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
subatomic particles, it is inconveniently large. 



The electron Volt is a unit of work (or energy) much smaller
than the Joule. 



Definition of the ElectronVolt 

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 6×10^{18} 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^{19}J
as the charge on one electron is 1.6×10^{19}C 



So, if you want to convert an energy in Joules to a number of eV 


