A charged particle of radius `5xx10^(-7)m` is located in a horizontal electric field of intensity `6.28xx10^(5)Vm^(-1)`. The surrounding medium has the coefficient of viscosity `eta=1.6xx10^(5)Nsm^(-2)`. The particle starts moving under the effect of electric field and finally attains a uniform horizontal speed of `0.02 ms^(-1)`. Find the number of electrons on it. Assume gravity free space.

Explanation

Since ball is moving with constant velocity
`F = F_(v)`
`qE = 6 pi eta r v`
where `n`: number of excess electrons , `q = n e` .
`n e E = 6 pi eta r v`
`n = (6 pi eta r v)/(eE) = 30`