In
physics, the
electron volt (symbol
eV; also written
electronvolt according to the
NIST,
IUPAC, and
BIPM) 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. Thus it is 1
volt (1
joule divided by 1
coulomb) multiplied by the
electron charge, . One electron volt is equal to .
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. It is commonly used with
SI prefixes 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 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/
c2, where
c is the
speed of light in a vacuum (from
E = mc2). Even more common is to use a system of
natural units and simply use 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/c
2, can
annihilate to yield 1.022 MeV of energy. The
proton has a mass of 0.938
GeV/c
2, 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 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
Conversion factors:
For comparison:
- 1 TeV: one teraelectronvolt (a million million electronvolts), or 1.602×10-7 J, about the kinetic energy of a flying mosquito
- 210 MeV: 200 megaelectronvolts, 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 petajoule per kilogram of product produced, which is equivalent to the energy released in a 100-kiloton explosion of TNT.
- 1 MeV: one million electron volts, or 1.602×10-13 J, about twice the rest mass-energy of an electron
- 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.
Relation to units of time and distance
In
particle physics, a system of units in which the speed of light in a vacuum
c and the
reduced Planck constant ħ 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:
ħ = .
The above relations also allow expressing the
mean lifetime τ of an unstable particle (in seconds) in terms of its
decay width Γ (in eV) via
Γ =
ħ/
τ. For example, the
B0 meson has a mean lifetime of 1.542(16)
picoseconds, or a decay width of , and its mean decay length is .
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
ν, and wavelength λ of a photon are related by
where
h is
Planck's constant,
c is the
speed of light. For quick calculations, this reduces to
A stream of photons with a wavelength of 532 nm (green light) would have an energy of approximately 2.33 eV. Similarly, 1 eV would correspond to a stream of infrared photons of wavelength 1240 nm, and so on.
1 eV = 8065.5447 cm
-1In scattering experiments
In a low-energy nuclear scattering experiment, it is conventional to refer to the nuclear recoil energy in units of eVr, keVr, etc. This distinguishes the nuclear recoil energy from the "electron equivalent" recoil energy (eVee, keVee, etc.) measured by
scintillation light. For example, the yield of a
phototube is measured in phe/keVee (
photoelectrons per keV electron-equivalent energy). The relationship between eV, eVr, and eVee depends on the medium the scattering takes place in, and must be established empirically for each material.
See also
