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

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??

Dr. Courtney
Gold Member
2020 Award
Why is one denominator x+8 instead of x-8?

Chandra Prayaga
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??

Is this a homework problem?

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!

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..

haruspex