Four particles form a square. The charges are and .
(a) What is if the net electrostatic force on particles 1 and 4 is zero?
(b) Is there any value of q that makes the net electrostatic force on each of the four particles zero? Explain.
The electric force between two particles is proportional to each of the charges and inversely proportional to the square of the distance between them
The force of q1 on q2 is the same as q2 on q1. This is action-reaction principle.
Think of it as gravitation. It is the same. Mass is gravitational charge and "charge" is electric charge. The force of the moon on the earth is the same as the earth on the moon. Effects are different because mass of each is different.
So the force of Q on Q is and the force of q on q is Notice that when the two charge are the same sign they tend to repulse each other. The opposite happen when they are of different sign. Here, Q and Q are obviously the same sign. Two charge q have to compensate for one charge Q that is farther.