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=mc*^{2} (1 kg = 90 petajoules). It is thus common in particle physics, where mass and energy are often interchanged, to use eV/*c*² 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).
## See also ## External link |