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Homework Help: How to find a charge on a sphere in equilibrium?

  1. Jun 16, 2011 #1
    1. The problem statement, all variables and given/known data

    A small, plastic sphere of mass m = 126 g is attached to a string as shown in the figure.

    There is an electric field of 151 N/C directed along the + x axis. If the string makes an angle 30 with the y axis when the sphere is in equilibrium, what is the charge on the sphere?

    2. Relevant equations

    kakz21.jpg

    3. The attempt at a solution

    mg = 0.126kg * 9.81m/s^2 = 1.24 N

    To make an angle of 30 degrees, the horizontal force must be 1.24 N * sin30 = 0.62 N

    So then i did .62 N/151N/C = .0041 C or 4.1 mC which is the units they want, i entered it but this isnt correct.

    Can someone tell me where i went wrong?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jun 16, 2011 #2

    vela

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    That's not correct.
    Start by drawing the free-body diagram for the sphere.
     
  4. Jun 16, 2011 #3
    Don't forget about the string! :wink:
     
  5. Jun 16, 2011 #4
    How do i incorporate the string in?!
     
  6. Jun 16, 2011 #5

    vela

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    The tension in the string exerts a force on the sphere, so it's another force in the free-body diagram.
     
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