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Application of Coloumb's Law to electroscope

  1. Jun 30, 2004 #1
    Hi there, I am having a bit of difficulty with this problem dealing with a little more difficult application of Couloumbs Law.

    A large electroscope has two wires that are 75-cm long w/22g balls at the ends. When charged, all the charge resides on the balls (Q/2 for each ball) If the wires each make a 30 degree angle with the vertical, what total charge Q must have been applied to the electroscope?

    So I set up my free body diagrams and I know how to figure out the weight force and i believe i can get tension--- but what should I be doing to know how much force electrical force is applied (Repulsion in this case since both charges are positive) to make that 30 degree angle to the vertical?

    I need to be thinking back to mechanics a little bit =P

    Any help would be appreciated--- thank you.
  2. jcsd
  3. Jul 1, 2004 #2


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    Use Coulomb's Law. Also observe the system is in static equilibrium.
  4. Jul 2, 2004 #3
    yes net force is = to 0 because a = 0. Thank you I noticed that actually right after i posted this--- silly me.
  5. Jul 17, 2004 #4
    Down ward force = mg applied to each side this must be resolved horizontally to equal the charge force which opposes it -- a little trigonometry.
  6. Jul 17, 2004 #5
    There is no horizontal component of a downward force.
  7. Jul 17, 2004 #6
    John not directly your right , but the leaves are held apart by a horizontal force and they are trying to fall due to a vertical force , obviously something connects these two.
  8. Jul 17, 2004 #7
    It isn't the force of gravity, but rather the tension in the leaves, that has the horizontal force component that opposes the charge force.
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