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Homework Help: Find the electric field at the origin

  1. Jul 15, 2017 #1
    1. The problem statement, all variables and given/known data
    Given two charged particles:

    1) 5x10^-9 C, (-.03 m, 0 m)
    2) -20x10^-9 C (.04 m, .02 m)

    find the electric field at the origin.

    2. Relevant equations
    F = (kQq)/r^2
    E = (kQ)/r^2

    3. The attempt at a solution

    So I know since 1)'s charge is plus, its field vector is

    [itex] ( (8.99x10^{-9}) (Q) ) / (.03)^2 [/itex] i hat, meaning its going to the right

    I use arctan to find the angle between the origin and 2), it is 26.57 deg, and pathagorean theorem to find the distance r to be .045m

    since 2) is - charged, its electric field is going away from the origin at 26.57 degrees.

    [itex] ( (8.99x10^{-9}) (Q) ) / (.045)^2 [/itex]

    Now I can break it down in to components, but how am I suppose to find Q, the charge of the origin?

    If they gave me a value for the electric force between the origin and one of the particles I can find Q, but with this information I dont see any way I can find it?
    Last edited: Jul 15, 2017
  2. jcsd
  3. Jul 15, 2017 #2


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    There is no charge at the origin. In the formula E = kQ/r2, Q is the charge that is producing the electric field. Each of the two charges that are given in the problem is producing electric field at the origin.
  4. Jul 16, 2017 #3
    Thank you sir. I was wasting my time on this question for 2 hours already.. now my answer matches my books answer. TY.
  5. Jul 16, 2017 #4


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    OK, good work.
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