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