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).

See also

• Orders of magnitude

External link

• BIPM's definition of the electronvolt (http://www1.bipm.org/en/si/si_brochure/chapter4/table7.html)
• Conversion Calculator for Units of ENERGY (http://www.ex.ac.uk/cimt/dictunit/ccenrgy.htm)
• http://physics.nist.gov/cuu/Constants physical constants reference; CODATA data