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Charge on a ball

  1. Jan 29, 2009 #1
    1. The problem statement, all variables and given/known data
    An electric field E=141000.0i N/C causes the 4.70 g ball in the picture to hang at a α=22.9° angle. What is the charge on the ball?

    2. Relevant equations

    3. The attempt at a solution
    I tried solving this question first by drawing a force diagram on the charged particle. On the diagram:
    W = T cos α -1
    F = T sin α -2

    I divided equation 2 by 1
    I get tan α = F/mg ; since F = 1/4pie * q/r^2; i subbed it in
    therefore: mgtanα4pie*r^2 = q

    but i am having trouble in solving for r; i tried solving for r by using the Pythagorus theorem, but i didnt workout... i'm kinda stuck, i guess its because the known value for E field is in a component form...

    can someone please guide me through this question

    picture is attatched
    thanks in advance.

    Attached Files:

    Last edited: Jan 29, 2009
  2. jcsd
  3. Jan 29, 2009 #2


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    Homework Helper

    What picture?
    How long is the string?
  4. Jan 29, 2009 #3
    length of the string is not given
  5. Jan 30, 2009 #4

    Doc Al

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    Staff: Mentor

    That's the field of a point charge--not relevant here. You are given the field.

    This is fine.
    This is not. What force does a field E exert on a charge q?
  6. Feb 3, 2009 #5
    k it should be f=1/4pieEo *q/r^2 then right?
  7. Feb 4, 2009 #6

    Doc Al

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    Staff: Mentor

    No. Again, that's the field surrounding a point charge--not relevant here. You have a uniform field given; you need to find the force that given field exerts on the charge. (What's the definition of electric field?)
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