In physics, the electron volt is a unit of energy equal to approximately 1.6×10−19
joule. By definition, it is the amount of energy gained by the charge of a single electron
moved across an electric potential difference of one volt. Thus it is 1 volt multiplied
by the elementary charge×10−19 C). Therefore, one electron volt is equal to 1.602176565(35)×10−19 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 V; 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, 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 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 SI prefixes milli-, kilo-, mega-, giga-, tera-, peta- or exa-.
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 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/c2, where c is the speed of light in
vacuum. 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 1.783×10−36 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 a convenient unit of mass for particle physics:
1 GeV/c2 = 1.783×10−27 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/c2 = 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. This gives rise to usage of eV as units of momentum, for the
energy supplied results in acceleration of the particle.
The dimensions of momentum units are LT−1M. The dimensions of energy units are L2T−2M.
Then, dividing the units of energy by a fundamental constant that has units of velocity, 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. 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 τ of an unstable particle
in terms of its decay width Γ via Γ = ħ/τ. For example, the B0 meson has a lifetime of
1.530(9) picoseconds, mean decay length is cτ = 459.7 µm, or a decay width of×10−4 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 kelvin is defined by using kB, the Boltzmann constant:
For example, a typical magnetic confinement fusion plasma is 15 keV, or 170 megakelvin.
As an approximation: kBT is about 0.025 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 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
measured by scintillation light. For example, the yield of a phototube is measured in phe/keVee.
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 5.25×1032 eV: total energy released from
a 20 kt nuclear fission device ~624 EeV: energy consumed by a single 100-watt
light bulb in one second 300 EeV: the so-called Oh-My-God particle
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
1 TeV: a trillion electronvolts, or 1.602×10−7 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: 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 kBT at room temperature;
one air molecule has an average kinetic energy 38 meV
230 µeV: the thermal energy kBT of the cosmic microwave background
See also Orders of magnitude
Notes and references
External links BIPM's definition of the electronvolt
http:physics.nist.govConstants physical constants reference; CODATA data