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E=mc^2 and fission and fusion

  1. Sep 29, 2012 #1
    Hi, this is my second year studying physics and few weeks ago we went in to nuclear physics very briefly because we are learning it next year.

    I'm not sure if the energy required to fission is equal to the energy required to fusion. For example is energy required to fission hydrogen atom, equal to fusion particles together to create hydrogen atom? (If fissioning result particles were same with fusioning particles)

    According to my friend who studies chemistry says it's not equal. (I do not study chemistry). Just according to E=mc2 if result of fission particles and particles required to fusion were the same, this means that mass and number of particles needed to fuse would be equal.

    I know that fusion is much harder to do, and we can't just put particles together to form an atom... Please give me a understandable answer.
  2. jcsd
  3. Sep 29, 2012 #2


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    "e= mc^2" has nothing to do with the energy required to cause fission or fusion. It is, rather, the energy you get out of fusion and fission. For example, Uranium 239 fissions into Neptunium 239 plus a neutron. The total mass of the the Neptunium 239 atom and the neutron is slightly less than the mass of a Uranium 239 atom. The difference in mass turns into energy, e= mc^2, and is sent off as a gamma ray.

    When two heavy hydrogen atoms combine to form a helium atom, the helium atom has slightly less mass that the total of the two hydrogen atoms. The difference in mass turns into energy.

    But the energy from one atom of Uranium fissioning is NOT the same as the energy from two hydrogen atoms fusioning. The fusion energy is about three to four times greater than the fission energy.
  4. Sep 29, 2012 #3
    That's the energy per nucleon. The fission energy of 1 atom of Uranium is about 200 MeV, more than 10 times the energy of any fusion reaction with hydrogen isotopes.
  5. Sep 30, 2012 #4


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    It is not really clear which processes you want to compare.

    A specific fusion process will always release/need exactly the energy the corresponding fission process will need/release.
    You can fuse deuterium+tritium (both are hydrogen isotopes) to helium+neutron, for example, this releases 17.6 MeV of energy. If you try to fission that helium nucleus again, you have to add a neutron and those 17.6 MeV of energy to get deuterium+tritium.

    Fusion of light elements (like hydrogen) gives more energy per mass than fission of heavy elements (like uranium), but that is comparing apples and oranges.

    Most hydrogen atoms are a single proton, you cannot fission them.
    That is "just" an engineering issue.
  6. Oct 1, 2012 #5
    Two hydrogen atoms fusing to make a helium atom release X amount of energy. The helium atom splitting into two hydrogens absorbs that same amount of energy. The magnitude (absolute value) of the energy is the same but the sign is opposite. So they aren't equal.

    The equilibrium favors helium so strongly that the reverse reaction virtually never happens and is always neglected.
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