### The Energy of Everything

Building blocks: All matter is composed of atoms, held together by chemical bonds; atoms are composed of nuclei and electrons, held together by electric forces; nuclei are composed of nucleons (protons and neutrons) held together by the strong nuclear force; nucleons are composed of quarks, held together by weak nuclear forces. The study of nuclear physics mostly concentrates on the behavior of the electrostatic and strong nuclear forces.

Nuclear Energy: When we think of nuclear energy we think of the relation E=mc2, where c is the speed of light in a vacuum. This relation defines a value called the rest mass energy. In nuclear physics, mass is a form of potential energy, we can turn mass into pure energy (heat or light for example) or we can turn energy into mass. The E=mc2 relation defines the maximum amount of energy we can get from mass, or vice-versa. Nuclear reactors work by changing mass into heat! In nuclear physics, the unit of energy is commonly in the range of mega electron-volts (MeV), and mass is given in MeV/c2. In these units the mass of the proton is 938.28 MeV/c2, the mass of the neutron is 939.57 MeV/c2, and the mass of the electron is 0.511 MeV/c2.

The mass of an atom is actually less than the combined masses of all protons, neutrons, and electrons. In fact, the simple act of the strong nuclear force holding all these particles together in the atom takes energy (and, by extension, mass) away from the atom. This energy needed to “bind” the atom is called binding energy:

One consequence of binding energy is that it is possible to release energy by either combining two small, weakly-bound nuclei into a heavier, more tightly bound one, or by splitting a heavy, weakly-bound nucleus into two lighter, more-bound nuclei. This is the idea behind fission and fusion energy, and the change in binding energy from initial to final states is what allows us to use nuclear energy.

Fission and Fusion-Harnessing Nuclear Energy:

Fission is the process by which a heavy nucleus breaks apart into smaller nuclei and releases energy. In nuclear reactors, fission doesn’t happen naturally, but is made to happen by adding an extra neutron to uranium-235. The extra neutron makes a new isotope, U-236, which has too much energy to remain stable and so breaks apart. By finding the difference in binding energy from splitting Uranium-236 into two smaller nuclei, we see that the energy released is about 200 MeV (relatively high!). Nuclear bombs have many of these reactions happen in a very short time, while nuclear power reactors have the rate of reaction spread over a long time; both resulting in a huge amount of energy for different purposes.

Fusion is the process that glues two nuclei together rather than tearing them apart. Two lighter nuclei can become more stable by joining together to form a larger, but more tightly bound nucleus. This is how our sun works! Hydrogen in the Sun becomes very hot and fuses together to form bigger elements up until iron and nickel. Since the final product has more binding energy, there is a large amount of energy that is free to escape. In the Sun, this energy is what allows us to live and enjoy suntans! One important fusion reaction is called D-T fusion, named for the initial nuclei deuterium and tritium, which produces helium and 17.6 MeV of energy.

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