Two small spheres each of mass 10 mg are suspended from a point by threads 0.5 m long. They are equally charged and repel each other to a distance of…

Two small spheres each of mass $\displaystyle 10$ mg are suspended from a point by threads $\displaystyle 0.5$ m long. They are equally charged and repel each other to a distance of $\displaystyle 0.20$ m . The charge on each of the sphere is $\displaystyle \frac{\mathrm{a}}{21} \times 10^{-8} \mathrm{C}$. The value of ' a ' will be \_\_\_\_. [Given $\displaystyle \mathrm{g}=10 \mathrm{~ms}^{-2}$ ]