The electron volt (eV) is a unit of equal to approximately 1.6×10−19 joule (J). By definition, it is the amount of energy gained (or lost) by the charge of a single electron moved across an difference of one volt.
Historically, the electron volt was devised as a standard unit of measure through its usefulness in electrostatic particle accelerator sciences because a particle with charge q has an energy E = qV after passing through the potential V; if q is quoted in integer units of the elementary charge and the terminal bias in volts, one gets an energy in eV.
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