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# Fusion and Fission Watch

1. Hello, so here's a paragraph from the textbook:

'When a heavy nucleus splits up, energy is released because the produced nuclei have a higher binding energy than the original nucleus. When two light nuclei fuse, energy is produced because the products again have a higher binding energy.'

What I don't understand is, if the products have a higher binding energy, doesn't that mean energy has been put into the nuclei to account for the higher binding energy? How can energy be released, surely it's been taken in?
2. (Original post by GlibByNature)
Hello, so here's a paragraph from the textbook:

'When a heavy nucleus splits up, energy is released because the produced nuclei have a higher binding energy than the original nucleus. When two light nuclei fuse, energy is produced because the products again have a higher binding energy.'

What I don't understand is, if the products have a higher binding energy, doesn't that mean energy has been put into the nuclei to account for the higher binding energy? How can energy be released, surely it's been taken in?
This always causes confusion.
Binding energy is released when the nucleus is formed from its constituents.
Binding energy is the energy which would need to be given to (absorbed by) the nucleus to break it up into its constituents.
Binding energy should actually be represented by a negative value.
A nucleus with a binding energy of, say, 200Mev needs that amount of energy to separate its constituents to infinity, where their energy is nominally zero.
A nucleus with a binding energy of, say, 200Mev, will release that amount of energy when it forms from its constituents.
So, strictly speaking, the binding energy is -200MeV, but is usually expressed as positive.
In other words, I have to give it 200MeV to get it to zero.
This is the same reason why an electron in an atom has a negative energy value in its ground state, for example. You need to give it energy to get it out of the atom (ionize it) where its value is zero.
This is what causes the confusion.
If a nucleus with 200MeV binding energy splits into 2 parts, each with, say, 110Mev binding energy, the total binding energy of the products is 220Mev and is (apparently) "greater" than that of the original nucleus.
The truth is, the original was -200MeV and the final is -220MeV
There has actually been a loss of energy in the nuclei equal to 20MeV (fallen from -200 to -220) and it is this energy that is released from the fission.
In the electron analogy, it would be like the electron falling to a lower energy level (more negative) and giving out the energy lost as a photon.
3. (Original post by Stonebridge)
x
Ah I see, so the total binding energy increases plus there's energy released; sounds like a win-win situation . Thanks for the help.

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Updated: December 22, 2010
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