# At what point along the x axis is the electric field zero?

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1. Jan 25, 2016

### A14

• Moved from another forum, so homework template missing
Hello, I've been trying to answering this question but my answer is always wrong.
Two point charges, -9.10 μC and 21.29 μC, are placed at x = 0 cm and x = 8.00 cm, respectively.
(a) At what point along the x axis is the electric field zero?

What i did: i figured that the point should be on the left of the first charge, so before x=0 (should be negative).
Then i resolve E1 + E2 =0
kQ1/r^2 + kQ2/r^2 =0
(-8,19*10^4)/(x^2) + (1,916*10^5)/((x+8)^2) = 0
(8,19*10^4)/(x^2) = (1,916*10^5)/((x+8)^2)
1.916*10^5*(x^2) = 8,19*10^4 ((x+8)^2)
Then i develop and get x1= 1.51*10^1 or x2= -3.16

What am i doing wrong??

2. Jan 25, 2016

### Dr. Courtney

Why is one denominator x+8 instead of x-8?

3. Jan 25, 2016

### Chandra Prayaga

Is this a homework problem?

4. Jan 25, 2016

### A14

I don't know, for me as it is on the left of Q1, the distance to Q2 would be x (distance from point to Q1) + 8 (distance between Q1&Q2)?

Yes it is a homework problem!

5. Jan 26, 2016

### A14

Yes i see why it has to be negative (it has to be on the left of Q1 which is at x=0)
But -3.16 is not the right answer..

6. Jan 26, 2016

### haruspex

Think again about that x+8. With x=-3.16, what does that make x+8?