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Energy is neother created nor lost? How about E=mc^2

  1. Dec 7, 2008 #1

    I know that energy is neither created nor lost based on everyday observation but how about E=mc^2 when mass converts to energy. Do physicist consider that as energy created because mass is loss while it creates a huge amount of energy.
  2. jcsd
  3. Dec 8, 2008 #2


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    Hi, Mattan.
    The laws of 'conservation of energy' and 'conservation of mass' have been combined into the law of 'conservation of mass/energy' because of that equivalence. While neither can be created nor destroyed, they can be converted to each other. One simple example is an H-bomb. Something like 1% of the matter is converted to energy, and it can get pretty loud. Particle accelerators run experiments of sufficient power that they can create particles from energy, and conversely can collide matter with antimatter to release the full energy potential. Professional particle physicists on this site can provide more specific answers, as can the theoretical physicists. I don't really know much about it.
  4. Dec 8, 2008 #3
    E=mc^2 means that mass IS a form of energy. So Mass does not convert to energy, because it is already energy. Turning Mass into heat is similar to turning light into heat.

    E=mc^2 just converts the units of kg to jewls* (and vice verca).

    *The unit used for mass on the sub-atomic level is eV/c^2 so when you put this into the equation, the c^2 's cancel and eV = eV. I dont know if this way of looking at it helps.

    edit: Actualy im not too confident about using jewls and kg in E=mc^2, so can someone either conferm that i am right or wrong...
    Last edited: Dec 8, 2008
  5. Dec 8, 2008 #4


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    Quite right. Mass is essentially 'bound' energy. Nice post. (Except that you spelled 'joules' wrong... :wink:)

    ...and 'versa'... :biggrin:
  6. Dec 8, 2008 #5
    Haha! Im a physisist/mathematition not an english student. (and im at school now, and the schools internet browser does not have a built in spell checker :redface:)
  7. Dec 8, 2008 #6


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    No worries, mate; you're still pretty articulate. Anyhow, most of those spell checkers use Yankee standards. Mine is automatic on this Macbook, and it yells at me every time I spell things like colour or manoeuvre correctly. :grumpy:
  8. Dec 8, 2008 #7
    people like to point to atom bombs & hydrogen bombs as examples of "E=mc^2" as is these are the best examples. They are not. These nuclear reactions release the energy stored in the binding energy between the components of a nucleus - they do not change the components of the nucleus (I mean, no neutrons or protons are being "converted" into energy). This is very similar to the way normal chemical reactions (burning coal for example) releases the binding energy between the atoms in a molecule. The most apparent difference between the nuclear reactions and the chemical reactions is the magnitude of the binding energy. Since this is about a million times higher for the nucleus, the corresponding change in mass is also a million times higher.

    If you want an example of 'matter' being 'converted' into energy, look instead to a proton - antiproton reaction.
  9. Dec 8, 2008 #8

    Ranger Mike

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    i continue to learn!
  10. Dec 8, 2008 #9

    D H

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    They certainly are examples of E=mc2. Fission bombs "convert" about 0.1% of the fuel mass to energy. There is binding energy that holds the nucleus together.

    Even chemical reactions are examples of E=mc2. The mass of a two hydrogen molecules plus the mass of one oxygen molecule differs slightly from the mass of two molecules of water. The reason conservation of mass appears to hold in the everyday world is because the change in mass that results from chemical reactions is practically immeasurable.
  11. Dec 8, 2008 #10
    I should also point out that while energy and mass are not individually conserved in general, as long as you aren't blowing atoms apart, they are excellent approximations.
  12. Dec 8, 2008 #11
    Any form of energy has a 'mass', right? Or at least a gravitational attraction. A beam of light exerts a gravitational pull, and a particle that is moving (has more kinetic energy) has a larger "mass" than it does when it is static. So when you say that 0.1% of the mass is turned to energy, is this the mass that comes from the binding energy, rather that the 'rest-mass' of any of the particles inside? In which case I can agree with "gmax137".

    He is not saying that mass is not lost, he is saying that the mass that is lost is not simply matter turning to energy. Do I make sense? Am I right?

    edit: So if energy has mass, then is the 'mass' of something only proportional to the energy it holds? In which case mass is always conserved, even in these extreme situations. Is is the total mass of the new forms of energy (like EM waves, or kinetic-energy/heat) equal to the 0.1% of the mass that is lost in the reaction?

    edit2: A photon has no mass, but it has a gravitational pull. So when I say mass, I mean its gravitational pull...
    Last edited: Dec 8, 2008
  13. Dec 9, 2008 #12
    DH, sorry if I wasn't clear. My point was simply that the nuclear and chemical reactions are not qualitatively different - in both cases "release" of binding energy is a change in mass. It's just more apparent in the case of the nuclear reactions since the binding energy is so much greater.
  14. Dec 10, 2008 #13


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    The binding energy of molecules is based on electrostatic forces, not the strong nuclear force. So they are completely different :)
  15. Dec 10, 2008 #14
    OK, in chemical reactions we see that certain arrangements of atoms actually weigh more or less than other arrangements of the same atoms. In nuclear reactions we see that certain arrangements of neutrons & protons weigh more or less than other arrangements. Do you see the similarity? The fact that the origin of the forces between the particles is different doesn't change the underlying idea. Now if we react a proton & antiproton, the result is *no* protons - what's left is photons and neutrinos, that is, objects with no or essentially no mass. The energy released in these reactions is essentially "all" of the original mass, and it is several orders of magnitude greater than the binding energy available in nuclear reactions and about ten orders of magnitude greater than the energy from chemical reactions.
  16. Dec 24, 2008 #15
    Hi, The Energy of these photons and neutrinos have energy, and that means that they exert a gravitational pull, they "fall" towards thinks with mass, and pull things towards them. Is the overall 'gravitational pull' conserved from before the annihilation to after? i.e. if the whole thing happened in a closed system, would the weight be the same from beginning to end?
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