In physics, an electronvolt, also written as electron-volt and electron volt, is a unit of measurement equivalent to the amount of kinetic energy gained by a single electron accelerating through an electric potential difference of one volt in a vacuum. When used as a unit of energy, the numerical value of 1 eV expressed in unit of joules is equal to the numerical value of the charge of an electron in coulombs. Under the 2019 revision of the SI, this sets 1 eV equal to the exact value 1.602176634×10−19 J. Historically, the electronvolt was devised as a standard unit of measure through its usefulness in electrostatic particle accelerator sciences, because a particle with electric charge q gains an energy E = qV after passing through a voltage of V.