A water droplet of radius 1 micron in Milikan oil drop appartus in first held stationary under the influence of an electric field of intensity `5.1xx10^(4) NC^(-1)`. How many excess electrons does it carry ? Take `e = 1.6xx10^(-19) C, g = 9.8 ms^(-2)` and density of water of `= 10^(3) kg m^(-3)`.

Answer

5

Explanation

Here `r = 1` micron `= 10^(-6) m, E = 5.1xx10^(4) N//C, n = ?`
From qE = mg
(ne) `E = (4)/(3) pi r^(3) rho g, n = (4)/(3) (pi r^(3) rho g)/(e E)`
`n = (4)/(3) xx (22)/(7) ((10^(-6))^(3) xx10^(3)xx9.8)/(1.6xx10^(-19)xx5.1xx10^(4)) = 5`