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Electronvolt
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In physics, the electron volt (eV) is a unit of energy. By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electrostatic potential difference of one volt.
1 eV = .
So an electron volt (electronvolt according to the NIST, IUPAC, BIPM) is 1 volt (1 joule divided by 1 coulomb) multiplied by the electron charge .
The electron volt is not an SI unit and its value must be obtained experimentally.
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Encyclopedia
In physics, the electron volt (eV) is a unit of energy. By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electrostatic potential difference of one volt.
1 eV = .
So an electron volt (electronvolt according to the NIST, IUPAC, BIPM) is 1 volt (1 joule divided by 1 coulomb) multiplied by the electron charge .
The electron volt is not an SI unit and its value must be obtained experimentally. It is the most common unit of energy within physics, widely used in solid state, atomic, nuclear, and particle physics, used with SI prefixes milli, kilo, mega, giga, tera, or peta (meV, keV, MeV, GeV, TeV and PeV respectively).
In a recorded lecture from 1961, Richard Feynman apologized to his students for using electron volts to measure energy instead of some multiple of the joule:
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 the ionization 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˛, 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, can annihilate to yield 1.022 MeV of energy. The proton has a mass of 0.938 GeV, making a gigaelectronvolt a very convenient unit of mass for particle physics.
1 GeV/c2 = 1.783 kg
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 = 931.46 MeV = 0.93146 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 volts"; it is equivalent to the GeV.
As a unit of energy
For comparison:
- 7 TeV: the energy of protons before collision in the LHC.
- 210 MeV: average energy released in fission of one Pu-239 atom.
- 200 MeV: total energy released in nuclear fission of one U-235 atom (on average; depends on the precise break up); this is 82 terajoules per kilogram, or twenty thousand tonnes of TNT equivalent per kilogram.
- 17.6 MeV: total energy released in fusion of deuterium and tritium to form helium-4 (also on average); this is 0.41 PJ/kg of product produced.
- 4MeV: amount of energy released by a snowflake hitting concrete pavement.
- 13.6 eV: energy required to ionize atomic hydrogen. Molecular bond energies are on the order of an eV per molecule.
- 1/40 eV: the thermal energy at room temperature. A single molecule in the air has an average kinetic energy 3/80 eV.
Conversion factor:
Relation to units of time and distance
In particle physics, a system of units in which the speed of light and the reduced Planck constant are dimensionless and equal to unity is widely used: . In these units, both distances and times are expressed in inverse energy units (while energy and mass are expressed in the same units, see Mass–energy equivalence). In particular, particle scattering lengths 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:
- = 6.582 118 99(16) x 10-16 eV s;
- = 197.326 9631(49) eV nm (or MeV fm).
The above relations also allow expressing the mean lifetime of an unstable particle (in seconds) 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)×10-4 eV, and its mean decay length is = 462 µm.
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 to kelvins (symbol: uppercase K) is defined by using kB, the Boltzmann constant:
For example, a typical magnetic confinement fusion plasma is 15 keV, or 170 megakelvins.
Photon properties
The energy E, frequency f, and wavelength ? of a photon are related by
where h is Planck's constant and c is the speed 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
to
.
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.
See also
External links
- physical constants reference; CODATA data
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