# Determining the magnitude of unknown charges

1. Nov 10, 2009

### crazuiee

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

A positive charge +q1 is 3.00m to the left of a negative charge -q2. The net electric field is zero 1.00m to the right of the negative charge. Determine the relative magnitude of the charges in a ratio q1/q2

2. Relevant equations
E=kQ/r^2

3. The attempt at a solution
I set it so that kQ1/r1^2=kQ2/r2^2 but I'm not sure how to solve for the ratio? would the k's cancel out and it'd just be a ratio of the distances? Are the distances r1=4.00m and r2=1.00m?

2. Nov 10, 2009

### rl.bhat

Find Q1/Q2.

3. Nov 11, 2009

### crazuiee

would the ratio just work out to be the ratio of the distances? so 4/1?

4. Nov 11, 2009

### rl.bhat

No. It should be square of the distances.

5. Nov 11, 2009

### crazuiee

So q1= +16 and q2= -1 ? Also there's a part of the question that asks to locate two spots where the potential is zero, in relation to the negative charge, any ideas how to find it?

6. Nov 12, 2009

### rl.bhat

In between the charges you can get zero potential. Similarly out side the charges near the smaller charge you can get another spot where then potential is zero.
If x is the distance where the potential is zero, then
kq1/(d-x) - kq2/x = 0