# Homework Help: Finding an unknown charge in a system of discrete point charges.

1. Jan 27, 2010

### Zift

A point particle that has a charge of 15.0 µC is located at x = 0, y = 0 (we'll call it q0 and a point particle that has a charge q is located at x = 12.0 cm, y = 0 (we'll call it q1. The electric force on a point particle that has a charge of 6.0 µC at x = 24.0 cm, y = 0 is -(19.7) N $$\hat{i}$$ (we'll call it q2. When this 6.0-µC charge is repositioned at x = 17.8 cm, y = 0, the electric force on it is zero. Determine the charge q.

First I tried to equate the electric force on q2 to the vector of the forces on q2 due to q0 and q1 using coulomb's law and then solve for q1, but this does not yield the correct value. I have been able to do all the other problems in the section on discrete charge distributions, but I'm completely stumped on this one. I also tried to do the same procedure but with the electric force on q2 equal to zero as indicated in the last sentence and but its yields a different (and still incorrect) value. what am I doing wrong. Am i missing something? Is there crucial information I should be deriving from the last sentence?

Last edited: Jan 27, 2010
2. Jan 28, 2010

### ehild

This problem looks over-defined, and I also found the results controversial.

ehild