1eV

1 eV, or one electronvolt, is a unit of energy equal to the amount of kinetic energy obtained by an electron when it is accelerated through an electric potential difference of one volt. It is defined exactly as 1.602176634 × 10⁻¹⁹ joules. The electronvolt is commonly used in particle physics, atomic physics, and chemistry, where energies are typically small compared to macroscopic scales but large compared to atomic and molecular energies.

In atomic and nuclear physics, the energy scale of electronvolts provides a convenient way to express ionization potentials, binding energies, and transition energies. For example, the ionization energy of hydrogen is 13.6 eV, and typical electronic transitions in visible light range from about 2 eV (ultraviolet) to about 4 eV (near infrared). In solid-state physics, the band gap of silicon is roughly 1.1 eV, and many semiconductor devices operate at energies on this order.

In high-energy physics, particle masses are often quoted in units of eV/c². The mass of the electron is 0.511 MeV/c², while masses of subatomic particles such as the muon, tau, and various mesons and baryons are expressed in GeV or TeV, still multiples of the eV scale. The rest mass of the proton is about 938 MeV/c².

Electronvolts are also used in astrophysics and cosmology. The energy of cosmic background radiation photons is about 0.0002 eV, while gamma-ray bursts produce photons in the keV to MeV range. Because the electronvolt neatly bridges microscopic and macroscopic energy scales, it remains a standard unit for expressing energies in many scientific disciplines.