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Homework Help: Where the total electric field is equal to zero?

  1. Nov 1, 2007 #1
    1. The problem statement, all variables and given/known data
    Two point charges of -2.5 µC and 6.0 µC lying along the x axis are 1.0 m apart. Locate the point (other than infinity) at which the total electric field is zero.

    2. Relevant equations
    I was thinking: E=F/q
    F=K (Q x q)/d^2
    But I'm not quite sure how to use them.

    3. The attempt at a solution
    Using the second equation I found F to be -.135. But I don't think that's right, and then when plugging that into the E=F/q. E would have to be zero, since I'm trying to find where the total electric field is zero? Then there's no solutions other than infinity, are there? And what exactly is q in the first equation. I really have no idea what's going on here.
  2. jcsd
  3. Nov 1, 2007 #2
    If the electric field is zero, is it the case that the force is also zero? So find the point(s) where the net electric force or field is zero.
  4. Nov 1, 2007 #3
    I found an example online and I think I substituted the right numbers in for this problem, is this how it would be solved? (I didn't understand the equation below and where it came from: KQ/x²-2.4KQ/(x+1.00)²)

    Enet=Esub1 + Esub2 = KQ/x²-2.4KQ/(x+1.00)²

    The Ks and Qs cancel out leaving:
    1/x² = 2.4/(x+1.00)²

    And then get it into a quadratic:
    1.4x² - 2x -1=0

    And find it to be 1.82 m to the left of charge Q. I ignored the other root, -.39m because that would be between the two charges where the fields cannot cancel out.
    Am I on the right track?
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