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Binding energy

  1. Sep 28, 2015 #1
    1. The problem statement, all variables and given/known data
    I've attached the question I'm having trouble with.

    2. Relevant equations



    3. The attempt at a solution

    I got an answer of 1.70375 * 10^-3 for the binding energy of a nucleus of uranium 235.
    For the third part, "When a uranium-235 nucleus undergoes fission..." I read the value of the binding energy per nucleon for a nucleus half the size of uranium's to be 8.5 *10^-13
    So I calculated:
    (1.70375 * 10 ^-3) -(2 * 117.5 * 8.5 * 10 ^-13) = difference in total binding energy.

    Then I converted this answer in MeV to joules but I got the wrong answer.
     

    Attached Files:

  2. jcsd
  3. Sep 28, 2015 #2

    mfb

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    2016 Award

    Staff: Mentor

    Use units! I'm sure that would have made the problem obvious.

    That quantity is missing units.
    What did you get, and what do you expect to get?
    The two different parts seem to use different units. That cannot work.
     
  4. Sep 28, 2015 #3
    The values on the vertical axis are already in MeV. Why do you need to convert? Can you show how you get these numbers?
     
  5. Sep 28, 2015 #4
    I did 7.25 MeV per nucleon * 235 nucleons. I've just realised that I used the wrong conversion factor.
    This gives me 1.70375 * 10 ^ 9 eV / 1703. 75 MeV
    Nasu, I needed to convert to eV because the question asked for it that way.
     
  6. Sep 28, 2015 #5
    Yes, now it makes sense. Your 10^(-3) did not look OK.
     
  7. Sep 28, 2015 #6
    Yeah, with that value I calculated:

    (8.25 * 235) - 1703.75 = 235 MeV

    235 MeV = 3.77 x 10 ^ 11
    !!!!!!!!
    Hurrah!
     
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