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I Does an H proton weigh more than an He proton?

  1. Jan 3, 2017 #1
    In his book Hyperspace, Michio Kaku is discussing hydrogen fusion and states that, "the protons in hydrogen weigh more than the protons in helium". Is this correct? I understand that atoms and nuclei will weigh different amounts, but I thought a proton was a proton, no matter where in the universe it was located.
     
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  3. Jan 3, 2017 #2

    Vanadium 50

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    The mass of a helium nucleus is less than the mass of two protons plus two neutrons. That's a fact. How you want to partition this difference is personal taste.
     
  4. Jan 3, 2017 #3
    Sounds like witchcraft (jk), any good source that explains this in more detail? I hate to rely only on Wikipedia. It makes sense that the mass of an atom can be more than the sum of its constituents due to binding energy, but being less sounds like there's some hidden field holding the remainder.
     
  5. Jan 3, 2017 #4

    mfb

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    Nuclear binding energy is negative. Energy is released when you form a helium nucleus.

    Same for binding electrons: a neutral hydrogen atom has a mass slightly below the sum of masses of proton plus electron.
     
  6. Jan 3, 2017 #5
    So then is it accurate to say nuclear fission of lighter elements (if even possible) would renergize the nucleons when freed?
     
  7. Jan 3, 2017 #6

    mfb

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    "renergize"?

    It would add energy to the system. It is possible, e. g. with high-energetic collisions.
     
  8. Jan 3, 2017 #7

    Vanadium 50

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    I would say this mixes cause and effect. To disassemble a nucleus like 4He requires that you add energy.
     
  9. Jan 3, 2017 #8
    Got it, thanks guys. (meant re-energize, that's what happens when I'm thinking physics in McDonalds)
     
  10. Jan 4, 2017 #9
    I did some research on this. Two atoms of deuterium (total of 2 protons, 2 neutrons, and 2 electrons) weights 4.028 u. One atom of helium-4 (total of 2 protons, 2 neutrons, and 2 electrons) weighs 4.003 u. What exactly causes this? Does the helium atom have less energy, which causes it to have less mass? I know that as an object speeds up, it weighs more because it has more energy.
     
  11. Jan 4, 2017 #10

    mfb

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    Correct.

    The difference is the nuclear binding energy.
    No it does not. "Mass" is defined in the rest frame of the object. The concept of a relativistic mass (which depends on speed) is not used any more.
     
  12. Jan 5, 2017 #11
    But why? Why can't we speak about the relativistic mass?
     
  13. Jan 5, 2017 #12

    mfb

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    We can, but it just leads to misconceptions, and there is no point in it - we have energy already, which is the same thing apart from a constant factor. Using relativistic mass is like using kilometers and miles in the same calculation for no reason.
     
  14. Jan 5, 2017 #13
    Oh yeah I get it.

    So its like in helium we have more binding energy than that in two deuterium.

    So micho kaku just divides this total mass among the protons

    Binding energy plays silently
     
  15. Jan 5, 2017 #14

    mfb

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    Please edit your posts if you want to add something, I merged them now.
    Correct.
    I guess so.
     
  16. Jan 5, 2017 #15
    You need to understand what most of that mass is. Protons are make of 3 quarks. Do how much mass do you think they have compared to the proton? 1/3 because there are three of them? It's actually about 1/300th. The total mass of the quarks in about 1% the mass of the proton.

    So where is the rest of it? It's energy in the strong force field.


    The strong field doesn't have exactly the required energy to hold 3 quarks together, it has extra (the leftover force holds the protons themselves together.). Since there is a little extra binding energy left over, it doesn't require twice as much in order to bind together twice as much stuff.
     
  17. Jan 11, 2017 #16

    PeterDonis

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    Several posts regarding relativistic mass have been deleted; relativistic mass is off topic for this thread. Please take note.
     
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