A Levels Physics

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?
ev
mgd
B
meV
gd
gd
A
D
mev
mgd
ev


Choice B is correct because:

Electric force on the charge acting upwards (FE) = Weight of the charge acting downwards (W)

Weight of the charge acting downwards (W) = Mass of charge (m) x Acceleration due to gravity (g)

W = mg

Voltage between the plates (V) = Work done per unit charge between the plates (W) / [Elementary charge (n) x Number of excess electrons (e)]

Work done per unit charge between the plates (W) = Electric force on the charge acting upwards (FE) x Distance between the plates (d)

V = W / (n x e)

V = (FE x d) / (n x e)

FE = (Vne) / d

(Vne) / d = mg

n = (mgd) / eV

Choices A, C & D are incorrect because they do not correctly state the value of n as (mgd) / eV.

Electric Fields | 2015 - M/J - 9702/12 | Q30