# Two charges located along x. Find all values?

1. Apr 18, 2015

### nawand

1. The problem statement, all variables and given/known data
The first one has charge Q is at "-a" from origin the second is at "+3a" which is an unknown charge.What are the values for unknown charge if at the origin the net electric field they produce with magnitude 2Ke(Q/a2)
2. Relevant equations
Ke (Q/a2)i - Ke(q/9a2)=2Ke(Q/a2)i
if charge is negative
or
if the charge is positive
Ke (Q/a2)i - Ke(q/9a2)=-2Ke(Q/a2)i
3. The attempt at a solution
The question is how to choose signs for equation.
Attempt:
Assuming charge one is pointed towards the origin along x axis so the sign will be "+i" and the second charge is pointed towards the origin in the negative x direction so tthe sign will be "-i".
Two possible values are at the origin: "+" or "-" 2Ke(Q/a2) .
If you choose "+" at the origin the answer will be -9Q. And for case chosen "-" sign in the origin, the answer will be 27Q so the particle has positive charge.
I wondering if this is the explanation?

2. Apr 18, 2015

### Staff: Mentor

Correct.

3. Apr 18, 2015

Thanks