$\left. \begin{matrix} {{U}_{1}}=\frac{1}{2}C{{V}^{2}}=\frac{1}{2}\times 2\times {{20}^{2}}=400\mu J  \\ {{U}_{2}}=\frac{1}{2}C{{V}^{2}}=\frac{1}{2}\times 2\times {{30}^{2}}=900\mu J  \\ \end{matrix} \right\}\Rightarrow {{U}_{2}}-{{U}_{1}}=900-400=500\mu J$