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Point Charge Location Question

  1. Apr 2, 2009 #1
    Sorry for the poor title - I'm not entirely sure how to describe the type of problem!

    1. The problem statement, all variables and given/known data

    A charge of -4.00 x 10-6 C is fixed in place. From a horizontal distance of 55.0cm a particle of mass 2.50 x 10-3 kg and charge -3.00 x 10-6 C is fired with an initial speed of 15.0m/s directly towards the fixed charge. How far does the particle travel before it stops and begins to return back?

    2. Relevant equations

    V = k(Q/r)

    PE = (QV)/2

    KE = (mv2)/2

    3. The attempt at a solution

    I've tried this a few times and haven't found anything that works so here is my latest attempt:

    A__________________________B_______________________C
    A = fixed charge
    B = unknown distance
    C = Starting point for moving charge

    Qa = -4.00 x 10-6 C
    Qc = -3.00 x 10-6 C
    v0c = 15m/s
    v1c = 0m/s
    mc = 2.50 x 10-3 kg
    rc = 0.55m
    rb = ?

    rb = k(Qa/Vc)
    Vc = 2PE/Qc
    PE = -KE
    KE = (mv2)/2

    KE = (2.50 x 10-3 kg x -15m/s)/2 = -0.009375
    PE = 0.009375

    Vc = 2(0.009375)/-3.00 x 10-6 C = -6.25 x 103

    rb = 9 x 109 (-4.00 x 10-6 C/-6.25 x 103) = 5.76 m
     
    Last edited: Apr 2, 2009
  2. jcsd
  3. Apr 2, 2009 #2

    rl.bhat

    User Avatar
    Homework Helper

    KE = 1/2*m*V^2
    PE = k*Qa*Qc/rb
    In this problem ra is not needed.
     
  4. Apr 2, 2009 #3

    Doc Al

    User Avatar

    Staff: Mentor

    Don't confuse the PE between two point charges with the energy stored in a capacitor. Look up the correct formula for PE.

    Good.

    Hint: Compare the PE at point C with the PE at point B.
     
  5. Apr 3, 2009 #4
    Thanks - I knew I was doing something wrong and just couldn't seem to sort out the problem correctly!
     
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