1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Energy Conservation & E=M*C2

  1. Mar 18, 2010 #1
    This is question about equation E=m*c2. Why this equation does not required any condition of matter?
    Case 1: I take two mole (4 Kg)H2 & one mole(32 Kg)O2 & make water & get energy.
    2H2 (4 Kg Hydrogen)+O2 (32 Kg Oxygen)=2H2O(36 Kg of water)+ 572 KJ Energy
    Now I convert this water into energy by equation E=m*c2.
    Total energy I get is QTotal= (36*c2) + 572 KJ Energy.

    »»» Now instead of I convert hydrogen & oxygen into water, I directly convert both gases into energy so total Energy I getting is,
    QTotal= m*c2 =(36*c2) Only.
    This energy is lesser as compared to first one by amount of “572 KJ Energy”.

    Case 2: I have one Kg of boiled water at 100 degree centigrade.
    Now I extract 418.7 KJ energy from the water & make it of zero degrees centigrade.
    Water at 100 C - 418.7 KJ Energy =Water at 0 C.
    Now I convert this zero degree water into energy,
    Total energy I getting is,
    QTotal=1*c2+418.7 KJ energy

    »»» Now I directly convert boiling water into energy,
    I getting energy is
    This energy is lesser as compared to first one by amount of “418.7 KJ Energy”.

    Why it is occurs? Why energy librated is different for the same initial & final condition of mass?
  2. jcsd
  3. Mar 18, 2010 #2


    Staff: Mentor

    Actually, you will get only 35.9999999999936 kg of water. Everything else then works out fine. This difference in mass between the products and the reactants is called the mass deficit and is closely related to the binding energy. See: http://en.wikipedia.org/wiki/Binding_energy. The mass deficit occurs for all interactions, although as you can see above it is usually pretty small and only noticeable for very strong interactions, like those involving the strong nuclear force.
  4. Mar 18, 2010 #3
    This means that boiling water possesses higher mass then cold water by extremely small amount which is neglected for practical purpose.
    OK, Thanks for giving answer.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Threads - Energy Conservation E=M*C2 Date
B Conservation of energy of a perfectly decoherent laser Today at 5:24 AM
I What does Noether's theorem actually say? Dec 18, 2017
B Does work energy theorem fail while dealing with friction? Sep 18, 2017