On electronVolts

Very often I need to explain briefly what electronVolts are. Here’s the explanation from Chapter 2 of Smashing Physics

An eV is an “electron-Volt”, and a “Volt” is a measure of electric potential. So a standard domestic battery might provide a potential of twelve Volts. If you allow a single electron to fall through this potential (that is, be repelled from the negative terminal and attracted to the positive terminal of the battery, because electrons have negative charge and like charges repel, opposites attract) it will pick up some speed.

Things that are travelling at speed have energy because of that speed, which we call “kinetic energy”. The kinetic energy this electron will have gained due to the speed it acquires accelerating through the voltage provided by the battery will be 12 eV – twelve electron-Volts. That is how an eV is defined – the kinetic energy picked up by an electron as it falls through an electrical potential of one Volt.