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Homework Help: Vector problem with electricity

  1. Feb 2, 2007 #1
    vector problem with electricity :(

    Ok, so i am having problems with my vectors in two homework problems (or maybe i went wrong some where else)...

    #1) Four identical point charges (q = +10.0 µC) are located on the corners of a rectangle. The dimensions of the rectangle are L = 70.0 cm and W = 25.0 cm. Calculate the magnitude and direction of the net electric force exerted on the charge at the lower left corner by the other three charges.

    ok, so i drew a lovely picture of the rectangle with all three force vectors as repulsion. my work goes something like this:
    F1= 8.99e9 * 10e-6 ^2 / .025^2 = 1438N
    F2= 8.99e9 * 10e-6 ^2 / (.025^2+ .07^2) = 163N
    F3= 8.99e9 * 10e-6 ^2 / .07^2 = 183.5N

    Then made a vector table:

    F1 x components = 0 and y components = 1438N
    F2 x components = 163cos19.65= 153N y components...=54.7N
    F3 x components = 183.5 N and y = 0

    I then added up the x to get Fx=337N and Fy=1493N

    then F= (337^2 + 1493^2) ^1/2 = 1530N (but thats wrong)

    but then i got the angle by taking hte inverse tan of 1493/337 = 257degrees and that is correct, so i dont really know why my force magnitude is worng.... and i have another problem on my hw that is similar and i keep getting it wrong too, so i think i am doing something wrong with my vectors... any ideas or help would be greatly appreciated. THANKS!!
  2. jcsd
  3. Feb 3, 2007 #2
    Check your unit conversions (specifically going from centimeters to meters).
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