Calculating the weight of 1 atomic mass unit

  • Thread starter JohnnyGui
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  • #26
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I feel like you are in general on the right track, but I don't like this statement. I don't think we can assign mass to nucleons in nuclei, at best we can calculate their average mass
What if I say that the average mass increases? I'm thinking this because the fusion of atoms heavier than Fe as well as the falling apart of atoms that are lighter than Fe both need energy while at the same time they both result in a lower binding energy release. Doesn't the net energy difference have to be turned into mass or do the newly formed atoms merely possess more kinetic energy?

I take it that my statements 1 to 3 are more or less correct?

What is the starting point of the triple alpha process?
It'd need two Helium atoms from what I know. I'm starting to get the idea that the difference is that the released binding energy is based on when you start from single protons and neutrons to form a 12C atom while the energy from the triple alpha process is based on when you start from 2 Helium atoms. It still amazes me though that starting from atoms would yield more energy than starting from the basic single particles.
 
  • #27
Borek
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What if I say that the average mass increases?
That's what the plot says (even if indirectly).

I take it that my statements 1 to 3 are more or less correct?
Yes, you are just using a bit convoluted approach, I am used to think in terms of binding energy, not mass changes (averaged, to make the argument even more difficult to follow).

It'd need two Helium atoms from what I know. I'm starting to get the idea that the difference is that the released binding energy is based on when you start from single protons and neutrons to form a 12C atom while the energy from the triple alpha process is based on when you start from 2 Helium atoms.
Not two He, three.

It still amazes me though that starting from atoms would yield more energy than starting from the basic single particles.
This is wrong, check your units/numbers.
 
  • #28
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This is wrong, check your units/numbers.
Ah, I totally ditched the units. So if I understand this correctly, making a 12C atom from 3 He atoms would yield 7.273 MeV (according to Wiki) of binding energy while making a 12C from 6 single neutrons and 6 single protons would yield 92161.753 keV which is around 92.162 MeV of binding energy? (also according to Wiki)
 
  • #29
Borek
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I have not checked the numbers, but at least now they seem to be consistent with what you have posted earlier.

If you look at the plot, it is obvious that most of the energy was already released when producing He (which is why it is so easy to burn H and produce He, but much more difficult to move further).
 
  • #30
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I have not checked the numbers, but at least now they seem to be consistent with what you have posted earlier.

If you look at the plot, it is obvious that most of the energy was already released when producing He (which is why it is so easy to burn H and produce He, but much more difficult to move further).
Do you mean that it's easier to burn H atoms than for example He since you're getting a large proportion of energy "back" in the form of binding energy after burning it? If so, shouldn't one look at the initial needed energy needed to burn atoms regardless of how much binding energy gets released afterwards?
 

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