Millikan produced an oil drop of radius 1.0X10^-6 m and introduced it between two horizontal plated separated by a distance of 5mm. The upper plate was at a potential V higher than the lower plate. It was found that by adjusting V to a value of 385V, the oil drop would remain stationary under the combined influence of the gravitational and electric forces. Find the charge on the oil drop(include the sign).
The density of the oil is 900kgm^-3.