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Homework Help: Coulomb's Law and insulating fiber

  1. Apr 2, 2006 #1
    A rigid, insulating fiber runs along a portion of the y-axis; the fiber isnot free to move. Gravity acts downward (g = 9.81 m/s2).A charge Qa = -3 µC is fixed to the fiber at the origin. A bead with a hole drilled through its center is slipped over the fiber andis free to move along the fiber without friction. The mass of the bead is m = 170 gand its charge is Qb. At equilibrium, the bead floats a distanceyb = 16 cm above the origin.
    ---
    the coulomb force must balance the gravitational force, so..
    the answer is in C, not µC, so Q(a) = 0.003 C.
    K * 0.003* Q(b) / 0.16^2 = 9.81 * 0.17
    solving for Q(b), i get 1.581226667e-9.
    i checked my solution, and it did equal to the gravitational force.
    what am i doing wrong?
    am i miscomprehending the problem, or the coulomb's law?
     
    Last edited: Apr 2, 2006
  2. jcsd
  3. Apr 2, 2006 #2
    check your units and signs,

    [tex] 0.003C \neq -3 \mu C[/tex]

    (there are two things that need to be changed to make this an equallity)
     
    Last edited: Apr 2, 2006
  4. Apr 2, 2006 #3
    doh!
    thanks. hehe.
     
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