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Source of energy in fusion reactions

  1. Nov 4, 2012 #1
    In the case of nuclear fission, when U-235 splits into Ba,Kr and three neutrons , then it is shown that the total mass of the products on the right side of the equation is slightly less than that of the reactants on the left side. So this missing mass-the mass defect – is converted into energy and released. But in the case of a nuclear fusion reaction, when 4 hydrogen nuclei fuse together and form a helium nuclei there also energy is released ie.27.6 MeV per fusion. But how this mechanism can be explained?

    Because in this reaction the total energy on the left side is calculated as 4.4 MeV and that of the product nuclei ie. He, is given as 28 MeV. Here the energy on the right hand side is found greater than that of the left side. Logically in this reaction it appears as if the energy is only absorbed, but not released. Then how we do we get the energy released in the fusion reactions as in the case of fission reactions?

    Can anybody kindly explain this contradiction in a simple manner?
  2. jcsd
  3. Nov 5, 2012 #2


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    If you calculate the masses on the left and right side, you'll see that the helium nucleus (plus positrons) is lighter than the 4 protons together. It is very similar to fission in that respect.
    The total energy in the process is of the order of 4 GeV, not MeV, but most of that remains in the nuclei.
    Also keep in mind that "4 protons -> Helium-4" as single process does not exist.
  4. Nov 5, 2012 #3


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    Also remember that somewhere along the way, two protons have been converted to two neutrons, and the positrons and neutrinos thus produced will be happy to carry away some of the energy in the form of kinetic energy.
  5. Nov 5, 2012 #4


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    Look up "binding energy curve" and "semi-empirical binding energy formula." Generally speaking (there are exceptions), at lower atomic masses, fusion releases energy and fission absorbs energy. At higher atomic masses, it's the other way around.
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