The electron-volt (eV) is a unit of:

The electron-volt (eV) is a unit of energy commonly used in atomic, nuclear and particle physics. Although its name contains the word "volt", it is not a unit of potential difference.

One electron-volt is defined as the amount of kinetic energy gained (or lost) by a single electron when it moves through an electric potential difference of one volt in a vacuum. Since energy = charge × potential difference, and the charge of an electron is about 1.602 × 10⁻¹⁹ coulomb:

1 eV = 1.602 × 10⁻¹⁹ joule.

The joule is the SI unit of energy, but it is far too large for describing the tiny energies involved in atoms and subatomic particles. The electron-volt is much more convenient at that scale. Larger multiples are also commonly used:

1 keV (kilo-electron-volt) = 10³ eV, used for X-ray energies.

1 MeV (mega-electron-volt) = 10⁶ eV, used for nuclear reactions. For example, the fission of one uranium-235 nucleus releases about 200 MeV.

1 GeV (giga-electron-volt) = 10⁹ eV, and 1 TeV (tera-electron-volt) = 10¹² eV, used in particle physics. The Large Hadron Collider (LHC) at CERN, near Geneva, accelerates protons to energies of several TeV.

Examples of energies in electron-volts:

• Photons of visible light have energies of about 1.6 to 3.3 eV.
• The ionisation energy of a hydrogen atom is 13.6 eV.
• The rest-mass energy of an electron is about 0.511 MeV, and of a proton about 938 MeV.

Because of Einstein's equation E = mc², particle physicists also express mass in units such as MeV/c² or GeV/c². For example, the Higgs boson, discovered at CERN in 2012, has a mass of about 125 GeV/c².

Other energy units include the calorie (about 4.184 joules), the kilowatt-hour (3.6 × 10⁶ joules, the "unit" on electricity bills) and the erg (10⁻⁷ joule).

Why not the other options?

• ✗Electric potential difference is measured in volts.
• ✗Electric charge is measured in coulombs.
• ✗Electric current is measured in amperes.