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Homework Help: Confuse over nuclear fusion

  1. Sep 14, 2011 #1
    1. The problem statement, all variables and given/known data
    This is the question:

    2H1 + 3H1 ==> 4He2 + 1n0 +Q

    Where Q = 17.7 MeV

    The binding energy per nucleon are given below:

    2H1 = 1.12 MeV
    1n0 = Null
    4He2 = 7.07 MeV

    " Determine the binding energy per nucleon, in MeV, of tritium (3H1)"

    2. Relevant equations

    3. The attempt at a solution

    This is how i do it,

    The B.E for:

    2H1 = 2 x 1.12 = 2.24 MeV
    4He2 = 7.07 x 4 = 28.28 MeV

    From chemical equation shown above, The energy before reaction is equal to energy after reaction. The energy is conversed before and after reaction:

    2.24 + 3(X) = 28.28 MeV + 17.7 MeV

    X = 14.58 MeV which is the B.E per nucleon for 3H1

    But when i check up the answer, it turns out to be 2.78 MeV. What wrong with my method of solving the questions? can anyone explain what my mistake?
  2. jcsd
  3. Sep 14, 2011 #2
    You are very close.

    The problem is that on the right side of the equation, binding energy and energy released cannot be added. Energy released is energy that is actually "there", where as binding energy represents mass or energy that's missing. So when you're combining binding energy and energy released in your very last calculation, they need different signs. Turn that + into a - and you should be home free!
  4. Sep 14, 2011 #3
    Thank for your reply. Is it true to explain this way, the total energy required to form helium is 28.28MeV meanwhile 17.7mev will be release from the 28.28mev..is this correct?
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