electronvolt

electronvolt. The electronvolt is a unit of energy equal to the amount of kinetic energy gained by a single electron when it accelerates through an electric potential difference of one volt in a vacuum. It is a common unit of energy in the fields of atomic physics, particle physics, and condensed matter physics due to its convenient scale for describing microscopic processes. The value of one electronvolt is defined by the elementary charge and the volt, making it a derived unit in the International System of Units.

Definition and equivalence

The electronvolt is defined as the product of the elementary charge, denoted by e, and one volt. The elementary charge, as defined by the 2019 redefinition of the SI base units, is exactly coulombs. Therefore, one electronvolt is exactly joules. This equivalence arises directly from the definition of the volt in terms of energy per charge, as established in electromagnetism. The unit is particularly useful because the energies involved in atomic-scale interactions, such as the ionization energy of the hydrogen atom (approximately 13.6 eV), are conveniently expressed in these terms. Its adoption is widespread in calculations involving particle accelerators like the Large Hadron Collider and in describing the band gap in semiconductors.

History

The concept of the electronvolt emerged in the early 20th century alongside the development of quantum mechanics and experimental particle physics. Early work on cathode rays by J. J. Thomson and measurements of the charge-to-mass ratio of the electron laid the groundwork. The unit became practically essential with Robert Millikan's famous oil-drop experiment, which precisely measured the elementary charge. As physicists began probing atomic nuclei with machines like Ernest Lawrence's cyclotron, the electronvolt provided a natural scale for discussing particle energies. The commissioning of ever-larger accelerators, such as the Stanford Linear Accelerator Center and CERN, solidified its use for describing the rest masses and kinetic energies of fundamental particles like the proton and electron.

Use in physics

In particle physics, the electronvolt is the standard unit for expressing the masses of elementary particles, utilizing Albert Einstein's mass–energy equivalence, E = mc². For instance, the mass of an electron is about 0.511 MeV/c², and that of a proton is approximately 938 MeV/c². It is central to describing collision energies in facilities like the Tevatron and Relativistic Heavy Ion Collider. In atomic and condensed matter physics, it is used to quantify binding energies, ionization potentials, and the energy of photons, with visible light photons having energies around 1.5 to 3 eV. The unit also appears in astrophysics and cosmology, for example, in describing the temperature of the cosmic microwave background or the energetic processes in supernova remnants.

Multiples and related units

Given the vast range of energies encountered, standard SI prefixes are used to form multiples of the electronvolt. Common multiples include the kiloelectronvolt (keV, 10³ eV), megaelectronvolt (MeV, 10⁶ eV), gigaelectronvolt (GeV, 10⁹ eV), and teraelectronvolt (TeV, 10¹² eV). In some contexts, the reciprocal centimeter (cm⁻¹), a unit from spectroscopy, is used as an equivalent measure of energy, with 1 eV approximately equal to 8065.5 cm⁻¹. For temperature, the electronvolt can be related to the kelvin via the Boltzmann constant, where 1 eV corresponds to about 11,605 kelvins, a conversion useful in plasma physics and discussions of stellar nucleosynthesis.

Examples and energy scales

Typical chemical reactions involve energy changes on the order of a few electronvolts per molecule. The photoelectric effect, explained by Einstein, involves photons with energies of a few eV ejecting electrons from metals. In nuclear physics, the energy released per nucleon in nuclear fission is around 1 MeV, while the mass of the Higgs boson discovered at the Large Hadron Collider is approximately 125 GeV/c². At the highest observed energies, ultra-high-energy cosmic rays can reach energies exceeding 10¹⁸ eV. Conversely, the very low energy scale of neutrino masses is often expressed in electronvolts or sub-electronvolt units, highlighting the unit's versatility across the breadth of physical phenomena. Category:Units of energy