A charged oil drop of mass $2.5\times {10}^{-7} \mathrm{kg}$ is in space between the two plates, each of area  $2\times {10}^{-2} {\mathrm{m}}^2$ of a parallel plate capacitor. When the upper plate has a charge of $5\times {10}^{-7 }\mathrm{C}$ and the lower plate has an equal negative charge, then the oil remains stationary. The charge of the oil drop is (take, $\mathrm{g}=10 \mathrm{m}/{\mathrm{s}}^2$)