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The role of binding energy in fusion

  1. Jul 12, 2014 #1
    I am doing a high school research project on nuclear fusion and i just wanted to double check if my understanding of the role of binding energy in fusion reactions was correct.

    So we add energy in the form of heat to a tritium atom of mass "3" approximately and deuterium of mass "2" , this is the energy required to overcome the strong nuclear force and break apart the atom into its nucleons and that energy is converted in to mass. So the nucleons have greater mass but less energy than the atom. Then we somehow get the nucleons to combine, i haven't researched how we do this yet, to form Helium of mass "4". The mass of the Helium "4" is smaller than the combined mass of our two starting products which have a mass of "5" This mass difference is the energy which is released during fusion and which we can harness.

    This is my current understanding but i have a feeling something is wrong? Because then how does fusion release energy when two deuteriums are used as the starting product?

    Another question i wanted to ask is if the mass of atoms which we have calculated using the semi-empirical mass formula is an approximation how do we know the mass of atoms is not a little bit larger and the same as the mass of the individual nucleons?
  2. jcsd
  3. Jul 12, 2014 #2


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    See above. (your description is unfortunately quite wrong).

    When H2 and H3 combine, the result is He4 and a neutron. The energy release comes from the mass difference between the incoming and the out going particles.

    In general, binding energy of any nucleus is the energy equivalent of the difference in mass of the sum of component nucleons and the nucleus.
  4. Jul 12, 2014 #3


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    Heating the atoms just makes them moving faster, it does not change their internal structure or break them apart. And the energy is way too low to do that anyway.
    Deuterium and Tritium repel each other (they are both positively charged), but as they are so fast, they can come very close to each other just by random chance - so close that the strong force attracts them and lets them fuse. The resulting collection of 2 protons and 3 neutrons is extremely unstable and emits a single neutron. This process releases a lot of energy and you get a very fast neutron and a fast He-4 nucleus. The sum of their masses is a bit smaller than the sum of tritium+deuterium mass.

    You just measure the mass of the nucleus.
    Good mass spectrometers can see the mass defect, and Penning traps can measure the mass down to the level of electronvolts (1 billionth of the proton mass!).
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