Electrostatics

A tiny spherical oil drop carrying a net charge \(q\) is balanced in still air, with a vertical uniform electric field of strength \(\frac{81}{7} \pi \times 10^5 \mathrm{~V} / \mathrm{m}\). When the field is switched OFF, the drop is observed to fall with terminal velocity \(2 \times 10^{-3} \mathrm{~ms}^{-1}\). Here \(g=9.8 \mathrm{~m} / \mathrm{s}^2\), viscosity of air is \(1.8 \times 10^{-5} \mathrm{Ns} / \mathrm{m}^2\) and density of oil is \(900 \mathrm{~kg} \mathrm{~m}^{-3}\). The magnitude of \(q\) is