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Finding the neutral point of two charges

  1. Sep 9, 2015 #1
    1. The problem statement, all variables and given/known data
    Two charges q1=+9c and q2+-1c are separated by 2m. Where is their neutral point?

    2. Relevant equations
    E = E(+) + E(-)
    E= k q/r^2

    3. The attempt at a solution
    k q1/(2+x)^2 = -(k q2/x^2)

    When I solve for this equation I end up square rooting a negative number which would then give i and I don't know how to work with this.
     
  2. jcsd
  3. Sep 9, 2015 #2

    SammyS

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    Doesn't the electric field vector point away from a positive charge and towards a negative charge?

    If so, the sign you use for the field from each of your charges depends upon the position relative to the charges.
     
  4. Sep 9, 2015 #3
    Correct. So the positive field flows right towards the negative which continues a field to the right. I'm looking for the neutral point nearest the negative charge. How can I use this info to solve this if q1 is at 0 and q2 is at x=2?
     
    Last edited: Sep 9, 2015
  5. Sep 9, 2015 #4

    SammyS

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    Show what you get when you put values for charge into the following and do a little simplifying.
    Also,
    What is the quadratic equation you get?
     
  6. Sep 9, 2015 #5
    k q1/(2+x)^2 = -(k q2/x^2)
    q1/(2+x)^2 = -(q2/x^2)
    +9/(2+x)^2 = -(-1/x^2)
    9/(2+x)^2 = 1/x^2
    x^2/(2+x)^2 = 1/9
    x/(2+x) = ± 1/3
    (2+x)/x = ± 3
    2/x + 1 = ± 3
    2/x = -1 ± 3
    So we have x=1 when -1 + 3

    Ah, that makes sense and I believe is the correct answer.
     
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