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Homework Help: HELP on electric field problem

  1. May 6, 2009 #1
    1. The problem statement, all variables and given/known data
    A 2g plastic ball is suspended by a 2cm string in a uniform electric field of 1.0 x 10^4 N/C (the arrows point to the right)

    a) is the balls charge positive of negative?
    (B) If the ball is in equilibrium when the string makes a 15 degree angle with the vertical, what is the net charge. (15 degree angle going towards where the E arrows point)

    2. Relevant equations

    3. The attempt at a solution
    a) i know it is positive
    b) i did work but i can never get the right answer which is 5.3*10^-7C
  2. jcsd
  3. May 6, 2009 #2


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    I'm not sure which direction your suspended object is going, but for b) consider the angle of the string. Won't it be the sum of m*g down and the electrostatic force horizontally? So isn't tan15 then the ratio of the 2 forces?
  4. May 6, 2009 #3
    its going right
  5. May 6, 2009 #4
    i dont understand. help, pleaseee
  6. May 6, 2009 #5


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    F = q*E


    q*E/(m*g) = tan15

    q = m*g*tan15/E

    Isn't that all there is to it?
  7. May 6, 2009 #6


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    The force on the charged ball due to electric field is Eq, and weight of the ball is mg. The ratio of the forces is Eq/mg = tanθ. Solve for q.
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