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

Proton-proton fusion?

  1. Aug 18, 2008 #1
    I've been thinking about nuclear fusion process in the sun, and stumbled upon something weird:

    Basically, why there is a net energy from PP fusion?

    We have an input of 4 protons and output of 2 protons and 2 neutrons, now, a mass of a neutron is slightly larger than a mass of proton, so the result appears to be more massive, than the input!

    Some online research shown that it doesn't go just that simple, but the PP-chain process have the same problem - the input is 6 protons, the output is 2 neutrons and 4 protons, which is again more massive than the input, and above that there are 2 positrons, neutrinos and gamma-quants of net output.

    Where does the energy come from?
    Kinetic energy of proton collisions?
    If yes, what is the point of fusion, if it only reemit parts of the heat that make it go in the first place?

    Something does not add up, where am i wrong?
  2. jcsd
  3. Aug 18, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    I thought one ended up with a He4 nucleus, and not 2p + 2n ;-)

    And the mass of He4 is smaller than mass(2p + 2n)
  4. Aug 18, 2008 #3
    Hm, and He4 nucleus is made out of 2p and 2n, which raises the question, why is it lighter?
    Are there different protons and neutrons in the atoms, than in hydrogen and free-flying neutrons?
  5. Aug 18, 2008 #4


    User Avatar
    Science Advisor
    Homework Helper

    This is basic nuclear physics, the mass of the nucleus has lower mass than its constituent particles due to the (negative)binding energy. Same holds for atoms aswell, an atomic nucleus + electrons has higher mass then the atom as a whole.





    Enjoy your reading, if you have any more questions regarding this, please ask hera again :-)
  6. Aug 18, 2008 #5
    Thank you for the links, that made some sense although somewhat counter-intuitive at first - energy having "negative mass"...
  7. Aug 18, 2008 #6
    While it takes energy to turn a proton into a neutron, there's also energy created when the positron produced from the beta decay of the proton as it turns into a neutron reacts with an electron creating an additional 1.022 MeV of energy. So while you have energy 'left over' when the constituent particles bind to create a larger particle, there's also a little extra from the electron-positron reaction.

  8. Jul 1, 2009 #7

    I'm sorry, could someone please help me to understand the conservation of charge and leptons that happens in the first step of Proton-Proton fusion?

    H(1) + H(1) --> H(2) + positron + neutrino.

    I think I'm missing something to do with an electron? My nucleons balance, but I'm a charge positive and a lepton short on the right hand side of the relation.

    Last edited: Jul 1, 2009
  9. Jul 2, 2009 #8


    User Avatar
    Science Advisor
    Gold Member

    Charges balance: on left +1 from each H1, on right +1 each from H2 and positron
    Lepton balance: positron is antilepton, neutrino is lepton - balance =0.
  10. Jul 2, 2009 #9
    So to clarify, we're assuming the atoms are not neutral. Thank you!
  11. Jul 3, 2009 #10


    User Avatar
    Homework Helper

    We're not assuming that; we're just ignoring the electrons because they don't take part in the nuclear reaction we're discussing. If you include them, they'd be on both the left and right sides, so charge still balances.

    However, it is actually true that the electrons in the Sun's core are not bound to nuclei. The temperature there is way too high for that.
  12. Jul 3, 2009 #11
    Okay, I see that. Except that if we did include them, we'd get a photon on the right because there would be a free electron and the positron. Yes? This was the original source of my confusion.
  13. Jul 3, 2009 #12
    It's a little appreciated fact but most of the Sun's energy is not from proton-proton fusion at all. That's the underlying source reaction of all the big energy producers, so it's vital, but the Sun's energy is mostly from making He4 out of D, He3 & T. Easy to overlook, but worth remembering.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?