Mechanism for Energy Released via the Strong Force in Fusion

[Jimmy87]

Hi,

I was wondering if there is a mechanism to explain how the strong force leads to an energy release when two light nuclei such as hydrogen fuse together. I get that the products of fusion have less mass than the reactants and that this "missing" mass is converted into energy in accordance with E=mc². I also get that the products are more "tightly bound" and have less binding energy per nucleon. I just wondered what the mechanism was for how the strong force actually does this? If we take gravity for example, to explain why energy is released when an asteroid falls to Earth we can say that the gravitational force does work on the asteroid. How does the strong force cause this energy release/mass defect?

Thanks for any insights/info offered.

 

[phyzguy]

Conceptually, I think it is no different than your asteroid example. If I start with the Earth and an asteroid at infinite separation and let the asteroid fall to Earth, the attractive gravitational force does work on the asteroid as it falls, and we end up with a bound configuration where the Earth and the asteroid, which are now closer together, have less gravitational energy than they did before. In the case of nuclear fusion, consider two deuterons at infinite separation and push them together. Once you have overcome their Coulomb repulsion, the attractive strong force takes over and pulls the two deuterons together. The strong force does work on the deuterons as they pull together, and we end up with a more tightly bound configuration than we started with. How is this conceptually different from the gravitational case?

 

[Jimmy87]

This is what I thought - I was just checking that the gravitational mechanism comparison is acceptable. Does it work out the same mathematically? For example, if a mass falls to the Earth then the gravitational force multiplied by the distance it falls is exactly the energy it releases. If you multiplied the strong nuclear force by the distance the nuclei move would you get the same answer as you would from using the missing mass of the products and multiplying it by the speed of light squared?

 

[phyzguy]

This is what I thought - I was just checking that the gravitational mechanism comparison is acceptable. Does it work out the same mathematically? For example, if a mass falls to the Earth then the gravitational force multiplied by the distance it falls is exactly the energy it releases. If you multiplied the strong nuclear force by the distance the nuclei move would you get the same answer as you would from using the missing mass of the products and multiplying it by the speed of light squared?

I think it's just not that simple. When a rock falls to the Earth, both the rock and the Earth maintain their identities. When two deuterons fuse together to make a helium nucleus, the helium nucleus is not just two deuterons resting against one another. It is not even two protons and two neutrons flying around. It is a constantly changing "soup" of quarks and gluons. So you can't even say how far the two deuterons "fell" when they assembled to make the helium nucleus.