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Homework Help: Electric Potential Energy Problem

  1. Jul 24, 2010 #1
    1. The problem statement, all variables and given/known data
    A charge of –4.00 µC is fixed in place. From a horizontal distance of
    55.0 cm, a particle of mass 2.50 × 10–3 kg and charge –3.00 µC is fired with an
    initial speed of 15.0 m/s directly toward the fixed charge. How far does the
    particle travel before it stops and begins to return back?

    2. Relevant equations
    KE = 1/2 mv^2
    Pba = -W = -qEd
    F = kQ1Q2/r^2

    3. The attempt at a solution
    1) Found the Kinetic energy of the moving particle :
    KE = 1/2mv^2
    = 1/2 (2.5x10^-3)(15)^2
    = 0.281 J

    2) Set the value I found for KE to PE and used the Potential Energy eqn:
    PE = -qEd

    Since E = kQ/d^2

    PE = -qd(kQ/d^2)

    Therefore: d = -qkQ/PE

    d = 0.38 m

    I'm not sure if I did that right. But the answer I came up with looks like it could work. Any help would be appreciated!
  2. jcsd
  3. Jul 25, 2010 #2
    You should not be finding [tex]\Delta[/tex]PE that way.
    [tex]\Delta[/tex]PE = -qEd only if E is constant. But E is not constant here.

    Write down V at the initial point and the final point.
    How is PE related to V?
    Then subtract the two to find [tex]\Delta[/tex]PE.

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