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In physics, the electronvolt〔(IUPAC Gold Book ), p. 75〕〔(''SI brochure'' ), Sec. 4.1 Table 7 〕 (symbol eV; also written electron volt) is a unit of energy equal to approximately 160 zeptojoules (symbol zJ) or joules (symbol J). By definition, it is the amount of energy gained (or lost) by the charge of a single electron moved across an electric potential difference of one volt. Thus it is 1 volt (1 joule per coulomb, ) multiplied by the elementary charge (''e'', or ()). Therefore, one electron volt is equal to Historically, the electron volt was devised as a standard unit of measure through its usefulness in electrostatic particle accelerator sciences because a particle with charge ''q'' has an energy after passing through the potential ''V''; if ''q'' is quoted in integer units of the elementary charge and the terminal bias in volts, one gets an energy in eV. The electron volt is not an SI unit, and its definition is empirical (unlike the litre, the light year and other such non-SI units), thus its value in SI units must be obtained experimentally. Like the elementary charge on which it is based, it is not an independent quantity but is equal to . It is a common unit of energy within physics, widely used in solid state, atomic, nuclear, and particle physics. It is commonly used with the metric prefixes milli-, kilo-, mega-, giga-, tera-, peta- or exa- (meV, keV, MeV, GeV, TeV, PeV and EeV respectively). Thus meV stands for milli-electron volt. In some older documents, and in the name Bevatron, the symbol BeV is used, which stands for billion electron volts; it is equivalent to the GeV. ==Mass== By mass–energy equivalence, the electronvolt is also a unit of mass. It is common in particle physics, where units of mass and energy are often interchanged, to express mass in units of eV/''c''2, where ''c'' is the speed of light in vacuum (from ). It is common to simply express mass in terms of "eV" as a unit of mass, effectively using a system of natural units with ''c'' set to 1.〔Barrow, J. D. "Natural Units Before Planck." Quarterly Journal of the Royal Astronomical Society 24 (1983): 24.〕 The mass equivalent of is : For example, an electron and a positron, each with a mass of , can annihilate to yield of energy. The proton has a mass of . In general, the masses of all hadrons are of the order of , which makes the GeV (gigaelectronvolt) a convenient unit of mass for particle physics: : = . The atomic mass unit, 1 gram divided by Avogadro's number, is almost the mass of a hydrogen atom, which is mostly the mass of the proton. To convert to megaelectronvolts, use the formula: :1 amu = = . 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「electronvolt」の詳細全文を読む スポンサード リンク
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