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In physics, nuclear fusion (a thermonuclear reaction) is a process in which two nuclei join, forming a larger nucleus and releasing energy. Nuclear fusion is the energy source which causes stars to shine, and hydrogen bombs to explode.It takes considerable energy to force nuclei to fuse, even those of the least massive element, hydrogen. But the fusion of lighter nuclei, which creates a heavier nucleus and a free neutron, will generally release even more energy than it took to force them together -- an exothermic process that can produce self-sustaining reactions.
The energy released in most nuclear reactions is much larger than that for chemical reactions, because the binding energy that glues a nucleus together is far greater than the energy that holds electrons to a nucleus. For example, the ionization energy gained by adding an electron to hydrogen is 13.6 electron volts -- less than one-millionth of the 17 MeVTo help compare different orders of magnitude this page lists energies between 10−12 joules and 10−11 joules (6. 2 MeV and 62 MeV) Weaker energies 1. 602 × 10−12 J 10 MeV Stronger energies External links Orders of magnitude (energy). released in the D-T reaction shown below.
1 Requirements for fusion
A substantial energy barrier opposes the fusion reaction. The positive electrical charges of the nuclei repel each other via the electromagnetic force, attempting to break any nuclei apart. Opposing this is the slightly more powerful strong nuclear force, which tries to hold them together. It is the tensionIn physics, tension is a force on a body directed to produce strain (extension); it can be considered to be negative compression. It is measured in according units ( newton, dynes, pound-forces, etc). Tension is the dominant static force acting on such ob between these two powerful forces that makes nuclear reactions so powerful.
The strong force only operates over short distances, unlike the electromagnetic force. As the nucleus grows by adding additional protonFor alternative meanings see proton (disambiguation). Proton Classification Subatomic particle Fermion Hadron Baryon Nucleon Proton Properties Mass: 938 MeV/ c2 Electric Charge: 1. 6 × 10−19 C Spin: 1/2 In physics, the proton is a subatomic particles and neutrons, it eventually reaches a size (at the element iron) where the strong force becomes overwhelmed and the nucleus spontaneously starts to "fall apart". This is what causes radioactivityRadioactive decay Radioactivity is the process by which unstable atomic nuclei decay. This process normally produces ionizing radiation with a relatively large amount of energy. This energy can be harnessed in the form of nuclear power, or it can be very in heavier elements.
In order to fuse nuclei, the electromagnetic repulsive force between protons must be overcome, and the nuclei brought close enough together for the strong force to start to act again. The combination of these two determines the threshold energy required for a fusion reaction. Since the repulsive force is generated solely by the electrically charged protons, this Coulomb barrierThe Coulomb barrier named after Coulomb, is the electrostatic repulsion of two point charges. Overcoming the Coulomb barrier is essential for achieving nuclear fusion. with :k Coulomb's constant :e charge of electron :r separation. is a minimum for hydrogen, which contains only one proton. The strong force is generated by both protons and neutrons, so the threshold energy is likewise minimized by adding neutrons. Thus it is lowest for heavy isotopes of hydrogen, deuterium (D) and tritium (T), which have only one proton keeping them apart, but several neutrons pulling them together.
In the D-T fuel, the resulting energy barrier is about 0.1 MeV. In comparison, the energy needed to remove an electron from hydrogen is 13eV, about 75000 times less energy. Once the fusion reaction is complete, the new nucleus drops to a lower-energy configuration and gives up additional energy by ejecting a neutron with 17.59 MeV, considerably more than what was needed to fuse them in the first place. This means that the D-T fusion reaction is very highly exothermic, making it a powerful energy source.
