Fission and fusion

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Thread starter 3 years ago
#1
Im having a hard time understanding why fission and fusion release energy. I know that there is an increase in binding energy per nucleon in each process but why is energy released when a nucleus is both formed and split?
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3 years ago
#2
(Original post by IDontKnowReally)
Im having a hard time understanding why fission and fusion release energy. I know that there is an increase in binding energy per nucleon in each process but why is energy released when a nucleus is both formed and split?
Well, it's not the same elements involved in the two cases.

The lowest energy nucleus and thus most stable nucleus is that of iron. Iron's mass is neither the largest nor the smallest in the periodic table.

If you take a more massive nucleus, then it will have more energy than iron (iron has the least). To create an iron nucleus from a bigger nucleus you have to split it into pieces (fission), and doing so will release energy, since you are creating fragments that are more similar to iron, and thus have less energy than the original nucleus had.

OTOH, if you take a smaller nucleus, then it will have more energy than iron. To create an iron nucleus from a smaller nucleus, you have to join it together with something else (fusion), and again this releases energy, since you are creating fragments that are more similar to iron.

So in general and with lots of handwaving: fuse small elements into iron to release energy, fission big elements into iron to release energy.

A very simplified justification for why iron has the least nuclear energy is that it is the nucleus where the repulsive Coulomb force of the protons is more-or-less balanced with the attractive strong nuclear force of the protons and neutrons.
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Thread starter 3 years ago
#3
(Original post by atsruser)
Well, it's not the same elements involved in the two cases.

The lowest energy nucleus and thus most stable nucleus is that of iron. Iron's mass is neither the largest nor the smallest in the periodic table.

If you take a more massive nucleus, then it will have more energy than iron (iron has the least). To create an iron nucleus from a bigger nucleus you have to split it into pieces (fission), and doing so will release energy, since you are creating fragments that are more similar to iron, and thus have less energy than the original nucleus had.

OTOH, if you take a smaller nucleus, then it will have more energy than iron. To create an iron nucleus from a smaller nucleus, you have to join it together with something else (fusion), and again this releases energy, since you are creating fragments that are more similar to iron.

So in general and with lots of handwaving: fuse small elements into iron to release energy, fission big elements into iron to release energy.

A very simplified justification for why iron has the least nuclear energy is that it is the nucleus where the repulsive Coulomb force of the protons is more-or-less balanced with the attractive strong nuclear force of the protons and neutrons.

Why would a smaller nucleus have more energy than iron? I thought that as the number of nucleons increased, the energy required to split the nucleus into neutrons and protons increased since more work has to be done against the strong nuclear force?
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3 years ago
#4
(Original post by IDontKnowReally)
Why would a smaller nucleus have more energy than iron? I thought that as the number of nucleons increased, the energy required to split the nucleus into neutrons and protons increased since more work has to be done against the strong nuclear force?
I can't explain that in any detail as I don't know much nuclear physics, and the reasons for it are not fully understood anyway, AFAIK. You have to treat the nucleus using quantum mechanics, of course, to have any hope of making a good calculation.

However, they are measurable facts about nuclei, so we don't need to justify it; we know that it is true.

I guess a handwaving explanation for a small nucleus is that it has proportionately too many protons, and not enough protons and neutrons. That's because there's a delicate balance between the weaker but long-range repulsive Coulomb force, and the stronger but short range attractive strong nuclear force. Only protons feel the former, but both protons and neutrons feel the latter, and you need the right mixture to cancel the two.
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3 years ago
#5
(Original post by IDontKnowReally)
Why would a smaller nucleus have more energy than iron? I thought that as the number of nucleons increased, the energy required to split the nucleus into neutrons and protons increased since more work has to be done against the strong nuclear force?
This answer on stackexchange may be helpful too:

https://physics.stackexchange.com/qu...edirect=1&lq=1
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Thread starter 3 years ago
#6
(Original post by atsruser)
I can't explain that in any detail as I don't know much nuclear physics, and the reasons for it are not fully understood anyway, AFAIK. You have to treat the nucleus using quantum mechanics, of course, to have any hope of making a good calculation.

However, they are measurable facts about nuclei, so we don't need to justify it; we know that it is true.

I guess a handwaving explanation for a small nucleus is that it has proportionately too many protons, and not enough protons and neutrons. That's because there's a delicate balance between the weaker but long-range repulsive Coulomb force, and the stronger but short range attractive strong nuclear force. Only protons feel the former, but both protons and neutrons feel the latter, and you need the right mixture to cancel the two.
That makes sense, thanks!
Although i should have been clearer in my question originally, sorry. What I meant to ask was, when a nucleus forms, energy is released, but why does this happen? The strong nuclear force does work to pull in the nucleons, so why is energy not taken in instead?
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3 years ago
#7
(Original post by IDontKnowReally)
That makes sense, thanks!
Although i should have been clearer in my question originally, sorry. What I meant to ask was, when a nucleus forms, energy is released, but why does this happen? The strong nuclear force does work to pull in the nucleons, so why is energy not taken in instead?
I would conjecture it is because E = mc^2, so work done is negligible.
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3 years ago
#8
Look at the binding energy per nucleon of different atoms and it will help you understand why.
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3 years ago
#9
Nuclei with low mass numbers may undergo nuclear fusion, where light nuclei are joined together under certain conditions so that the final product may have a greater binding energy per nucleon. The change in mass is converted into energy.

Nuclei with high mass numbers may undergo nuclear fission, where the nucleus split to give two daughter nuclei with the release of neutrons. The daughter nuclei will possess a greater binding energy per nucleon.

The mass defect of a nucleus is the difference between the mass of the nucleus nd the mass of its constituent particles. The mass defect is related to the binding energy as follows Binding energy (J) = mass defect (kg) × c^2 which is basically E=mc^2

you need to be able to apply this to the binding energy graph and why fusion occurs in small nuclei and fission on large.

Does this make it easier to understand? You will also need to know how to calculate the mass thus energy as a result of this.
Energy in Mev
Mass in Mev/c^2
Momentum in Mev/c

Any other question then feel free to ask.
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3 years ago
#10
(Original post by IDontKnowReally)
That makes sense, thanks!
Although i should have been clearer in my question originally, sorry. What I meant to ask was, when a nucleus forms, energy is released, but why does this happen? The strong nuclear force does work to pull in the nucleons, so why is energy not taken in instead?
I can only repeat what I said earlier: when fusion occurs, the fragments form a new nucleus of lower energy, and the process of fusion will therefore produce either smaller fragments carrying kinetic energy, or photons carrying electromagnetic energy.

As to why the new nucleus has lower energy, well, you probably won't get a better elementary answer than the one about the semi-empirical mass formula in the stackexchange link. The details of the precise energy of a nucleus are complex, and I have no idea how good the modern theory of this stuff is. You'll need to talk to a nuclear physicist, I'm afraid.

There's a nice overview of the terms of the SEMF here too:

https://en.wikipedia.org/wiki/Semi-e...l_mass_formula
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3 years ago
#11
It seems that students tend to have the following thought:
“The change in mass (mass defect in nuclear physics) is converted into energy.”

I strongly discourage such thought. Have a look at this post.
https://www.thestudentroom.co.uk/sho...4#post71387574

OR
https://stanford.library.sydney.edu....tries/equivME/
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