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The Electron Volt
 The S.I. unit for energy is the Joule. 1J is the quantity of work done (energy converted) when a force of 1N moves 1m in the direction of the force. This is a convenient unit for measuring quantities of energy in everyday situations; energy from food, energy for lighting heating etc. However, for the quantities of energy possessed by atoms and sub-atomic particles, it is inconveniently large. The electron Volt is a unit of work (or energy) much smaller than the Joule. Definition of the Electron-Volt 1eV is the quantity of work done (energy converted) when 1 electron moves through a p.d. of 1V Now, 1V is equivalent to 1JC-1 which means that we can say that: 1J is the quantity of work done (energy converted) when 1C of charge moves through a p.d. of 1V Comparing these two statements leads us to conclude that the J is about 6×1018 times as big as the eV as this is the number of electrons needed to have 1C of charge. Alternatively, we can say that 1eV is equal to 1.6×10-19J as the charge on one electron is 1.6×10-19C So, if you want to convert an energy in Joules to a number of eV

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