What Is the Correct Value of q in Coulomb's Law Problem?

[jenduncan]

The net force on a 1nC charge is 0. What is q?

There are 3 forces. q is 2cm in the y direction away from a charge of 1nC. Another charge of 2nC is 4cm below q and 3cm to the right. The final charge is 4cm below q and 3cm to the left. Using Coulomb's law, I found the force of the two bottom charges to be 9.79e-6i+9.79e-6j and -9.79e-6i+9.79e-6j respectively. The final charge I found to be -22500q in the j direction. By adding all the forces together I find q to be 0.870nC. The answer in the back of the book is 0.68nC. Where have I gone wrong?

My final equation to solve for q was 1.96e-5 = 22500q

 

[whozum]

Force from first charge + Force fromsecond charge + Force from q = Force on 1nC charge = 0.

I think you missed this part which is why your off.