Superposition Principle

The principle of superposition is to calculate the electric force experienced by a charge q1 due to other charges q2, q3 ……. qn. 

The total force on a given charge is the vector sum of the forces exerted on it due to all other charges.

The force on q1 due to q2 

Vector F12 = k q1 q2 / (r21)^2     (r21 cap)

similarly,  force on q1 due to q3 

Vector F21 = k q1 q2 / (r31)^2     (r31 cap)

The total force F1 on the charge q1 by all other charges is, 

Vector  F1 = Vector [F12+F13+F14+.........+F1n]

So 

Vector F1=1/4πεo[q1q2/r21^2 (r21cap) + q1q3/r31^2  (r31cap) + ..... + q1q n/r n1^2   (r n1 cap )]

Note:

A torsion balance is a sensitive device to measure force. It was also used later by Cavendish to measure the very feeble gravitational force between two objects, to verify Newton’s Law of Gravitation.