A charged dust particle of radius 5×10−7 m is moving in a horizontal electric field of intensity 6.28×105V/M. The surrounding medium is air with coefficient of viscosity η=1.6×10−5N−s/m2. If this particle is moving with a uniform horizontal speed of 0.02 m/s, the number of excess electrons on the drop are 6k. The value of k is _________.