A charged dust particle of radius 5x10-7 m is located in a horizontal electric field having an intensity of 6.28 x 105 Vm-1. The surrounding medium is air with coefficient of viscosity η=1.6×10−5 Nsm−2. If this particle moves with a uniform horizontal speed 0.02 ms-1, find the number of electrons in it.

The horizontal force on dust particle is F = qEIf v is speed of particle in air, then by Stoke's Law,Viscous force =6πηrvFor uniform speed v, we have qE=6πηrvIf n is the number of excess electrons, then q=ne⇒ neE=6πηrv⇒ n=6πηrveESubstituting given values n=6×3.14×1.6×10−5×5×10−7×0.021.6×10−19×6.28×105⇒ n = 30