Electronvolt

An electronvolt (symbol eV, or, rarely and incorrectly, ev) is the amount of kinetic energy gained by a single unbound electron when it passes through an electrostatic potential difference of one volt, in vacuum. The one-word spelling is the modern recommendation although the use of the earlier electron volt still exists.

One electronvolt 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 unit of energy, accepted (but not encouraged) for use with SI.

Using electronvolts to measure mass

Einstein reasoned 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 only strictly valid when working in natural units where c=1.)

For example, an electron and a positron, each with a mass of 0.511 MeV/c², can annihilate to yield 1.022 MeV of energy. The proton (which is a member of the baryon family of particles) has a mass of 0.938 GeV, making GeV a very convenient unit of mass for particle physics.

1 eV/c² = 5.902×10⁻³⁶ 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

In some older documents, one sometime encounters the symbol "BeV", which stands for "billion-electron-volt"; it is equivalent to the GeV (gigaelectronvolt).

Electronvolts and kinetic energy

For comparison:

• 3.2×10⁻¹¹ joule or 200 MeV - total energy released in nuclear fission of one U-235 atom (on average, it depends on the precise break up)
• 3.5×10⁻¹¹ joule or 210 MeV - total energy released in fission of one Pu-239 atom (on average, it depends on the precise break up)
• Molecular bond energies are on the order of an electronvolt per molecule.
• The typical atmospheric molecule has an energy of about 0.03 eV. This corresponds to room temperature.

Electronvolts and temperature

In certain fields, such as plasma physics, it is convenient to use the electronvolt as an unit of temperature. The conversion is defined using k, the Boltzmann constant.

${1{\mbox{ eV}} \over k}={1.6022\times 10^{-19}{\mbox{J}} \over 1.380650\times 10^{-23}{\mbox{J/K}}}=11605{\mbox{ K}}$

For example, a typical magnetic confinement fusion plasma is 15 keV, or 174 megakelvins.

See also

• Orders of magnitude (energy)

External links

• BIPM's definition of the electronvolt
• Conversion Calculator for Units of ENERGY
• http://physics.nist.gov/cuu/Constants physical constants reference; CODATA data