Source of Nuclear Energy
Date: Winter 2013-14
If nuclear energy comes from splitting nuclei into protons and neutrons, where does the actual energy come from? Does it stem from the weak nuclear force? If that is the case, than how come splitting protons and neutrons into quarks doesn't produce so much more energy, if it involves splitting the color/strong nuclear force?
The energy comes from the nuclear force (the strong force). You do not always get energy by splitting a nucleus into smaller nuclei, by the way. That only works if the nucleus is large. Small nuclei release energy when you fuse them together. The dividing size is roughly that of an iron nucleus.
The reason for this comes in two parts. One part is pretty easy to understand; the other part is just the way quantum mechanics works, and is not very intuitive.
Here is the easy part: The nuclear force is short range. Nucleons (neutrons or protons) feel a very strong attraction from their nearest neighbors, and much less attraction from nucleons that are further away. If you have a nucleus with lots of nucleons, and add another one, the force binding the new guy is mostly from the relatively small number of nucleons in its immediate neighborhood. It is not that much different than if you had added the new guy to a small nucleus.
However, one of the central principles of quantum mechanics is that you cannot have two nucleons in the same state in a nucleus. The states that a nucleon can be in can be sorted by their energies, and all the nucleons are normally in the lowest available states. So if you add a nucleon, and all the low-energy states are occupied, the new guy has to go into a high-energy state.The more nucleons, the higher the energy of the lowest unoccupied state.
It is a raw deal - energetically speaking - for many of the nucleons in a big nucleus. They have to be in high-energy states, because of all the other nucleons; but they do not feel a binding attraction from most of those nucleons, because the nuclear force is short range.
If you break the nucleus into two parts, then nobody has to go into those high-energy states, because there are now twice as many low-energy states. That is the energy that gets released in fission.
Thanks for the question. Nuclear energy comes from splitting large nuclei into smaller nuclei. The smaller nuclei are lower in energy than the large nuclei, so energy (about 200 MeV) is released in the process. I will state the fission (or splitting) of nuclei is only energetically feasible for nuclei larger than iron. A small amount of energy (say 15 MeV) of the 200 MeV noted above actually comes from the weak force. The energy released by the weak force is the beta and gamma decay of the fission products. It is for this reason that spent nuclear fuel is still radioactive and needs to be cooled and kept shielded.
Nuclear energy does not directly involve the quarks and colors of sub-atomic particles. It would take a tremendous amount of energy (on the order of GeVs) to split protons and neutrons in quarks.
I hope this helps.
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