Английская Википедия:Electronvolt
Шаблон:Short description Шаблон:Redirect-several
In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacuum. When used as a unit of energy, the numerical value of 1 eV in joules (symbol J) is equivalent to the numerical value of the charge of an electron in coulombs (symbol C). Under the 2019 redefinition of the SI base units, this sets 1 eV equal to the exact value Шаблон:Physconst
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 gains an energy Шаблон:Nowrap after passing through a voltage of V. Since q must be an integer multiple of the elementary charge e for any isolated particle, the gained energy in units of electronvolts conveniently equals that integer times the voltage.
Definition and use
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, Шаблон:Val, multiplied by the elementary charge Шаблон:Physconst Therefore, one electronvolt is equal to Шаблон:Physconst
The electronvolt (eV) is a unit of energy, but is not an SI unit. It is a common unit of energy within physics, widely used in solid state, atomic, nuclear, and particle physics, and high-energy astrophysics. It is commonly used with SI prefixes milli-, kilo-, mega-, giga-, tera-, peta- or exa- (meV, keV, MeV, GeV, TeV, PeV and EeV respectively). The SI unit of energy is the joule (J).
In some older documents, and in the name Bevatron, the symbol BeV is used, where the "B" stands for billion. The symbol BeV is therefore equivalent to the GeV.
Relation to other physical properties and units
| Measurement | Unit | SI value of unit |
|---|---|---|
| Energy | eV | Шаблон:Val |
| Mass | eV/c2 | Шаблон:Val |
| Momentum | eV/c | Шаблон:Val |
| Temperature | eV/kB | Шаблон:Val |
| Time | ħ/eV | Шаблон:Val |
| Distance | ħc/eV | Шаблон:Val |
Mass
By mass–energy equivalence, the electronvolt corresponds to 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 [[Mass–energy equivalence|Шаблон:Nowrap]]). It is common to informally express mass in terms of eV as a unit of mass, effectively using a system of natural units with c set to 1.[1] The kilogram equivalent of Шаблон:Val is:
<math display="block">1\; \text{eV}/c^2 = \frac{(1.602\ 176\ 634 \times 10^{-19} \, \text{C}) \times 1 \, \text{V}}{(299\ 792\ 458\; \mathrm{m/s})^2} = 1.782\ 661\ 92 \times 10^{-36}\; \text{kg}.</math>
For example, an electron and a positron, each with a mass of Шаблон:Val, can annihilate to yield Шаблон:Val of energy. A proton has a mass of Шаблон:Val. In general, the masses of all hadrons are of the order of Шаблон:Val, which makes the GeV/c2 a convenient unit of mass for particle physics:[2] Шаблон:Block indent
The atomic mass constant (mu), one twelfth of the mass a carbon-12 atom, is close to the mass of a proton. To convert to electronvolt mass-equivalent, use the formula: Шаблон:Block indent
Momentum
By dividing a particle's kinetic energy in electronvolts by the fundamental constant c (the speed of light), one can describe the particle's momentum in units of eV/c.[3] In natural units in which the fundamental velocity constant c is numerically 1, the c may informally be omitted to express momentum as electronvolts.
<math display="block">E^2 = p^2 c^2 + m_0^2 c^4</math>
in natural units (with <math>c=1</math>)
<math display="block">E^2 = p^2 + m_0^2</math>
is a Pythagorean equation. When a relatively high energy is applied to a particle with relatively low rest mass, it can be approximated as <math>E \simeq p</math> in high-energy physics such that an applied energy in units of eV conveniently results in an approximately equivalent change of momentum in units of eV/c.
The dimensions of momentum units are Шаблон:Dimanalysis. The dimensions of energy units are Шаблон:Dimanalysis. Dividing the units of energy (such as eV) by a fundamental constant (such as the speed of light) that has units of velocity (Шаблон:Dimanalysis) facilitates the required conversion for using energy units to describe momentum.
For example, if the momentum p of an electron is said to be Шаблон:Val, then the conversion to MKS system of units can be achieved by: <math display="block">p = 1\; \text{GeV}/c = \frac{(1 \times 10^9) \times (1.602\ 176\ 634 \times 10^{-19} \; \text{C}) \times (1 \; \text{V})}{2.99\ 792\ 458 \times 10^8\; \text{m}/\text{s}} = 5.344\ 286 \times 10^{-19}\; \text{kg} {\cdot} \text{m}/\text{s}.</math>
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: Шаблон:Nowrap. 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: <math display="block">\hbar = 1.054\ 571\ 817\ 646\times 10^{-34}\ \mathrm{J{\cdot}s} = 6.582\ 119\ 569\ 509\times 10^{-16}\ \mathrm{eV{\cdot}s}.</math>
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 Шаблон:Nowrap. For example, the [[B meson|Шаблон:Subatomic particle meson]] has a lifetime of 1.530(9) picoseconds, mean decay length is Шаблон:Nowrap, or a decay width of Шаблон:Val.
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: <math display="block">\frac{1\; \text{eV}}{hc} = \frac{1.602\ 176\ 634 \times 10^{-19} \; \text{J}}{(2.99\ 792\ 458 \times 10^{10}\; \text{cm}/\text{s}) \times (6.62\ 607\ 015 \times 10^{-34}\; \text{J} {\cdot} \text{s})} \thickapprox 8065.5439 \; \text{cm}^{-1}.</math>
Temperature
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: <math display="block">{1 \,\mathrm{eV} / k_{\text{B}}} = {1.602\ 176\ 634 \times 10^{-19} \text{ J} \over 1.380\ 649 \times 10^{-23} \text{ J/K}} = 11\ 604.518\ 12 \text{ K},</math>
where kB is the Boltzmann constant.
The kB is assumed when using the electronvolt to express temperature, for example, a typical magnetic confinement fusion plasma is Шаблон:Val (kiloelectronvolt), which is equal to 174 MK (megakelvin).
As an approximation: kBT is about Шаблон:Val (≈ Шаблон:Sfrac) at a temperature of Шаблон:Val.
Wavelength
The energy E, frequency v, and wavelength λ of a photon are related by
<math display="block">E = h\nu = \frac{hc}{\lambda} = \frac{4.135\, 667\, 516 \times 10^{-15}\,\mathrm{eV{\cdot}s} \times 299\, 792\, 458\,\mathrm{m/s}}{\lambda}</math>
where h is the Planck constant, c is the speed of light. This reduces to[4] <math display="block">\begin{align} E\mathrm{(eV)} &=4.135\, 667\, 516 \times 10^{-15}\,\mathrm{eV{\cdot}s}\times\nu \\[4pt] &=\frac{1\ 239.841\ 93\,\text{eV}{\cdot}\text{nm}}{\lambda}. \end{align}</math> A photon with a wavelength of Шаблон:Val (green light) would have an energy of approximately Шаблон:Val. Similarly, Шаблон:Val would correspond to an infrared photon of wavelength Шаблон:Val or frequency Шаблон:Val.
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
| 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. |
| Freq: Frequency |
| Energy | Source |
|---|---|
| Шаблон:Val | total energy released from a 20 kt nuclear fission device |
| 12.2 ReV (Шаблон:Val) | the Planck energy |
| 10 YeV (Шаблон:Val) | approximate grand unification energy |
| ~624 EeV (Шаблон:Val) | energy consumed by a single 100-watt light bulb in one second (Шаблон:Val = Шаблон:Val ≈ Шаблон:Val) |
| 300 EeV (Шаблон:Val = ~Шаблон:Val) | The first ultra-high-energy cosmic ray particle observed, the so-called Oh-My-God particle.[8] |
| Шаблон:Val | two petaelectronvolts, the highest-energy neutrino detected by the IceCube neutrino telescope in Antarctica[9] |
| Шаблон:Val | designed proton center-of-mass collision energy at the Large Hadron Collider (operated at 3.5 TeV since its start on 30 March 2010, reached 13 TeV in May 2015) |
| Шаблон:Val | a trillion electronvolts, or Шаблон:Val, about the kinetic energy of a flying mosquito[10] |
| 172 GeV | rest energy of top quark, the heaviest measured elementary particle |
| 125.1±0.2 GeV | 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[11] |
| Шаблон:Val | average energy released in fission of one Pu-239 atom |
| Шаблон:Val | approximate average energy released in nuclear fission fission fragments of one U-235 atom. |
| 105.7 MeV | rest energy of a muon |
| Шаблон:Val | average energy released in the nuclear fusion of deuterium and tritium to form He-4; this is Шаблон:Val per kilogram of product produced |
| 2 MeV | approximate average energy released in a nuclear fission neutron released from one U-235 atom. |
| 1.9 MeV | rest energy of up quark, the lowest mass quark. |
| Шаблон:Val (Шаблон:Val) | about twice the rest energy of an electron |
| 1 to 10 keV | approximate thermal temperature, <math>k_\text{B}T</math>, in nuclear fusion systems, like the core of the sun, magnetically confined plasma, inertial confinement and nuclear weapons |
| Шаблон:Val | the energy required to ionize atomic hydrogen; molecular bond energies are on the order of Шаблон:Val to Шаблон:Val per bond |
| Шаблон:Val to Шаблон:Val | the photon energy of visible light |
| Шаблон:Val | energy <math>E_g</math> required to break a covalent bond in silicon |
| Шаблон:Val | energy <math>E_g</math> required to break a covalent bond in germanium |
| < Шаблон:Val | approximate rest energy of neutrinos (sum of 3 flavors)[12] |
| Шаблон:Val | thermal energy, <math>k_\text{B}T</math>, at room temperature; one air molecule has an average kinetic energy Шаблон:Val |
| Шаблон:Val | thermal energy, <math>k_\text{B}T</math>, of the cosmic microwave background |
Per mole
One mole of particles given 1 eV of energy each has approximately 96.5 kJ of energy – this corresponds to the Faraday constant (F ≈ Шаблон:Val), where the energy in joules of n moles of particles each with energy E eV is equal to E·F·n.
See also
References
External links
- ↑ Шаблон:Cite journal
- ↑ Шаблон:Cite web
- ↑ Шаблон:Cite web
- ↑ Шаблон:Cite web
- ↑ What is Light? Шаблон:Webarchive – UC Davis lecture slides
- ↑ Шаблон:Cite web
- ↑ Шаблон:Cite web
- ↑ Open Questions in Physics. Шаблон:Webarchive German Electron-Synchrotron. A Research Centre of the Helmholtz Association. Updated March 2006 by JCB. Original by John Baez.
- ↑ Шаблон:Cite web
- ↑ Glossary Шаблон:Webarchive - CMS Collaboration, CERN
- ↑ Шаблон:Cite journal
- ↑ Шаблон:Cite journal
