meV, keV, MeV, GeV, TeV and PeV redirect here. For other uses, see MEV, KEV, GEV, TEV and PEV.
In physics, the electronvolt (symbol eV, also written electron-volt and electron volt) is a unit of energy equal to approximately 6981160000000000000♠1.6×10−19 joules (symbol J) in SI units.
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 E = qV 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 1 J/C √2hα / μ0c0. 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 (109) electronvolts; it is equivalent to the GeV.
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, 7000100000000000000♠1 J/C, multiplied by the electron's elementary charge e, 6981160217662079999♠1.6021766208(98)×10−19 C. Therefore, one electronvolt is equal to 6981160217662079999♠1.6021766208(98)×10−19 J. The electronvolt, as opposed to volt, is not an SI unit. Its derivation is empirical, which means its value in SI units must be obtained by experiment and is therefore not known exactly, unlike the litre, the light-year and such other non-SI units. Electronvolt (eV) is a unit of energy whereas volt (V) is the derived SI unit of electric potential. The SI unit for energy is joule (J). 1 eV is equal to 6981160217662079999♠1.6021766208(98)×10−19 J.
7000100000000000000♠1 GeV/c2 = 6973178300000000000♠1.783×10−27 kg.
1 u = 7002931494100000000♠931.4941 MeV/c2 = 6999931494100000000♠0.9314941 GeV/c2.
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., 6981160217648700000♠1 eV). 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 L2MT−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.
In particle physics, a system of "natural units" in which the speed of light in vacuum c and the reduced Planck constant ħ are dimensionless and equal to unity is widely used: c = ħ = 1. 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.
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 lifetime of 1.530(9) picoseconds, mean decay length is cτ = 6996459699999999999♠459.7 μm, or a decay width of 6977689256324707400♠(4.302±25)×10−4 eV.
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.
For example, a typical magnetic confinement fusion plasma is 6985240326473049999♠15 keV, or 170 MK.
As an approximation: kBT is about 6979400544121750000♠0.025 eV (≈ 290 K/11604 K/eV) at a temperature of 7002293150000000000♠20 °C.
A photon with a wavelength of 6993532000000000000♠532 nm (green light) would have an energy of approximately 6981373307121471000♠2.33 eV. Similarly, 6981160217648700000♠1 eV would correspond to an infrared photon of wavelength 6994124000000000000♠1240 nm or frequency 7014241800000000000♠241.8 THz.
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.
γ: Gamma rays MIR: Mid infrared HF: High freq.
HX: Hard X-rays FIR: Far infrared MF: Medium freq.
SX: Soft X-rays Radio waves LF: Low freq.
EUV: Extreme ultraviolet EHF: Extremely high freq. VLF: Very low freq.
NUV: Near ultraviolet SHF: Super high freq. VF/ULF: Voice freq.
Visible light UHF: Ultra high freq. SLF: Super low freq.
NIR: Near Infrared VHF: Very high freq. ELF: Extremely low freq.
One mole of particles given 1 eV of energy has approximately 96.5 kJ of energy – this corresponds to the Faraday constant (F ≈ 7004964850000000000♠96485 C mol−1) where the energy in joules of N moles of particles each with energy X eV is X·F·N.
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