ElectronvoltEdit
The electronvolt is a practical unit of energy that recurs across many branches of physics, from the quantum scales of atoms and molecules to the high-energy frontiers of particle physics. It is defined as the amount of energy gained by a single electron when it is moved across an electric potential difference of one volt. Because the elementary charge is a fixed, exact quantity in the International System of Units, 1 electronvolt is exactly 1.602176634×10^-19 joules. This exact conversion makes the electronvolt convenient for expressing energies that are otherwise cumbersome to write in joules, especially when dealing with electronic structure, spectroscopy, and particle energetics. The electronvolt is not itself an SI base unit, but it is a widely accepted and deeply entrenched unit in scientific practice.
In practice, the electronvolt covers a broad energy range that is particularly relevant for atomic, molecular, and solid-state phenomena, as well as for high-energy physics. At the atomic and molecular level, energies are typically in the eV range, for example in chemical bond energies and electronic transitions. In solid-state physics, band gaps and crystal-field splittings are routinely expressed in eV. In particle physics, energies encountered in accelerators and detectors are often quoted in MeV (megaelectronvolts), GeV (gigaelectronvolts), or TeV (teraelectronvolts). The use of eV and its multiples reflects a natural alignment with energy scales that arise from electronic charge interactions and quantum phenomena.
Definition and nomenclature
An electronvolt (eV) is defined as the energy gained by an electron when it moves through electric potential difference of one volt. Equivalently, it is the product of the elementary charge e and a one-volt potential difference: 1 eV = e × 1 V. The elementary charge e is exactly 1.602176634×10^-19 coulombs, so 1 eV equals exactly 1.602176634×10^-19 joules. Because energy scales in atomic and subatomic systems are often small, the electronvolt is more convenient than joules for everyday physics practice. In common usage, powers of ten are attached to the eV to form keV, MeV, GeV, and TeV, where 1 keV = 1000 eV, 1 MeV = 10^6 eV, and so on.
In many contexts, the symbol eV is used without capitalization, and it appears in relation to other energy expressions such as electron mass-energy, kinetic energy, and photon energies. The term “electronvolt” may be encountered in both scientific literature and educational materials, and it is closely tied to the fundamental relation between energy, charge, and electric potential. See also the concepts of Planck constant, elementary charge, and joule for broader context on energy quantization and unit definitions.
Physical meaning and scales
The electronvolt is a unit that naturally matches several physical situations:
Electronic structure: energies associated with electron orbitals in atoms and molecules are commonly in the eV range. For example, the ionization energy of hydrogen is about 13.6 eV, and many valence-band energies in solids lie within a few eV of the Fermi level.
Spectroscopy and photonics: photon energies are often measured in eV, since a photon's energy E = hν can be expressed in electronvolts using the Planck constant h (see Planck constant). This makes it convenient to compare photon energies with electronic transition energies.
Solid-state physics: band gaps in semiconductors and insulators are typically quoted in eV, reflecting the energy required to promote an electron across the forbidden region between valence and conduction bands.
High-energy physics: when particle energies reach relativistic magnitudes, the scale is frequently stated in MeV, GeV, or TeV, with the GeV being a common reference in collider experiments and phenomenology.
Mass-energy equivalence: in discussions of relativistic energy, masses are often expressed in eV/c^2, especially in particle physics where natural energy scales are set by the effective masses of particles in a given interaction.
Common conversions and related units help bridge contexts. For example, 1 eV ≈ 1.602×10^-19 joules, and 1 eV corresponds to approximately 1.160×10^4 kelvin in the sense of kBT ~ energy, where kB is the Boltzmann constant. In many practical problems, the energy scale of interest is explicitly compared against characteristic energies such as binding energies, photon energies, or rest-mass energies, so the eV remains a concise shorthand that keeps discussions readable and physically intuitive.
Examples and typical values that illustrate the range of the electronvolt include: - Chemical bond energies on the order of a few eV. - Hydrogen ionization energy 13.6 eV. - Typical semiconductor band gaps from about 0.1 to 3 eV. - Rest-mass energy of the electron is about 0.511 MeV (511,000 eV). - Particle collider energies commonly span GeV to TeV scales.
See also the discussions of photon, band gap, and Fermi energy for how energy scales expressed in eV connect to broader physical concepts.
Uses in science and technology
The electronvolt appears across multiple disciplines and practical domains:
In chemistry and atomic physics, it is a natural unit for describing ionization energies, excitation energies, and reaction energetics. Spectroscopic data are routinely reported in eV to facilitate comparison with electronic energy levels.
In solid-state physics and materials science, eV is the standard unit for band structure, defect levels, and work functions. Device engineers commonly quote operating thresholds and energy barriers in eV to relate electronic behavior to material properties.
In particle physics and cosmology, large-energy processes are described using MeV, GeV, or TeV scales, but eV remains a reference point for low-energy phenomena, background radiation, and certain astrophysical spectra.
In metrology and standardization, precise definitions of energy units influence the interpretation of experiments, calibration procedures, and the consistency of data across laboratories. The exact conversion to joules ensures that energy measurements retain coherence with the SI system.
In radiology and medical imaging, electronvolt-sized energy scales appear in the context of photon-tunings, detectors, and the interpretation of energy deposition in tissue.
See also Joule and High-energy physics for broader connections to energy measurement and experimentation, and Solid-state physics for applications in materials.
Historical development
The practical adoption of the electronvolt grew with advances in quantum theory and the ability to quantify electronic energies in atoms, molecules, and solids. Early quantum chemistry and spectroscopy benefited from expressing energies in eV as a way to align theoretical predictions with experimental observables. The exact numerical definition of the electronvolt as e × 1 volt—using the exact value of the elementary charge—was solidified within the broader evolution of the SI system, which standardizes unit definitions to ensure consistency across disciplines.
Over time, the electronvolt gained prominence as a unit that directly reflects electrostatic processes at the particle level, rather than requiring repeated conversions from joules. This convenience helped unify discussions across chemistry, condensed matter, and high-energy physics, even as researchers continued to perform calculations in natural or SI units depending on the problem at hand.