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Mass-Energy Equivalence

  1. Jun 19, 2011 #1
    I was reading WikiPedia's entry on this, and there was one paragraph that surprised me:

    E = mc2 has sometimes been used as an explanation for the origin of energy in nuclear processes, but mass–energy equivalence does not explain the origin of such energies. Instead, this relationship merely indicates that the large amounts of energy released in such reactions may exhibit enough mass that the mass-loss may be measured, when the released energy (and its mass) have been removed from the system.


    My question is, is this valid information? If so, could someone elucidate on where the energy from nuclear reactions comes from if it is not E=mc2?

    Many thanks!
  2. jcsd
  3. Jun 19, 2011 #2
    What they mean is that the energy comes from things like nuclear binding forces. E=mc^2 is always true but doesn't tell you the origin of the E. For nuclear masses it's the strong force. For chemical reactions the (tiny) mass deficit is electrostatic in origin. Etc.
  4. Jun 19, 2011 #3

    What I believe the article is describing is what is called the "Mass Defect." In short the Mass Defect is rumored to be the origin of E = mc^2.

    In concept the "Mass Defect" states that; "The measured mass is less than the sum of its parts!"

    For example a Helium Atom is comprised of: two (2) electrons, two (2) protons, and two (2) neutrons.

    If you know the mass of each particle, and you sum up the mass, and write that Net Mass calculation down. Then you go and measure the mass of the Helium Atom, you will find that the Net Mass of the Helium is less than your calculated value.

    The difference between your calculated value Net Mass and your measured value of the Net Mass, will be the "Mass Defect" equal to E = mc^2

    Given by the following equation

    [tex] E_{Defect} = \Delta m c^2_{Light} = E_{calculated} - E_{measured} [/tex]

    [tex] E_{Defect} = \Delta m c^2_{Light} = m_{calculated}c^2_{Light} - m_{measured}c^2_{Light} [/tex]
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