In quantum and particle physics, quantities such as energy and momentum are extremely small and so it is logical to have a standard unit of energies at this scale. It saves time to write 1eV, as opposed to 1.6*(10^-19)J in every sum as well as making quantities easier to compare. An electron volt is described as the energy gained by an electron when passed through a charge of 1 volt and holds the quantity of 1.6*(10^-19)J. Note that this is numerically equivalent to the charge of an electron as E = qV. Therefore, to convert eV to Joules, you simply multiply by this number. To convert Joules to eV, you divide the energy by this constant.