Electronvolt
Unit of energy gained by an electron through one volt.
The electronvolt (symbol eV) is a unit of energy defined as the kinetic energy a single electron picks up when it accelerates across a one-volt electric potential difference in a vacuum. Because one volt equals one joule per coulomb, and the elementary charge is exactly 1.602176634×10⁻¹⁹ C, one electronvolt equals exactly 1.602176634×10⁻¹⁹ J. While not an SI unit, the electronvolt is standard in solid-state, atomic, nuclear, and particle physics, as well as high-energy astrophysics. It is often combined with SI prefixes like milli- (meV), kilo- (keV), mega- (MeV), giga- (GeV), tera- (TeV), peta- (PeV), exa- (EeV), zetta- (ZeV), yotta- (YeV), ronna- (ReV), and quetta- (QeV). In older texts, the symbol BeV (for billion electronvolts) appears, equivalent to GeV.
The unit originated in electrostatic particle accelerator work, where a particle with charge q gains energy E = qV after passing through voltage V. By mass–energy equivalence, the electronvolt also corresponds to a mass unit: 1 eV/c² equals about 1.78266192×10⁻³⁶ kg. For instance, an electron and positron each have a mass of 0.511 MeV/c², and their annihilation yields 1.022 MeV of energy; a proton’s mass is 0.938 GeV/c². Particle physicists often express mass simply in eV, effectively setting c = 1. Momentum can be given in eV/c, and in natural units where c = 1, the energy–momentum relation E² = p²c² + m₀²c⁴ simplifies to E² = p² + m₀², a Pythagorean equation.
- field
- Physics
- unit_type
- Energy
- symbol
- eV
- common_prefixes
- meV, keV, MeV, GeV, TeV, PeV, EeV, ZeV, YeV, ReV, QeV
- related_units
- eV/c² (mass), eV/c (momentum)
Lore & Background
The electronvolt is a unit of energy equal to the kinetic energy gained by a single electron when it accelerates through a potential difference of one volt in a vacuum. Its numerical value in joules is identical to the numerical value of the elementary charge in coulombs, a relationship fixed by the 2019 revision of the SI. Historically, the unit was developed for its practical use in electrostatic particle accelerators, where a particle with charge q gains energy E = qV after passing through a voltage V. While not an SI unit, the electronvolt is widely employed in solid-state, atomic, nuclear, and particle physics, as well as high-energy astrophysics. It is commonly combined with SI prefixes, including milli-, kilo-, mega-, giga-, tera-, peta-, exa-, zetta-, yotta-, ronna-, and quetta- (symbols meV, keV, MeV, GeV, TeV, PeV, EeV, ZeV, YeV, ReV, and QeV). In older documents, the symbol BeV (for billion electronvolts) appears, equivalent to GeV. Through mass–energy equivalence, the electronvolt corresponds to a unit of mass, often expressed as eV/c²; for example, an electron and positron each have a mass of 0.511 MeV/c², and a proton has a mass of 0.938 GeV/c². Momentum can be expressed in eV/c, and in natural units where c and ħ are set to 1, distances and times are expressed in inverse energy units. The electronvolt is also used to express temperature by dividing by the Boltzmann constant, so a typical fusion plasma of 1 keV corresponds to roughly 174 megakelvins. Photon energy in electronvolts relates to wavelength via the Planck constant and speed of light.
Reader's Guide
The electronvolt is a fundamental unit in modern physics, enabling convenient expression of energies at atomic and subatomic scales. Its definition ties directly to the elementary charge and the volt, making it intuitive for describing particle accelerators and quantum phenomena. Similarly, momentum is often expressed in eV/c. The unit's flexibility with SI prefixes allows it to span from milli-electronvolts (meV) in solid-state physics to quetta-electronvolts (QeV) in high-energy astrophysics. Its adoption in the 2019 SI revision as an exact value ensures precision in calculations across disciplines.
Did You Know?
- The electronvolt is not an SI unit but is widely used in physics.
- In some older documents, the symbol BeV (billion electronvolt) is used, equivalent to GeV.
Frequently Asked Questions
What exactly is an electronvolt?
It is the kinetic energy a single electron acquires when it is accelerated across a one-volt potential difference in a vacuum. Physicists and chemists use it as a compact energy unit spanning atomic, nuclear, solid-state, and high-energy astrophysics contexts.
What prefixes do people actually pair with eV?
The full ladder runs from meV (milli) up through keV, MeV, GeV, TeV, PeV, EeV, ZeV, YeV, ReV, and QeV. In practice, particle physicists gravitate toward GeV and TeV, while solid-state and atomic researchers tend to stay in the meV-to-keV range.
Why do chemists reach for eV instead of joules?
Bond dissociation energies, ionization potentials, and semiconductor band gaps land at tidy one- or two-digit values when expressed in eV. That makes comparisons and calculations far more intuitive than juggling 10⁻¹⁹-joule figures.
Can eV express mass or momentum as well?
Yes—dividing an energy by c² yields a mass in eV/c², and dividing by c gives a momentum in eV/c. This shorthand is standard in particle physics, where quoting kilograms or kg·m/s would produce absurdly small numbers.
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