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Homework Help: Square array (electric field question)

  1. Oct 3, 2007 #1
    1. The problem statement, all variables and given/known data

    Four charges q1 = q3 = -q and q2 = q4 = +q, where q = 6 µC, are fixed at the corners of a square with sides a = 1.3 m. (see attached .gif)

    Calculate the x- and y-components of the net electric field at the midpoint M of the bottom side of the square.

    2. Relevant equations
    Why dont the charges cancel in the field??
    The distances and angles, are all equal
    The charges are opposite.

    SO how can there be a net electric field at that point (M)?

    3. The attempt at a solution
    1. The problem statement, all variables and given/known data
    havent calc'd anything cause i dont get why field shouldnt be zero

    2. Relevant equations
    im totally stuck
    thanks for any help sum1.

    3. The attempt at a solution

    Attached Files:

  2. jcsd
  3. Oct 3, 2007 #2

    Doc Al

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    Staff: Mentor

    If you were asked to find the field in the middle of the square, then you'd be correct: The field would be zero.

    But not at point M. Note that on one side of M there's a +q and on the other side there's a -q. They don't cancel! (If M were in between two similar charges, then you could say that their field cancels.)
  4. Oct 3, 2007 #3


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    Science Advisor
    Homework Helper
    Gold Member

    It's always a good idea to go back to the basics, especially when confused, rather than seeking the shortcut that might not be there.

    For each point charge, draw a vector representing its electric field at point M (...paying attention to direction and some sense of relative scale). Draw the vector sum of those electric field vectors. Does it look like the [total] electric field vector at M is zero?
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