A charged particle of radius 5×10−7m is located in a horizontal electric field of intensity 6.28×105V/m. The surrounding medium has the coefficient of viscosity  η=1.6×105N−s/m2. The particle start moving under the effect of electric field and finally attains a uniform horizontal speed of  0.02 m/s. Find the _______×108 number of electrons on it. Assume gravity free space