JEE Advanced 2020 Paper 2

Two large circular discs separated by a distance of 0.01 m are connected to a battery via a switchas shown in the figure. Charged oil drops of density 900 kg m−3 are released through a tiny hole at the center of the top disc. Once some oil drops achieve terminal velocity, the switch is closed to apply a voltage of 200 V across the discs. As a result, an oil drop of radius 8×10−7 m stops moving vertically and floats between the discs. The number of electrons present in this oil drop is _______.
(neglect the buoyancy force, take acceleration due to gravity =10 ms−2 and charge on an electron(e)=1.6×10−19 C)