Electronvolt

The electron-volt or electron volt, symbol eV, is used to measure energy. It is defined as the amount of energy an electron gains after being accelerated by 1 volt of electricity. Joules are used often for energy measurement, but it is sometimes useful to use electron-volts for very small amounts of energy, such as that carried by a single subatomic particle. To convert joules into electron-volts, divide joules by the charge of an electron, which is about 1.602 coulomb.
E.g. 5 J = 5/1.602 5 J = 3.121 eV.
The electronvolt is often used in atomic, nuclear and particle physics. For example, the energy of photons can be measured by the voltage necessary to overcome their photoelectric effect.