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Homework Help: Electric Field Problem

  1. May 5, 2008 #1
    1. The problem statement, all variables and given/known data
    A +8·µC charge is located at coordinates (3,5)·m, and a -2·µC charge is located at coordinates (3,-2)·m. Find the electric field at the origin.
    E= ? V/m x + ? V/m y

    (It asks for the solution in x and y components)

    2. Relevant equations
    I was using E=kq/r^2, where E is the electric field vector (Away if positive, towards if negative) K was 9x10^9, q is the value of the charge.

    3. The attempt at a solution
    First I used pythagorean theorem to determine distance to origin. Then I used this value for r.
    Using the formula for the +8 charge I get E=2117.61 and for the -2 charge I got E=1376.63. Then I used the trig functions from the triangles to solve for the components, which I thern added together for the wrong answer. Where have I gone wrong here?
  2. jcsd
  3. May 5, 2008 #2
    -2 charge, E=1376.63, it should be negative, or pointing in to the right so when you add the 2 E vectors you will be subtracting them.

    I get the same values for E's, just that negative sign, Hope that's the problem!
  4. May 5, 2008 #3
    The angles from the original triangles are used to calculate the components, correct? I was treating the +8 charge as my negative vector as electric fields point outwards from positive charges, right?
  5. May 5, 2008 #4

    I think if you use the 8 charge as the negative, you will get the wrong answer, maybe just -1*answer wil fix it but I'm not 100% sure on that.

    You should find the 2 vector pointing away, then inverse it so it points inward and then find the x-y components and subtract or add the components.

    Also for your -2 charge, I got E=1384.62 not 1376.63
  6. May 6, 2008 #5


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    Homework Helper

    Hi negatifzeo,

    What components and wrong answer did you get?
  7. May 6, 2008 #6
    I finally got the correct answer, my problem was rounding a bit too much.
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