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Homework Help: 4 charges on rectangles

  1. Jul 8, 2009 #1
    1. The problem statement, all variables and given/known data
    Four charges are placed on the corners of a rectangle. What is the resultant force on the positive charge (a = 1.4 m, b = 0.9 m, q = 2.7 × 10-9C)?

    the rectangle has a charge
    -q____+q on the 4 corners of the rectangle ant a line r from the +q to the -q diagonally
    the bottom is labeled a and the left side is labeled b

    2. Relevant equations
    kq/ r^2
    Fx and Fy components

    3. The attempt at a solution

    i started by using k and q 8.99*10^9 * 2.7*10^-9

    then i divided it by the values of a and b and r which i got by using tan inverse of 1.4/ .9 i got my three numbers

    -8.091*10^-8 x component
    -3.34*10^-8 y component
    -2.3499*10^-8 both

    and since there is only 2 to add together for each i found F of x and y by multiplying by sin theta which is 32.74 degrees for the y components and cos theta for x components

    next i found those numbers and plugged it into the formula F = square root of Fx^2 + Fy^2 and i got 7.143*10^-8 which is wrong i dont know where i went wrong

  2. jcsd
  3. Jul 8, 2009 #2


    User Avatar
    Homework Helper

    The Coulomb froce law incorporates a product of two charges; you only have one.

    The diagonal distance should be obtained from the Pythagorean theorem; inverse tangent will give you an angle.

    I have no idea what you did after that.

    There are three Coulomb forces acting on the positive charge. Determine them, and then add them up vectorially. Try to work it out symbolically, first.
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