Hi. I would appreciate some help with this point charge problem. I think I understand the general set up, but don't know where to go from there.(adsbygoogle = window.adsbygoogle || []).push({});

1. The problem statement, all variables and given/known data

Two point charges are placed on the x axis as follows: Charge q1 = +4.00nC is at x=0.400m, and charge q2 = +2.00nC is at x=-0.200m. State, in x and y coordinates, where a negative point charge, q3 = -4.73nC, must be placed to have a zero net force exerted on it by q1 and q2.

2. Relevant equations

I know that F = (k*q*Q)/r^{2}and that the force from q1 acting on q3 (F13) has to be equal to the force from q2 acting on q3 (F23) for the net force to = 0.

3. The attempt at a solution

Where I am getting lost is in relation to the distances between the charges. If I don't know the distance between q1 and q3, and q2 and q3, how can I do the problem? And I can't rely on the Pythagorean theorem because there is no guarantee that a right angle is formed. I know my best chance at solving this is a quadratic equation, but I don't know how to get to that point. It has been a LONG time since I took algebra, so I may be overlooking something very simple. This is what I have:

F13 = (k*q1*q3)/r_{13}^{2}= F23 = (k*q2*q3)/r_{23}^{2}

things cancel and it simplifies to

q1/r_{13}^{2}= q2/r_{23}^{2}

From here I have absolutely no idea where to go. Thanks for your help.

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Point Charges and Force vector addition

**Physics Forums | Science Articles, Homework Help, Discussion**