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

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