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Homework Help: Coulombs law not working

  1. Sep 30, 2007 #1
    1. The problem statement, all variables and given/known data

    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 = -7 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 = 250g and its charge is Qb. At equilibrium, the bead floats a distanceyb = 11 cm above the origin.

    Calculate the charge on the bead.

    2. Relevant equations

    Why isn't coulonmbs law working for me????

    3. The attempt at a solution

    OK heres what i did

    F = (KQa * Qb) / r^2

    (absolute values for the charges)
    (i got F by realizing that it must balance the opposing downward force (drew FBD)


    (.245)9.81 = (9.0*10^9) (7*10^-6)Qb / ((.011)^2)

    So Qb comes to 4.71*10-9
    =wrong for sum reason

    Please help and THANKS!
  2. jcsd
  3. Sep 30, 2007 #2
    i was saying that 11cm = .011m that was prob.

    One other question though.
    Why does the opposing charge have to be negative??
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