9702/May Jun/12/2015/Q30

A charged oil drop of mass m, with n excess electrons, is held stationary in the uniform electric
field between two horizontal plates separated by a distance d.

The voltage between the plates is V, the elementary charge is e and the acceleration of free fall
is g.

What is the value of n ?

Solution:
Answer: B.

Electric field intensity = F/ne = V/d

F = mg

mg/ne = V/d

mgd = neV

n = mgd /eV

Reference: PYQ - May/Jun 2015 Paper 12 Q30