electronvolt

The electronvolt (symbol eV) is a unit of energy used primarily in atomic, nuclear, and condensed matter physics. It is defined as the amount of work required to move a single elementary charge across an electric potential difference of one volt. Since the elementary charge is exactly 1.602176634×10^-19 coulombs, one electronvolt equals 1.602176634×10^-19 joules. The electronvolt is not an SI base unit, but it is accepted for use with the SI and is widely used for energies on atomic and subatomic scales.

In practice, the electronvolt is convenient for expressing energies of electrons, photons, and electronic transitions. For photons, energy is given by E = hν = hc/λ, and the relation E(eV) ≈ 1240/λ(nm) allows quick conversion between wavelength and energy. Typical visible photons have energies of a few electronvolts, while ultraviolet and X-ray photons have much higher energies.

Common energy scales are described using multiples of the electronvolt. Room-temperature thermal energy is about 0.0259 eV (kT at 300 K). Semiconductor band gaps are on the order of 1 eV, while kinetic energies of electrons in many solid-state contexts are several eV. In high-energy physics, energies are often expressed in keV, MeV, GeV, or TeV, with 1 keV = 10^3 eV, 1 MeV = 10^6 eV, 1 GeV = 10^9 eV, and 1 TeV = 10^12 eV.

The electronvolt remains a practical unit for relating microscopic energy scales to measurable quantities in experiments and theoretical models.