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Homework Help: Two small metal spheres

  1. Sep 20, 2009 #1
    1. The problem statement, all variables and given/known data
    A small metal sphere, carrying a net charge of q_1 = -2.60 \mu C, is held in a stationary position by insulating supports. A second small metal sphere, with a net charge of q_2 = -7.70 \mu C and mass 1.40 g, is projected toward q_{1}. When the two spheres are 0.800 \rm m apart, q_{2} is moving toward q_{1} with speed 22.0 \rm m/s . Assume that the two spheres can be treated as point charges. You can ignore the force of gravity.
    What is the speed of q_{2} when the spheres are 0.450 m apart
    ?How close does q_{2} get to q_{1}?

    2. Relevant equations

    3. The attempt at a solution
    I figured this was a simple conservation of energy problem, so i began it at such.
    .5m2Vi^2 + q2Va = .5m2Vf^2 + q2Vb

    after simplifying it all down i got:
    sqrt((m2vi^2 + q2(Va-Vb))/m2) = Vf

    I calculated Va to be -29233V and Vb to be -51952V. Im not sure what i did wrong here.
    to calculate V i used q1, the charge of motionless sphere, since V is independent of the observer charge. For the r i used the distance from the moving sphere to sphere, and i used both values. when multiplying (Va-Vb) by q i used the q value of the moving sphere. Please help!
  2. jcsd
  3. Sep 20, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor

    What's the electrical potential energy between two point charges?
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