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The electronvolt (symbol eV, or, rarely and incorrectly, ev) is a unit of energy. It is the amount of kinetic energy gained by a single unbound electron when it passes through an electrostatic potential difference of one volt, in vacuum. In other words, it is equal to one volt (1 volt = 1 joule per coulomb) times the charge of a single electron (in coulombs). The one-word spelling is the modern recommendation although the use of the earlier electron volt still exists. One electronvolt is a very small amount of energy: 1 eV = 1.602 176 53(14){{e|−19}} J. (Source: CODATA 2002 recommended values) The unit electronvolt is accepted (but not encouraged) for use with SI. It is widely used in solid state, atomic, nuclear, and particle physics, often with prefixes m, k, M, G or T.
Using electronvolts to measure mass Albert Einstein reasoned that energy is equivalent to mass, as famously expressed in the formula E=mc² (1 kg = 90 petajoules). It is thus common in particle physics, where mass and energy are often interchanged, to use eV/c² or even simply eV as a unit of mass. For example, an electron and a positron, each with a mass of 0.511 MeV/c², can annihilate to yield 1.022 MeV of energy. The proton has a mass of 0.938 GeV, making GeV a very convenient unit of mass for particle physics. 1 eV/c² = 1.783 kg 1 keV/c² = 1.783 kg 1 MeV/c² = 1.783 kg 1 GeV/c² = 1.783 kg 1 TeV/c² = 1.783 kg 1 PeV/c² = 1.783 kg 1 EeV/c² = 1.783 kg See: Orders of magnitude (mass) In some older documents, and in the name Bevatron, the symbol "BeV" is used, which stands for "billion-electron-volt"; it is equivalent to the GeV (gigaelectronvolt). Electronvolts and kinetic energy For comparison: Electronvolts and temperature In certain fields, such as plasma physics, it is convenient to use the electronvolt as a unit of temperature. The conversion to kelvins (symbol: uppercase K) is defined, in part, by using kb, the Boltzmann constant. For example, a typical magnetic confinement fusion plasma is 15 keV, or 174 megakelvins. Using electronvolts to measure time and distance In particle physics, distances and times are sometimes expressed in inverse electronvolts via the conversion factors In these units, the mean lifetime of an unstable particle can be reexpressed in terms of its decay width (in eV) via . For example, the B0 meson has a mean lifetime of 1.542(16) picoseconds, or a decay width of 4.269(44) x 10-4 eV, and its mean decay length is = 462 m. Reference See also | ||||||||
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