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Homework Help: Electric Potential Energy Question

  1. Feb 17, 2012 #1
    1. The problem statement, all variables and given/known data

    A uranium nucleus in a reactor captures a slow neutron and divides, or fissions, into two smaller daughter nuclei. Assuming the nucleus divides into two equal daughters with charge Q=46e and diameter d=2x10-14m, calculate their electric potential energy.

    2. Relevant equations

    k = (9x10^9)Nm^2/C^2

    V = kQ/r

    U = qV

    3. The attempt at a solution

    I've tried using the two equations above to find the U, but it's not right with both:

    k(46)/(1x10^-14) & k(46^2)/(1x10^-14).

    Am I using the wrong formulas? Not doing enough work?
     
    Last edited: Feb 17, 2012
  2. jcsd
  3. Feb 18, 2012 #2
    May be you are using wrong units? What is the unit of charge?
     
  4. Feb 18, 2012 #3
    For the charge it just says Q=46e. So I just plug that in to the equation:

    U = (k*Q)/(r)

    = (9x10^9)*(46^2)/(1x10^-4)

    //Using the constant k, two charges of Q since there are two daughters, and dividing the diameter by 2 to get the radius.

    Given the formulas they gave us in class, I would think they would be relevant to the question.... this is why physics frustrates me! :(
     
  5. Feb 18, 2012 #4
    e = Charge of an electron = -1.6 * 10 ^-19 C
     
    Last edited: Feb 18, 2012
  6. Feb 18, 2012 #5

    ehild

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    Homework Helper

    In Q=46e e means the elementary charge, 1.6x10-19 C. Are you sure that the formula for U is valid in this case? That charge 46e means 46 protons in both nucleus.
    ehild
     
    Last edited: Feb 18, 2012
  7. Feb 18, 2012 #6
    Ah, stupid mistake again. Clearly these questions are clouding my basic understanding...

    So let me try to understand, Q=46e = 46(1.6x10[itex]^{-19}[/itex]) ?

    And I have this formula too:

    U= k*Q[itex]_{1}[/itex]*Q[itex]_{2}[/itex]*e[itex]^{2}[/itex]/d[itex]^{2}[/itex]

    (Using d[itex]^{2}[/itex] since it should be both radius added together anyways)

    So therefore:

    U = (9x10[itex]^{9}[/itex])*46*46*e[itex]^{2}[/itex]/(2x10[itex]^{-14}[/itex])

    ?

    Yes seems so! Thanks again guys, I really appreciate your time for helping me understand. I'm trying my hardest to learn.
     
    Last edited: Feb 18, 2012
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