electron-volt, abbr. eV, unit of energy used in atomic and nuclear physics; 1 electron-volt is the energy transferred in moving a unit charge, positive or negative and equal to that charge on the electron, through a potential difference of 1 volt. The maximum energy of a particle accelerator is usually expressed in multiples of the electron-volt, such as million electron-volts (MeV) or billion electron-volts (GeV). Because mass is a form of energy (see relativity), the masses of elementary particles are sometimes expressed in electron-volts; e.g., the mass of the electron, the lightest particle with measurable rest mass, is 0.51 MeV/c2, where c is the speed of light.

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