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Homework Help: Electric Field homework problem

  1. Jan 16, 2007 #1
    Hello all. I am new to this forum and I'm glad I found this place! Anywho, here's the problem I'm stuck on...

    Three equal charges 4.4 micro coulombs are located in the xy-plane, one at (0m, 63m), another at (52m, 0m), and th third at (49m, -53m). Find the magnitude of the electric field at the origin due to these three charges.

    Here's how I tried to solve it:

    d1 = 63m; d2 = 52m; d3 = 72.18m (derived using the distance formula)

    E = kq/d^2
    Enet = E1+E2+E3 = kq (1/d1^2+1/d2^2+1/d3^2)

    But when I plug the numbers in, I end up with ~32 N/C as my answer but the correct answer is 20.25 N/C. What am I doing wrong? Any help would be greatly appreciated.
  2. jcsd
  3. Jan 16, 2007 #2


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    1. Welcome to PF. Notice that we have subforums dedicated to Homework Help. Please use one of those from the next thread on.

    2. The electric field is a vector quantity (with a magnitude and a direction in the XY plane), not a scalar. Do you knoow how to add vectors using cartesian (or rectangular) components?
  4. Jan 16, 2007 #3
    I apologize for posting in the wrong sub-forum..thank you for moving it here. As for the problem, I thought about splitting them into components...field 1 does not have an x component and field 2 does not have a y component. For the third field, it does have x and y compoenets, but how do I find the angle needed for the calculation?

    So basically, is it:

    Ex = kq2/r2^2 + kq3/r3x^2 cos (?)
    Ey = kq1/r1^2 + kq3/r3y^2 cos (?)

    sqrt(Ex^2 + Ey^2) = E

    where ? = mystery angle

    I already tried using this...but didnt know what angle to use and the answer I got was close but not exactly to the answer given in the sample problem. Is this correct? If so, any hints on how I can find the mystery angle?
  5. Jan 16, 2007 #4


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    Do you really need an angle? You already have the x and y distance components...
  6. Jan 16, 2007 #5
    Figured it out using the angles....thanks!