A copper ball of density 8.6 g cm^-3, 1 cm in diameter is immersed in oil of density 0.8 g cm^-3. If the ball remains suspended in oil in a uniform electric field of intensity 36000 Nc^-1 acting in upward direction, what is the charge on the ball?

Apparent weight of the ball = mg -upthrust

$\begin{array}{c}=\frac{4}{3}{\mathrm{\pi r}}^3(\mathrm{\rho }-\mathrm{\sigma })\mathrm{g}=\frac{4}{3}\mathrm{\pi }\times (0.5{)}^3\times (8.6-0.8)\times 980\\ =4,002.4\text{ dyne }=4.0024\times {10}^{-2}\mathrm{N}\end{array}$

Let q be the charge on the ball. As the ball remains suspended, 
qE = apparent weight of the ball

$\begin{array}{c}\mathrm{q}=\frac{\text{ apparent weight }}{\mathrm{E}}\frac{4.0024\times {10}^{-2}}{36,000}\\ =1.11\times {10}^{-6}\mathrm{C}\end{array}$