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Homework Help: Electric Charge Force Diagram

  1. Sep 14, 2014 #1

    I drew a force diagram of one charge with gravity downward, electric force to the right, and tension at 20 degrees left of +y. I summed the forces in the x and y directions:

    Fy = Tsin20° - mg = 0

    T = 0.1434 N

    Fx = kq^2 / r^2 - Tcos20° = 0

    q = 6.29 x 10^-12 C

    The correct answer is 2.9 x 10^-7 C, though my answer is a multiple choice option so whatever I did wrong probably happens often. I'm not sure what exactly I did wrong, only that I believe q is supposed to be equal to 2Lsinθ√((mg/k)tanθ) through some derivation. Some pointers would be appreciated!
  2. jcsd
  3. Sep 14, 2014 #2


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    Did you use the correct trig functions?
  4. Sep 14, 2014 #3
    Thank you! I mixed my sine and cosine up and I also didn't solve for q properly (didn't take square root of everything). Plugging everything back in I got the right answer.
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