# Fission and fusion

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#1
Okay guys, I'm having a hard time understanding this concept and need your help

So as I understand when nucleons join together they form an atom, the mass of individual seperate nucleons that make up the atom is greater than the atom formed from them.

So seperate nucleons have greater mass than the atom formed from them, the excess mass (mass defect) is converted into energy. E=mc^2

1. So is this an example of fusion or is fusion when atoms join to form larger atoms, as opposed to nuceons joining to form an nucleus?

2. If fusion releases energy, which I understand to be due to the mass defect then how can fission also produce energy, please could you explain this in terms of mass defect and binding energy?

Many thanks guys, REP will be given
0
7 years ago
#2
(Original post by MrDoctor)
Okay guys, I'm having a hard time understanding this concept and need your help

So as I understand when nucleons join together they form an atom, the mass of individual seperate nucleons that make up the atom is greater than the atom formed from them.

So seperate nucleons have greater mass than the atom formed from them, the excess mass (mass defect) is converted into energy. E=mc^2
Yes that's correct.

1. So is this an example of fusion or is fusion when atoms join to form larger atoms, as opposed to nuceons joining to form an nucleus?
It's fusion whether or not it's single nucleons or larger atoms. Fusion just means joining together.
2. If fusion releases energy, which I understand to be due to the mass defect then how can fission also produce energy, please could you explain this in terms of mass defect and binding energy?

Many thanks guys, REP will be given
It all depends on the difference between the initial mass of all the items involved and the final mass of all the items involved.

In the case of fusion, energy will only be released if the mass of the product is less than the sum of that of the original nuclei. This happens when the original nuclei are light and combine to form a nucleus with mass less than that of iron 56.*

In the case of fission, the sum of the final mass of the products must be less than that of the original single nucleus. This happens when the original nucleus is heavy and the final products have a mass greater than about iron 56*.

*For the significance of iron 56 you need to look at the standard graph of binding energy of a nucleus against nucleon number. If you haven't studied this yet, then I imagine doing this will answer your question. Energy is released when lighter nuclei fuse or heavier nuclei fission. Fe56 is the nucleus with the greatest binding energy per nucleon and represent a very stable configuration.

2
7 years ago
#3
(Original post by MrDoctor)
Okay guys, I'm having a hard time understanding this concept and need your help

So as I understand when nucleons join together they form an atom, the mass of individual seperate nucleons that make up the atom is greater than the atom formed from them.

So seperate nucleons have greater mass than the atom formed from them, the excess mass (mass defect) is converted into energy. E=mc^2

1. So is this an example of fusion or is fusion when atoms join to form larger atoms, as opposed to nuceons joining to form an nucleus?

2. If fusion releases energy, which I understand to be due to the mass defect then how can fission also produce energy, please could you explain this in terms of mass defect and binding energy?

Many thanks guys, REP will be given
For 1, nucleons are just the things that make up the nucleus, hence the name, which consist of protons and neutrons. An atom has less mass than it separate nucleons. This difference is given by the mass deficit/defect. Binding energy is the energy is needed to split a nucleus into its separate nucleons. Yes, as you mentioned there, joining separate nucleons therefore has to mean that overall there's mass loss. To account for this, by E=mc^2, there's extra energy. Nuclear fusion involves lighter nuclei/nucleons fusing together to form a single heavier nucleus. The reaction must involve the LHS mass>RHS mass, so there's an overall decrease in mass, so by E=mc^2, energy is formed.

For 2 I got confused as well, but now Stonebridge's post explains it nicely.
1
#4
(Original post by Stonebridge)
Yes that's correct.

It's fusion whether or not it's single nucleons or larger atoms. Fusion just means joining together.

It all depends on the difference between the initial mass of all the items involved and the final mass of all the items involved.

In the case of fusion, energy will only be released if the mass of the product is less than the sum of that of the original nuclei. This happens when the original nuclei are light and combine to form a nucleus with mass less than that of iron 56.*

In the case of fission, the sum of the final mass of the products must be less than that of the original single nucleus. This happens when the original nucleus is heavy and the final products have a mass greater than about iron 56*.

*For the significance of iron 56 you need to look at the standard graph of binding energy of a nucleus against nucleon number. If you haven't studied this yet, then I imagine doing this will answer your question. Energy is released when lighter nuclei fuse or heavier nuclei fission. Fe56 is the nucleus with the greatest binding energy per nucleon and represent a very stable configuration.

Hey thanks for the reply I just came across another doubt and hopefully you will be able to answer. In nuclear fission and fusion it makes sense now that the mass of the products must be less than the mass of the reactants for energy to be released.

But I thought that the total mass of smaller nuclei formed in fission is always greater than the mass of the nuclei that they formed from, isnt that the whole point of mass defect (all atoms are lighter than the sum of their masses of their constituent protons neutrons and electron) so how would this make sense for fission to release energy at all?

This part of physics: really not my strong point thanks alot!
0
7 years ago
#5
(Original post by MrDoctor)

But I thought that the total mass of smaller nuclei formed in fission is always greater than the mass of the nuclei that they formed from,
Let's take a simple case.
A nucleus with nucleon number 200 splits exactly in half and forms two nuclei of number 100.
From the graph a nucleus of mass 200 has approx 8MeV per nucleon binding energy.
So its total binding energy is 200 x 8MeV = 1600 MeV

The two product nuclei have mass 100 each.
Look at the graph again.
From the graph these have a binding energy of approx. 8.5MeV per nucleon.
Total binding energy of these two is
100 x 8.5MeV + 100 x 8.5MeV

This is 850MeV + 850MeV = 1700MeV

So the result is that in this reaction there is an overall increase in binding energy. (of 100MeV)

Binding energy is by definition the energy released when the nuclei form. So this reaction releases energy. If you think about it, it's a consequence of the graph being higher in the middle.
You can try yourself the case where, say, two smaller nuclei join to form a larger one. You only get energy out if you move up the slope of the graph.
You don't get energy from joining two Fe nuclei together or splitting a Fe nucleus in half.
Check it out.
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