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Homework Help: Coulomb's Law of a cobweb

  1. Nov 6, 2008 #1
    1. The problem statement, all variables and given/known data
    A dust cobweb is drawn from an initial vertical position toward a nearby wall by an electrostatic force. Assume the cobweb to be like a single dust ball of mass 2.0 x 10-7g suspended on a massless string of length 0.42 m connected a horizontal distance of 0.35 m from the wall, as shown below. The tethered dust ball is drawn to the wall by another similar dust ball of opposite charge, -q = -3.0 x 10-6C, as shown.


    a) Draw a free-body diagram for the tethered dust ball in its final resting position.

    b) Transfer the force information from the free-body diagram to a triangle. (I think they mean a component triangle)

    c) What is the final charge on the cobweb?

    2. Relevant equations
    Fe = [tex]\frac{Kq_{1}q_{2}}{d^{2}}[/tex]

    where Fe is the electric force between two objects
    K is Coulomb's constant (9.0 x 109 Nm2/C2)
    q1 is object 1's electric charge
    q2 is object 2's electric charge
    d is the distance between centers of charged objects

    3. The attempt at a solution
    a) just draw the Normal Force, Force of Gravity, and the Electric force?

    b)unsure, but i could figure this out alone

    c) Would I just solve for the distance between the two dust balls and then sub that into the equation above, using +q for q1 and -q for q2 and 9.0 x 109 for K?

    edit: bah, nevermind that failed. need help for c)
    Last edited: Nov 6, 2008
  2. jcsd
  3. Nov 7, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Instead of "normal force", that should be tension.
    You would use that formula (Coulomb's law) to solve for the magnitude of the electric force in terms of the charges. (In what direction does that force act?) The negative charge is given; the positive charge is unknown--that's what you are trying to solve for. (It's a bit confusing that they called the charges +q and -q. If that were true, you wouldn't need to solve for anything!)

    Yes, you'll have to compute the distance between the charges.

    Hint: You'll need to analyze both vertical and horizontal force components on the tethered dustball.
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