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Homework Help: At what point along the x axis is the electric field zero?

  1. Jan 25, 2016 #1


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    • 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??

    Thank you for your help!
  2. jcsd
  3. Jan 25, 2016 #2
    Why is one denominator x+8 instead of x-8?
  4. Jan 25, 2016 #3
    Is this a homework problem?
  5. Jan 25, 2016 #4


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    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!
  6. Jan 26, 2016 #5


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    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..
  7. Jan 26, 2016 #6


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