Electronvolt

The electronvolt (eV), also commonly spelled as electron volt (two words), is a unit of energy used in many branches of physics, especially in atomic, nuclear, and particle physics.

One electronvolt is the amount of potential energy that one electron in an electric potential field &Phi; loses (because an electron carries negative charge) when the field &Phi; is increased by one volt. The potential energy of a charge q in &Phi; is equal to  q&Phi;. The charge of an electron is &minus;e (minus the elementary charge), so that the energy of the electron in the field is &minus;e&Phi;. When &Delta;&Phi; = 1 volt, the potential energy loss is e, and it follows that in SI units, in which the energy unit joule is coulomb times volt ,
 * 1 eV = e coulomb &times; 1 volt = 1.602&thinsp;176&thinsp;487 &times; 10&minus;19 joule.

The loss in potential energy of a single unbound electron that passes  in vacuum through an electric potential difference of 1 volt is the gain in its kinetic energy (provided radiation damping is negligible).