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 Quote by afulldevnull 1. The problem statement, all variables and given/known data Two particles lie on the x axis. The first particle is at the origin with a charge of +1.6uC and the other has a charge of -3.1uC and is +10.5cm away. If particle 3 of unknown charge q3 is to be located such that the net electrostatic force on it from particles 1 and 2 is zero. what is the coordinates of the third particle? 2. Relevant equations Coloumbs Law: F = k (q1 q2) \ r^2 3. The attempt at a solution Since the charges of the particles are opposite, and because particle 1 has a smaller magnitude, then the third particle must go to the left of particle 1 and must have a negative charge. This is because for the net to be 0, then the strong force at a distance must repel as strongly as the small force nearby attracts. If the net force is to be 0, then the forces must have the same magnitude. So: k(1.6q)/d^2 = k(3.1q) / (10.5+d)^2. I get the 26 cm, but that can't be right. Any thoughts?
You said that d was on the left of the origin - so is negative.
The solution of the quadratic equation yields approx 26 and also around -4.
Perhaps it is the "-4" you are after.

Also, the charge of q3 can be either positive or negative.