The electron volt (eV) is a unit of energy,
much smaller than a joule or erg,
defined to be the amount of energy necessary to move
an electron through an electric field
to a point where the field has an
electric potential one volt more negative.
Based upon the current (since 2019) SI definitions,
this is exactly 1.602176634 × 10-19 joules
(which is roughly 1.6 × 10-12 ergs).
The electron volt is commonly used in atomic-scale physics.
They are commonly used when citing the energy of individual photons:
EMRphoton energy is directly proportional to its
frequency, and is commonly cited when characterizing
very-high-frequency/short-wavelength EMR
such as X-rays and gamma rays.
Electron volts are also commonly used to cite energy differences
necessary for atomic excitation or ionization.
They are also commonly used for particlekinetic energy
(e.g., for cosmic rays),
and for particle rest mass (according to the mass/energy equivalence,
e=mc², i.e., 1.782662×10-36 kg).
The electron volt also serves as a unit for other quantities
that have a direct relation to energy,
for example, as a large unit of temperature often used for
plasma, corresponding to 1.160451812×104 K
(as per the equation e=kBT).