electron volt
Energyexact by definition
The electronvolt is the kinetic energy gained by an electron accelerated through a potential difference of one volt, exactly 1.602176634 × 10⁻¹⁹ joules. Since the 2019 SI revision fixed the elementary charge at exactly 1.602176634 × 10⁻¹⁹ coulombs, this conversion is now exact by definition. Before 2019 it was a measured quantity carrying an experimental uncertainty.
| 1 eV | 1.6021766e-19 J |
The unit came out of early twentieth century atomic and particle physics, where laboratory apparatus naturally accelerated charges through known voltages and the energy scale of atomic processes happened to land conveniently near one volt's worth.
About the electron volt
The electronvolt is a rare case where the natural laboratory operation defines the unit. Put an electron across a one-volt gap and it arrives with an energy of one electronvolt, by the definition of the volt as a joule per coulomb. The unit is therefore the product of the elementary charge and one volt, and since 2019 the elementary charge is exactly 1.602176634 × 10⁻¹⁹ C by definition of the ampere, so the electronvolt's value in joules is now exact. This is a real change: pre-2019 tables carry an uncertainty on the last digits that no longer exists.
Its scale is what makes it useful. Chemical bonds and visible photons sit at a few eV, which is why chemistry and optics are comfortable here: green light at 550 nm carries about 2.25 eV, and a silicon band gap is 1.12 eV. Nuclear and X-ray processes run in keV, particle physics in MeV, GeV and TeV. The Large Hadron Collider's beams reach several TeV per proton, which sounds enormous and is about the kinetic energy of a flying mosquito, concentrated into a single particle.
Particle physicists also use the electronvolt for mass, writing masses as eV/c² from \(E = mc^2\) and often dropping the \(c^2\) in speech. An electron is 511 keV/c², a proton 938 MeV/c². Converting to kilograms is possible but rarely useful, because every calculation in that field is in the same currency.