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Homework Help: Find the speed of the charge

  1. Mar 12, 2012 #1
    For this question, I have solved letters a, b and c. I'm stuck at letter d.

    1. The problem statement, all variables and given/known data
    Two point charges ql = +2.40 nC and q2 = -6.50 nC are 0.100 m apart. Point A is midway between them; point B is 0.050 m from ql and 0.060 m from q2 (Fig). Take the electric potential to be zero at infinity. Find
    (a) The potential at point A;
    (b) The potential at point B;
    (c) The work done by the electric field on a charge of 2.50 nC that travels from point B to point A.
    (d) The speed of the 2.50 nC when it reaches point A, if it starts from rest at point B

    2. Relevant equations

    KE = 1/2 m v2 ?

    3. The attempt at a solution
    I have solved a, b and c

    a) VA = -737 V
    b) VB = -704 V
    c) WB->A = +8.2 x 10-8 J

    Now I'm stuck at letter D, how do i find the speed? I assume that I might need the equation of KE = 1/2 m v2.. But I dont know how to do it.

    What I did was to put +8.2 x 10-8 J as my KE. However, I do not have a mass, so how do I do this?
  2. jcsd
  3. Mar 12, 2012 #2


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    Your method is fine. You can't solve this problem without having the mass of the charge.
  4. Mar 12, 2012 #3
    Oh you mean that the problem is missing the mass? If that's so then I cannot solve this.
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