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Homework Help: Electric field problem

  1. Jan 31, 2009 #1
    i have been stuck on this problem for days and can't figure out where i'm going wrong. i have a rectangle with sides 6cm and 8cm. 3 points of the triangle have a point charge on them with the following values:

    q1 = -2 * 10^(-8) C
    q2 = 8 * 10^(-8) C
    q3 = -4 * 10^(-8) C

    The point charges are arranged such that q1 is on top right, q2 on top left, q3 on bottom left. Find the field intensity at the 4th point (bottom right).

    So what I did is use E = kq/r^2 (k = 9*10^9) to find the field intensity on the 4th point. For q2 I broke it into x- and y- components with E2x = E2 cos 36.87 and E2y = E2 sin 36.87. For q2 I found the angle of 36.87 by doing the inverse tan of 0.06/0.08 = 0.75 and i found the distance between the two points by using the pythagorean theorum (a^2 + b^2 = r^2).

    Once I have both E1 and E3 as well as E2 broken into components, I add up the ones on the x-axis (E2x, E3) and the ones on the y-axis (E2y, E1). Then again I use the pythagoren theorum to add up the resultant (Ex^2 + Ey^2 = R^2)

    I keep getting the wrong answer and I dont know why. Could you please point out where I'm going wrong or list the steps I should be taking so I could compare. The answer on the sheet is 6.93 * 10^3 at an angle of 78.8 under the horizontal

    thanks everyone!
  2. jcsd
  3. Jan 31, 2009 #2


    User Avatar

    Staff: Mentor

    Sounds like your approach is right. The rectangle is 6 high by 8 wide?

    Can you list the x and y components of E that you get for each of the 3 sources, so that we can check your math? E1 just points up, and E3 just points left, and E2 points down and to the right, correct?
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