Electronvolt
A non-SI unit accepted for use · quantity: energy
01 · Definition
The electronvolt was born in accelerator laboratories: counting the energy of particles in joules is as awkward as measuring the distance between cities in microns.
One electronvolt is the energy an electron gains after passing through a potential difference of one volt. The definition turns straight into a number: the charge of the electron times the volt gives 1.602 176 634·10⁻¹⁹ joule — exact since 2019, because the elementary charge is fixed by the definition of the ampere. The unit covers the whole physics of small scales: the binding energy of a molecule is a few electronvolts, an X-ray quantum is tens of kiloelectronvolts, a proton in the Large Hadron Collider is seven teraelectronvolts.
The energy of a photon in electronvolts is 1240 divided by the wavelength in nanometres. Green light at 550 nm is 2.25 eV, and everything visible fits between 1.65 and 3.26 eV. One number replaces Planck’s constant in your pocket.
02 · Conversion
Enter an energy — the sheet converts it
The electronvolt is the one unit that converts neatly into four scales at once: energy, temperature, wavelength and mass. Physicists use all four as synonyms — «a two-electronvolt transition» and «a line at 620 nanometres» describe the same thing.
Through E = mc² the electronvolt becomes a measure of mass as well. The electron weighs 511 keV/c², the proton 938 MeV/c², the Higgs boson 125 GeV/c². In particle physics the division by c² is usually left out.
| Unit | Name | Value | Where it is met |
|---|---|---|---|
| eV | electronvolt | 1 | molecular bonds, atomic spectra |
| meV | millielectronvolt | 1000 | lattice vibrations, thermal noise |
| keV | kiloelectronvolt | 0.001 | X-rays, inner shells |
| MeV | megaelectronvolt | 1·10⁻⁶ | nuclear transitions, particle masses |
| GeV | gigaelectronvolt | 1·10⁻⁹ | colliders, the proton mass |
| TeV | teraelectronvolt | 1·10⁻¹² | the limit of accelerators, cosmic rays |
One and the same line reads four ways: as energy, as the wavelength of a photon, as the temperature kT and as mass in eV/c². Physicists use them as synonyms.
03 · Orders of magnitude
from thermal noise to cosmic rayschemical bonds and visible light
04 · Measuring instruments
What measures electronvolts
Spreads radiation out by the energy of its quanta. A germanium detector tells lines apart to within a kiloelectronvolt.
The energy of the beam is set by the total voltage the particle has passed: that is where the unit got its name. The LHC accelerates a proton to 7 TeV.
Measures the energy of electrons knocked out by light: the difference between the quantum and the kinetic energy gives the binding energy in the substance.
Absorbs the particle whole and measures the energy released in gigaelectronvolts. That is how the Higgs boson was found at 125 GeV.
05 · Writing rules
e lower case, V upper case, the prefix before both
The symbol is written as one: keV, MeV, GeV — the prefix goes before the whole unit, not between its letters. A capital E would mean exa, a lower-case v would mean nothing. Local spellings of the name are allowed in running text, but in formulae and tables the Latin symbol is preferred.
Particle physics uses a system where c = 1, and mass is written simply in GeV without dividing by the square of the speed of light. Outside that field c² may not be left out: eV and eV/c² are different quantities.
06 · Neighbouring units
The electronvolt stands where three SI units meet: the charge of the electron sets the factor, the volt gives the definition, the joule gives the result. Through the constants of Planck and Boltzmann it converts into frequency and temperature.
sets the acceleration
e · V
1.602·10⁻¹⁹
At 300 K the thermal energy kT is 25.9 meV — this value sets the scale of thermal processes in electronics and chemistry.
07 · Historical section
Neighbours on the atomic scales
The unit of energy of the CGS system. For atomic scales it proved far too large: the electronvolt is six hundred billion times smaller than the erg.
The ionisation energy of hydrogen: 13.6 eV. Atomic spectra are still counted in rydbergs and reciprocal centimetres.
The atomic unit of energy, twice the rydberg. Molecular calculations are carried out in it — that way all the constants vanish from the equations.
Binding energy reckoned per mole of substance. Biochemistry and organic chemistry hold on to it; physics has moved to electronvolts.
A non-SI unit that in 2019 became more exact than the joule
Before the SI reform the electronvolt was approximate: its value depended on the measured charge of the electron and was refined with every new experiment. In 2019 the elementary charge was fixed by definition — exactly 1.602 176 634·10⁻¹⁹ coulomb — and the conversion factor from electronvolts to joules became an exact number with no uncertainty. A curious situation resulted: a non-SI unit, one that the SI merely «accepts for use», is now tied to the joule absolutely strictly, while many quantities inside the system itself are measured to finite precision.
Catalogue · units of measurement
A passport for every quantity
Seven SI base units, twenty-two derived ones with names of their own, and the non-SI units that neither engineering nor daily life can do without. Each gets its own sheet: definition, conversion, instruments, writing rules. In 5 languages.
Non-SI units
open the electronvolt sheetSI base units
in terracotta — those that stand behind the electronvoltSI derived units with names of their own
twenty-two sheetsPassport language
5 languages. The symbol eV and the formulae are not translated — they are the same in every locale; the prose, the examples and the captions are.