First slide
NA
Question

There are n electrons on a drop of oil. It is in equilibrium in an electric field of intensity E. If the density of the oil is ρ, then radius of the drop will be

Easy
Solution

In equilibrium, q E = m g
There are n electrons on the drop hence, q = n e
 neE=43πr3ρg
or    r=3neE4πρg1/3

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