# Zero field location

1. Aug 28, 2013

### hacker804

1. The problem statement, all variables and given/known data
Two charges of q1=4μC and q2=-1μC are separted by a distance of 3 m.Find and justify the zero field location.

2. Relevant equations

$E=\frac{kQ}{r^{2}}$

3. The attempt at a solution
Consider a point x to the right of charge q1 where the field is zero.The distance of x from q1 is x and from q2 is 3+x.
I solved the problem and the answer comes out as -6m.The answer given by the book is 3m.Is my answer correct??

Thanks

2. Aug 28, 2013

### voko

You and the book should agree on how the charges are located and from which point the distance is measured.

3. Aug 28, 2013

### ehild

No, your answer is not correct. If the point in question is at distance x to the right from q1, a negative value means that it is on the left, and it is impossible. Perhaps, you will show your work in detail.

ehild

4. Aug 28, 2013

### hacker804

So it is 6m to the left from q1 which means it is not on the line between the two charges which is only 3m.

5. Aug 28, 2013

### LeonhardEu

First of all there are two such positions, one between the charges and one outside from the side of the smallest. The two positions come from the dependance of E from x squared ( or only by physics as we like, as you go close to q2<q1 the field of q2 becomes stronger. In other cases the field of the bigger charge is bigger than that of q2. But you can go from both sides. So there are two such points). Just find the expression of E at any location in the line of the two charges and solve E = 0 for x. Your book finds the distance of the point from q2.