Two charges $q_1$ and $q_2$ are placed $30\mathrm{ }\mathrm{c}\mathrm{m}$ apart, as shown in the figure. A third charge $q_3$ is moved along the arc of a circle of radius $40\mathrm{ }\mathrm{c}\mathrm{m}$ from $C$ to $D$. The change in the potential energy of the system is $\frac{q_3}{4\pi {\in }_o}k$ where $k$ is -