- Electron volt
In
physics , the electron volt (eV) is a unit ofenergy . By definition, it is equal to the amount of energy gained by a single unboundelectron when it accelerates through an electrostatic potential difference of onevolt . In SI units, it is the number which measures the charge of the electron with the unit changed from C to J.:: 1 eV = 1.602 176 53(14)e|−19 J. [cite journal | author=Peter J. Mohr and Barry N. Taylor | title=CODATA recommended values of the fundamental physical constants: 2002 | journal=Reviews of Modern Physics | month=January | year=2005 | volume=77 | pages=1–107 | url=http://www.atomwave.org/rmparticle/ao%20refs/aifm%20refs%20sorted%20by%20topic/other%20rmp%20articles/CODATA2005.pdf | format=PDF | accessdate = 2006-07-01 | doi=10.1103/RevModPhys.77.1 An in-depth discussion of how the CODATA constants were selected and determined.]
So an electron volt (electronvolt according to the NIST) is 1
volt ( 1joule / 1coulomb ) multiplied by the electron charge ( 1.602 176 53(14)e|−19coulomb ).The electron volt is now accepted within SI [cite web|url=http://www.bipm.org/en/si/si_brochure/chapter4/table7.html|title=Non-SI units whose values in SI units must be obtained experimentally|accessdate=2008-09-10|work=
International Bureau of Weights and Measures ] . It is the most common unit of energy within physics, widely used in solid state, atomic, nuclear, andparticle physics , often withSI prefix es milli, kilo, mega, giga, tera, or peta (meV, keV, MeV, GeV, TeV and PeV respectively).In
chemistry , it is often useful to have the molar equivalent, that is the kinetic energy that would be gained by a mole of electrons passing through a potential difference of one volt. This is equal to 96.48538(2) kJ/mol. Atomic properties like theionization energy are often quoted in electron volts.As a unit of mass
By
mass-energy equivalence , the electron volt is also a unit of mass. It is common in particle physics, where mass and energy are often interchanged, to use eV/"c"², or more commonly simply eV with c set to 1, as a unit of mass.For example, an electron and a
positron , each with a mass of 0.511 MeV, canannihilate to yield 1.022 MeV of energy. Theproton has a mass of 0.938 GeV, making a GeV a very convenient unit of mass forparticle physics .:: 1 GeV = 1.783e|−27 kgThe
atomic mass unit , 1 gram divided byAvogadro's number , is almost the mass of a hydrogen atom, which is mostly the mass of the proton. To convert to MeV,use the formula:::1 amu = 931.4 MeV = .9314 GeV::1 MeV = 1.074·10-3 amu
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.Since MeV as a unit is often used in "nuclear energy equations", for example as in the stellar nuclear fusion process of carbon burning, among others the equation
:
As a unit of energy
For comparison:
* 1 TeV: about the energy of motion of a flying mosquito [ [http://askanexpert.web.cern.ch/AskAnExpert/en/Accelerators/LHCgeneral-en.html#8 CERN: The Large Hadron Collider in general] ] .
* 210 MeV: average energy released in fission of one Pu-239 atom.
* 200 MeV: total energy released innuclear fission of one U-235 atom (on average; depends on the precise break up); this is 82 TJ/kg, or twenty thousand tonnes ofTNT equivalent per kilogram.
* 17.6 MeV: total energy released in fusion ofdeuterium andtritium to form helium-4 (also on average); this is 0.41 PJ/kg of product produced.
* 13.6 eV: energy required to ionize atomic hydrogen. Molecular bond energies are on the order of an eV per molecule.
* 1/40 eV: thethermal energy at room temperature. A single molecule in the air has an average kinetic energy 3/80 eV.Conversion factor:
*1 eV per amu is 96.5 MJ/kgRelation to units of time and distance
In
particle physics , a system of units in which thespeed of light c and the reduced Planck constant hbar are dimensionless and equal to unity is widely used: c=hbar=1. In these units, both distances and times are expressed in inverse energy units (while energy and mass are expressed in the same units, seeMass–energy equivalence ). In particular, particlescattering length s are often presented in units of inverse particle masses.Outside this system of units, the conversion factors between electronvolt, second, and nanometer are the following: [http://pdg.lbl.gov/2008/reviews/consrpp.pdf C. Amsler et al., "Review of Particle Physics", Phys. Lett. B667, 1 (2008)] ]
* hbar = 6.582 118 99(16) x 10-16 eV s;
* hbar c = 197.326 9631(49) eV nm (or MeV fm).The above relations also allow expressing the
mean lifetime au of an unstable particle (in seconds) in terms of its decay width Gamma (in eV) via Gamma = hbar/ au. For example, the B0 meson has a mean lifetime of 1.542(16)picosecond s, or a decay width of 4.269(44) x 10-4 eV, and its mean decay length is c au = 462 mum.As a unit of temperature
In certain fields, such as
plasma physics , it is convenient to use the electronvolt as a unit of temperature. The conversion tokelvin s (symbol: uppercase K) is defined by using "kB", theBoltzmann constant ::1 mbox{ eV} over k_B} = {1.60217653(14) imes 10^{-19} mbox{J} over 1.3806505(24) imes 10^{-23} mbox{J/K = 11604.505(20) mbox{ kelvins}.
For example, a typical
magnetic confinement fusion plasma is 15 keV, or 174 megakelvins.Photon properties
The energy "E", frequency "f", and wavelength λ of a photon are related by
:E=hf=frac{hc}{lambda}= frac{1240~ m{nm{lambda} m~eV
where "h" is
Planck's constant and "c" is thespeed of light . For example, the spectrum of visible light consists of wavelengths ranging from 400 nm to 700 nm. Photons of visible light therefore have energies ranging from:E_mbox{min} = frac{1240~ m{nm{700~ m{nm m~eV= 1.77~ m{eV}
to
:E_mbox{max} = frac{1240~ m{nm{400~ m{nm m~eV = 3.10~ m{eV}.
An electron volt is also the energy of an infrared photon with a wavelength of approximately 1240 nm. Similarly, 10 eV would correspond to ultraviolet of wavelength 124 nm, and so on.
References
ee also
*
Orders of magnitude (energy)
*Thermodynamics External links
* [http://www1.bipm.org/en/si/si_brochure/chapter4/table7.html BIPM's definition of the electronvolt]
*http://physics.nist.gov/cuu/Constants physical constants reference; CODATA data
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