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Electron Volts

We can also take advantage of the fact that Nature supplies electric charges in integer multiples of a fixed quantity of charge^17.11 to define some more "natural" units. For instance, the electric charge of an electron is -e [where e is the charge of a proton, defined in Eq. (2)]. An ELECTRON VOLT (eV) is the kinetic energy gained by an electron [or any other particle with the same size charge] when it is accelerated through a one volt (1 V) electric potential. Moving a charge of 1 C through a potential of 1 V takes 1 J of work (and will produce 1 J of kinetic energy), so we know immediately from Eq. (2) that

$$1 \; \hbox{\rm eV} \; = \; 1.60217733(49) \times 10^{-19} \; \hbox{\rm J}$$ (17.17)

This is not much energy if you are a toaster, but for an electron (which is an incredibly tiny particle) it is enough to get it up to a velocity of 419.3828 km/s, which is 0.14% of the speed of light! Another way of looking at it is to recall that we can express temperature in energy units using Boltzmann's constant as a conversion factor. You can easily show for yourself that 1 eV is equivalent to a temperature of 11,604 degrees Kelvin or about 11,331$^\circ$C. So in the microscopic world of electrons the eV is a pretty convenient (or "natural") unit. But not in the world of toasters and light bulbs. So let's get back to "conventional" units.