Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Mass to energy

  1. Sep 16, 2004 #1
    According to Einstein's theory, during the process of fusiom some of the mass of the element(s) gets converted to energy, therefore the over all mass of the obnject is less due to the mass to energy convertion. This is true yes?
  2. jcsd
  3. Sep 16, 2004 #2


    User Avatar
    Science Advisor

    Yes. Example (all numbers are atomic mass units).
    neutron 1.0086649
    H1 1.007825
    He4 4.0026032
    Add up 2 neutrons and 2 H and get 4.0329798, leaving a difference (converted to energy) of .0303766.
    Last edited: Sep 16, 2004
  4. Sep 16, 2004 #3
    To mathman

    Thanks for your reply.
    Another question: Outside of the big bang are there any examples where energy gets converted to mass?
  5. Sep 16, 2004 #4
    Sure, lots. Another nuclear example: In nuclear fission (as opposed to fusion), a neutron is absorbed by a fissionable nucleus such as u-235, or pu-239. The resulting nucleus is unstable and splits, generally into two large fragments and releases some more neutrons. The sum of the masses of the fragments and the released neutrons is less than the mass of the nucleus and the absorbed neutron before the reaction. This "mass defect" is converted to energy. This is how a nuclear weapon works and how a nuclear power plant generates energy.
  6. Sep 16, 2004 #5


    User Avatar
    Staff Emeritus
    Science Advisor

    Yes, though the energy has to get radiated away before the mass will actually decrease.
  7. Sep 17, 2004 #6
    The sum of the rest mass of the individual particles change. But the total mass remains unchanged. See http://www.geocities.com/physics_world/sr/nuclear_energy.htm

    The conservation of mass holds true whether you think of mass as relativist mass or as invariant mass. In the case of the later the invariant mass equals the energy in the inertial frame divided by c^2. Since energy is conserved then so too does the invariant mass. In the former case the total mass is the sum of the masses.

    Note: Relativistic mass is the m in p = mv. Given this definition it can be shown that this is a conserved quantity and to show this one does not need to rely on the conservation of energy.

  8. Sep 17, 2004 #7


    User Avatar
    Science Advisor

    Pair production (gamma ray to electron-positron pairs) is an absorption mechanism used in shielding against gamma rays.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook