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Binding Energy

  1. Jan 29, 2008 #1
    I was working at the energy released through the triplealpha process, and came to a slight problem. I worked out for Helium first, using:

    (2 x 1.007825u) + (2 x 1.008665u) - 4.002602u = 0.030378u

    which when x3 and converted gives ~84.9MeV.

    Doing the same for carbon, but using 12.011u for the atomic mass (as is given in most places), I end up with ~81.9MeV, when I should be getting ~92.2MeV. But when I do the same equation using 12u, I get the right answer! What's going on?
  2. jcsd
  3. Jan 29, 2008 #2


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    12u is DEFINED to be the mass of one carbon-12 atom, it cant be anything else! 12.011u is the average atomic mass..(averaging over all C-isotopes and their relative abundances).

    You should also realise that the triple alpha process takes place inside hot stars, so you must be careful to check if you have the correct number of electrons on LHS as RHS (not saying that this is the error you made, but one should always check that)
    Last edited: Jan 29, 2008
  4. Jan 29, 2008 #3
    But surely 4.002602u is the average atomic mass of helium, so you'd have to use 4u in the first equation and then get a total of ~92.2MeV, which is wrong...

    Or is it okay to use 4.002602u for the helium and 12u for the carbon?
  5. Jan 29, 2008 #4


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    One alpha atom (He-4) has mass of 4.002602u.

    You have to DEFINE the concept of u (atomic mass unit), so one has DEFINED it to be the mass of ONE C-12 atom.
  6. Jan 29, 2008 #5
    New problem now - when doing the same from 12C + 4He -> 16O, I get ~10.25MeV. I think it's meant to be 7.162MeV. Where am I going wrong this time?
  7. Jan 29, 2008 #6


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    How am I supposed to know that when you dont show your calculations and what values you used?

    Even though this is not posted in HW-thread, it really smells like homework. And even if you are only checkning these things for fun, show some detailed calculations, please!
  8. Jan 30, 2008 #7
    Good point, sorry! Not homework (this time!) just some really crap lecture notes, not explaining things.

    C is actually formed from 3xHe (the original reaction), so O must be formed from 4xHe.
    Taking Q(He) = 0.030378u, Q(4xHe) = 0.121512 ~113.19MeV

    Oxygen: (8 x 1.007825u) + (8 x 1.008665u) - 15.9994u = 0.13252u

    which is ~123.44MeV. Last time it was simply the difference between the two, but this gives me ~10.25MeV. I looked at it as Q(4xHe) = Q(3xHe) + Q(C), but that made the difference even larger. I really can't see where she's got her numbers from.
  9. Jan 30, 2008 #8


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    in the reaction C-12 + He-4 -> O-16, we have the same amount of electrons on LHS as RHS, so we can use atomic masses.

    O-16 atom has mass: 15.9949u
    He-4 atom has mass: 4.00260u
    C-12 atom has mass 12.0000u

    mass difference = 0.0077u

    1u = 931.49MeV

    Energy released in this reaction = 7.17MeV

    You cant put C-12 = 3*He-4 !!! If the reaction is C-12 + He-4 -> O-16; then you must use these nuclei!!
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