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Homework Help: Finding final velocity given 2 charges

  1. Sep 16, 2009 #1
    1. The problem statement, all variables and given/known data

    A small metal sphere, carrying a net charge of q_1 = -2.70 \mu C, is held in a stationary position by insulating supports. A second small metal sphere, with a net charge of q_2 = -7.60 \mu C and mass 1.50 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.


    What is the speed of q2 when the spheres are 0.410 m apart?
    How close does q2 get to q1?

    2. Relevant equations


    3. The attempt at a solution

    I don't really know how to do this problem but i started it in this process-

    The potential energy is the work done in moving the charge through the electric field. That Potential energy is donated U. So i know that the work done in the system will equal a force x distance. F = ma. ( Work= Fd). so the U = ma x distance. Then using this i could find the acceleration and the velocity... problem is i have not time intervals. so im lost. please help
    Last edited: Sep 16, 2009
  2. jcsd
  3. Sep 16, 2009 #2


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    What is required in the problem?
  4. Sep 16, 2009 #3


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    You don't say what the question is, but it looks like you need to use conservation of mechanical energy, time is not an issue here. KE + PE at one point is equal to KE + PE at another.
    Last edited: Sep 16, 2009
  5. Sep 16, 2009 #4
    so i need to find the electrical potential energy. Then this is equal to the KE= 1/2mv^2;

    so K(Q1Q2)/r=1/2mv^2 and solve for v at r .410m for the first question

    then how do i find how close they get to each other. Using the same equations and solving for r where ke=u? if so, how do i find v^2 for the KE?
  6. Sep 16, 2009 #5
    The kinetic energy of the charge q2 is zero at the point of closest approach since it is instantaneously at rest there.
  7. Sep 16, 2009 #6
    well i worked out part 1 of the problem and got 24.5m/s which is wrong. When finding the potential energy at the point .410, which lies between the 2 charges, do i use .410 for the distance r in the equation;

    U=(8.85e9)*(-2.6e-6 * -7.6e-6)/r = 1/2mv^2 ?
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