{{\left( {5 \times {{10}^{ - 6}}} \right)}^2}}}{2}\left( {\frac{1}{{2 \times {{10}^{ - 6}}}} - \frac{1}{{5 \times {{10}^{ - 6}}}}} \right)\)
\( = \frac{{15}}{4} \times {10^{ - 6}}\)
\( = 3.75 \times {10^{ - 6}}\,{\text{J}}\)