An electronvolt (symbol: eV) is the amount of energy gained by a single unbound electron when it falls through an electrostatic potential difference of one volt. This is a very small amount of energy:
- 1 eV = 1.602 176 53 (14) × 10−19 J. (Source: CODATA 2002 recommended values)
It is a non-SI unit of energy, accepted for use with SI.
Using electronvolts to measure mass
Einstein taught us that energy is equivalent to (rest) mass, as famously expressed in the formula E=mc2 (1 kg = 90 petajoules). It is thus common in particle physics, where mass and energy are often interchanged, to use eV/c² or even simply eV as a unit of mass. (The latter is often paired with natural units where c=1, but this is not strictly necessary.)
For example, an electron and a positron, each with a mass of 511 keV, can annihilate to yield 1.022 MeV of energy. The proton, a typical baryon, has a mass of 0.938 GeV, making GeV (often pronounced jev) a very convenient unit of mass for particle physics.
- 1 eV/c² = 1.783 × 10−36 kg
- 1 keV/c² = 1.783 × 10−33 kg
- 1 MeV/c² = 1.783 × 10−30 kg
- 1 GeV/c² = 1.783 × 10−27 kg
For comparison, charged particles in a nuclear explosion range from 0.3 to 3 MeV. The typical atmospheric molecule has an energy of about 0.03 eV.
To convert a particle's energy in electronvolts into its temperature in kelvins, multiply by 11,605 (see Boltzmann constant).