# Help! I don’t understand nuclear fusion!

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Thread starter 8 months ago
#1
So with nuclear fission energy is released because the smaller nuclei produced will have higher average binding energy per nucleon. That makes sense because smaller nuclei will have a smaller nucleon number so more binding energy per nucleon.

But how is it that with nuclear fusion, the larger nucleus produced has a higher binding energy per nucleon as well?? Surely because the nucleon number has increased there will be less binding energy per nucleon because the binding energy is spread out more between all of the nucleons??

Can someone please explain?
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Thread starter 8 months ago
#2
Bump?
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8 months ago
#3
(Original post by Icykitten)
So with nuclear fission energy is released because the smaller nuclei produced will have higher average binding energy per nucleon. That makes sense because smaller nuclei will have a smaller nucleon number so more binding energy per nucleon.

But how is it that with nuclear fusion, the larger nucleus produced has a higher binding energy per nucleon as well?? Surely because the nucleon number has increased there will be less binding energy per nucleon because the binding energy is spread out more between all of the nucleons??

Can someone please explain?
For any given nucleus there is an overall binding energy. Dividing this by the number of nucleons in this nucleus gives its binding energy per nucleon. So, saying it the other way round, the overall binding energy of a nucleus is it's binding energy per nucleon multiplied by the number of nucleons it contains.

For small nuclei we find that the binding energy per nucleon increases. Therefore if we can make one larger nucleus out of two smaller ones then the overall binding energy will increase. This gives us fusion. For large nuclei we find that binding energy per nucleon decreases. So if we can split a larger nucleus into two smaller ones then the overall binding energy will increase. This gives us fission.
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8 months ago
#4

To go with David Getling's point, here is the graph that should be quite useful to understand the concept. You see you do Fusion with the ones on the LHS, but Fission with the ones on the RHS.
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Thread starter 8 months ago
#5
Thank you both, I think I’m understanding it now
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