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Homework Help: Where along line is electric field zero?

  1. Jan 24, 2009 #1
    1. The problem statement, all variables and given/known data
    Two charges of +4microC and +9microC are 30cm apart. Where on the line joining the charges is the electric field zero?

    2. Relevant equations

    3. The attempt at a solution
    Since charges are alike, put Ep=E1-E2=0, then E1=E2
    Substituted for E1 and E2
    eventually leads to q1(r^2-2rx+x^2)=q2x^2

    He told us to use quadratic since it's more secure, but that requires the (r^2-2rx+x^2) to be set equal to zero, which i cannot figure out how to do
  2. jcsd
  3. Jan 24, 2009 #2


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    Homework Helper

    The way I look at it you get something like:

    4/x2 = 9/y2

    Taking the square root of both sides yields

    2/x = 3/y

    That looks a little easier to deal with doesn't it?
  4. Jan 24, 2009 #3
    Never mind, i got it. I plugged in the values for q1 and q2, distributed, subtracted the right from left, then used quadratic. Thanks for your help anyway!
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