In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the amount of kinetic energy gained (or lost) by a single electron accelerating from rest through an electric potential difference of one volt in vacuum. When used as a unit of energy, BIPM has fixed the definition of the electronvolt equal to joules (symbol J), even though the exact measurement of the charge of an electron continues to be improved and as of 2014^{[update]} is .
Historically, the electronvolt was devised as a standard unit of measure through its usefulness in electrostatic particle accelerator sciences, because a particle with electric charge q has an energy after passing through the potential V; if q is quoted in integer units of the elementary charge and the potential in volts, one gets an energy in eV.
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). In some older documents, and in the name Bevatron, the symbol BeV is used, which stands for billion (10^{9}) electronvolts; it is equivalent to the GeV.
Measurement | Unit | SI value of unit |
---|---|---|
Energy | eV | |
Mass | eV/c^{2} | |
Momentum | eV/c | |
Temperature | eV/k_{B} | |
Time | ?/eV | |
Distance | ?c/eV |
An electronvolt is the amount of kinetic energy gained or lost by a single electron accelerating from rest through an electric potential difference of one volt in vacuum. Hence, it has a value of one volt, , multiplied by the electron's elementary charge e, .^{[1]} Therefore, one electronvolt is equal to .^{[2]}
The electronvolt, as opposed to the volt, is not an SI unit. The electronvolt (eV) is a unit of energy whereas the volt (V) is the derived SI unit of electric potential. The SI unit for energy is the joule (J).
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.^{[3]} 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 unified atomic mass unit (u), almost exactly 1 gram divided by the Avogadro number, is almost the mass of a hydrogen atom, which is mostly the mass of the proton. To convert to electron volts, use the formula:
In high-energy physics, the electronvolt is often used as a unit of momentum. A potential difference of 1 volt causes an electron to gain an amount of energy (i.e., ). This gives rise to usage of eV (and keV, MeV, GeV or TeV) as units of momentum, for the energy supplied results in acceleration of the particle.
The dimensions of momentum units are LMT^{-1}. The dimensions of energy units are L^{2}MT^{-2}. Then, dividing the units of energy (such as eV) by a fundamental constant that has units of velocity (LT^{-1}), facilitates the required conversion of using energy units to describe momentum. In the field of high-energy particle physics, the fundamental velocity unit is the speed of light in vacuum c.
By dividing energy in eV by the speed of light, one can describe the momentum of an electron in units of eV/c.^{[4]}^{[5]}
The fundamental velocity constant c is often dropped from the units of momentum by way of defining units of length such that the value of c is unity. For example, if the momentum p of an electron is said to be , then the conversion to MKS can be achieved by:
In particle physics, a system of "natural units" in which the speed of light in vacuum c 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:
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 B^{0} meson has a lifetime of 1.530(9) picoseconds, mean decay length is , or a decay width of .
Conversely, the tiny meson mass differences responsible for meson oscillations are often expressed in the more convenient inverse picoseconds.
Energy in electronvolts is sometimes expressed through the wavelength of light with photons of the same energy: 1 eV = 8065.544005(49) cm^{-1}.
In certain fields, such as plasma physics, it is convenient to use the electronvolt to express temperature. The electronvolt is divided by the Boltzmann constant to convert to the Kelvin scale:
Where k_{B} is the Boltzmann constant, K is Kelvin, J is Joules, eV is electronvolts.
The k_{B} is assumed when using the electronvolt to express temperature, for example, a typical magnetic confinement fusion plasma is (kilo-electronvolts), which is equal to 170 MK (million degrees Kelvin).
As an approximation: k_{B}T is about (? ) at a temperature of .
The energy E, frequency v, and wavelength ? of a photon are related by
where h is the Planck constant, c is the speed of light. This reduces to
A photon with a wavelength of (green light) would have an energy of approximately . Similarly, would correspond to an infrared photon of wavelength or frequency .
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.
Energy | Source |
---|---|
total energy released from a 20 kt nuclear fission device | |
the Planck energy | |
10 YeV | the approximate grand unification energy |
~624 EeV | energy consumed by a single 100-watt light bulb in one second ( = ? ) |
300 EeV ( = ~) | ^{[10]} the so-called Oh-My-God particle (the most energetic cosmic ray particle ever observed) |
two petaelectronvolts, the most high-energetic neutrino detected by the IceCube neutrino telescope in Antarctica^{[11]} | |
the designed proton collision energy at the Large Hadron Collider (operated at about half of this energy since 30 March 2010, reached 13 TeV in May 2015) | |
a trillion electronvolts, or , about the kinetic energy of a flying mosquito^{[12]} | |
125.1±0.2 GeV | the energy corresponding to the mass of the Higgs boson, as measured by two separate detectors at the LHC to a certainty better than 5 sigma^{[13]} |
the average energy released in fission of one Pu-239 atom | |
the average energy released in nuclear fission of one U-235 atom | |
the average energy released in the fusion of deuterium and tritium to form He-4; this is per kilogram of product produced | |
about twice the rest energy of an electron | |
the energy required to ionize atomic hydrogen; molecular bond energies are on the order of to per bond | |
to | the photon energy of visible light |
the energy E_{G} required to break a covalent bond in silicon | |
the energy E_{G} required to break a covalent bond in germanium | |
the thermal energy k_{B}T at room temperature; one air molecule has an average kinetic energy | |
the thermal energy k_{B}T of the cosmic microwave background |
One mole of particles given 1 eV of energy has approximately 96.5 kJ of energy -- this corresponds to the Faraday constant (F ≈ ), where the energy in joules of n moles of particles each with energy E eV is equal to E·F·n.