Electronvolt

In physics, the electronvolt (symbol eV; also written electron volt) is a unit of energy equal to approximately 160 zeptojoules (symbol zJ) or 1.6×10^âˆ'19 joules (symbol J). By definition, it is the amount of energy gained (or lost) by the charge of a single electron moved across an electric potential difference of one volt. Thus it is 1 volt (1 joule per coulomb, 1 J/C) multiplied by the elementary charge (e, or 1.602176565(35)×10^âˆ'19 C). Therefore, one electron volt is equal to 1.602176565(35)×10^âˆ'19 J. Historically, the electron volt was devised as a standard unit of measure through its usefulness in electrostatic particle accelerator sciences because a particle with charge q has an energy E = qV after passing through the potential Vi>; if q is quoted in integer units of the elementary charge and the terminal bias in volts, one gets an energy in eV.

The electron volt is not an SI unit, and its definition is empirical (unlike the litre, the light year and other such non-SI units), thus its value in SI units must be obtained experimentally. 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). Thus meV stands for milli-electron volt.

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.

§Mass

By massâ€"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/c², where c is the speed of light in vacuum (from E = mc²). 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.

The mass equivalent of 1 eV is 1.783×10^âˆ'36 kg.

For example, an electron and a positron, each with a mass of 0.511 MeV/c², can annihilate to yield 1.022 MeV of energy. The proton has a mass of 0.938 GeV/c². In general, the masses of all hadrons are of the order of 1 GeV/c², which makes the GeV (gigaelectronvolt) a convenient unit of mass for particle physics:

1 GeV/c²Â = 1.783×10^âˆ'27 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 = 931.4941 MeV/c²Â = 0.9314941 GeV/c².

§Momentum

In high-energy physics, the electron volt 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., 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 LT^âˆ'1M. The dimensions of energy units are L²T^âˆ'2M. Then, dividing the units of energy (such as eV) by a fundamental constant that has units of velocity (LT^âˆ'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. Thus, dividing energy in eV by the speed of light, one can describe the momentum of an electron in units of eV/c.

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 1 GeV, then the conversion to MKS can be achieved by:

§Distance

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 massâ€"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 Ï„ of an unstable particle (in seconds) in terms of its decay width Î" (in eV) via Î" = ħ/Ï„. For example, the B⁰ meson has a lifetime of 1.530(9) picoseconds, mean decay length is cÏ„ = 459.7 µm, or a decay width of (4.302±25)×10^âˆ'4 eV.

Conversely, the tiny meson mass differences responsible for meson oscillations are often expressed in the more convenient inverse picoseconds.

§Temperature

In certain fields, such as plasma physics, it is convenient to use the electronvolt as a unit of temperature. The conversion to the Kelvin scale is defined by using k_B, the Boltzmann constant:

For example, a typical magnetic confinement fusion plasma is 15 keV, or 170 megakelvin.

As an approximation: k_BT is about 0.025 eV (≈ 290 K/11604 K/eV) at a temperature of 20 °C.

§Properties

The energy E, frequency v, and wavelength λ 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 532 nm (green light) would have an energy of approximately 2.33 eV. Similarly, 1 eV would correspond to an infrared photon of wavelength 1240 nm, and so on.

§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.

§Energy comparisons

• 5.25×10³²Â eV: total energy released from a 20 kt nuclear fission device
• 1.22×10²⁸ eV: the energy at the Planck scale
• 1×10²⁵ eV: the approximate grand unification energy
• ~624 EeV (6.24×10²⁰ eV): energy consumed by a single 100-watt light bulb in one second (100 W = 100 J/s ≈ 6.24×10²⁰ eV/s)
• 300 EeV (3×10²⁰ eV = ~50 J): the so-called Oh-My-God particle (the most energetic cosmic ray particle ever observed)
• 1 PeV: one petaelectronvolt, the amount of energy measured in each of two different cosmic neutrino candidates detected by the IceCube neutrino telescope in Antarctica
• 14 TeV: the designed proton collision energy at the Large Hadron Collider (which has operated at half of this energy since 30 March 2010)
• 1 TeV: a trillion electronvolts, or 1.602×10^âˆ'7 J, about the kinetic energy of a flying mosquito
• 125.3±0.6 GeV: the energy emitted by the decay of the Higgs boson, as measured by two separate detectors at the LHC to a certainty of 5 sigma
• 210 MeV: the average energy released in fission of one Pu-239 atom
• 200 MeV: the average energy released in nuclear fission of one U-235 atom
• 17.6 MeV: the average energy released in the fusion of deuterium and tritium to form He-4; this is 0.41 PJ per kilogram of product produced
• 1 MeV (1.602×10^âˆ'13 J): about twice the rest energy of an electron
• 13.6 eV: the energy required to ionize atomic hydrogen; molecular bond energies are on the order of 1 eV to 10 eV per bond
• 1.6 eV to 3.4 eV: the photon energy of visible light
• 25 meV: the thermal energy k_BT at room temperature; one air molecule has an average kinetic energy 38 meV
• 230 µeV: the thermal energy k_BT of the cosmic microwave background

§See also

• Orders of magnitude (energy)

§Notes and references

§External links

• BIPM's definition of the electronvolt
• physical constants reference; CODATA data

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