A charged dust particle of radius $5\times {10}^{-7} \mathrm{m}$ is located in a horizontal electric field having an intensity of $6.28\times {10}^5 \mathrm{V} {\mathrm{m}}^{-1}$. The surrounding medium in air with coefficient of viscosity $\mathrm{\eta }=1.6\times {10}^{-15}\mathrm{N} \mathrm{s} {\mathrm{m}}^{-2}.$ If this particle moves with a uniform horizontal speed of $0.01 \mathrm{m} {\mathrm{s}}^{-1},$ the number of electrons on it will be