If an electron is accelerated by a potential difference of 1 volt then its energy gain is given by:
 |
(1) |
In "normal" physics units this is 1.6x10-19 Joules. You could equally say though that the electron has gained 1eV of energy. An electronvolt (eV) is the amount of energy gained by an electron when it moves through a potential difference of 1 volt.
Now which unit is easier to keep using all day long???
Particle accelerators work with singly charged particles and big voltages. An accelerator with a beam energy of 10MeV is simply something that takes particles with one unit of charge and shifts them through a pd of 1MV ( 1 000 000 Volts)
10MeV is much easier to write and work with than 1.6x10-12 J
Particle physicists use large numbers much of the time (oftentimes in their budgets but not in their salaries!)
| k= x1000 or 103 M=x1 000 000 or 106 G=x1 000 000 000 or 109 etc! |
The Einstein mass energy equivalence equation is so fundamental to particle physics that it helps to have "weird" mass units too.
 |
(2) |
This rearranges to:
 |
(3) |
In normal physics units you would convert mass to energy by taking an energy in Joules and dividing by 9x1016 to get a mass in kg.
A particle physicist would take an energy in eV and simply convert to mass in eV/c2
So an electron has a rest mass of 0.51MeV/c2 , and yields 0.51MeV of energy if its mass is annihilated.
In sensible units you would need to do sums to convert between these quantities.
Electron rest mass = 0.51 x 1.6 x 10-13 / 9 x 1016 = 9.07 x 10-31 kg
If you measure momentum in units of eV/c
as well then you can eliminate lots of messing about with very small
numbers.
Summary
Energy: eV, MeV or even GeV. (e.g. an electron accelerated by 10MeV has energy of 10MeV.)
Mass: eV/c2, MeV/c2 or GeV/c2. (e.g. an electron accelerated by 10MeV could be converted to a particles of mass 10.0+0.51 MeV/c2 in a suitable reaction.)
Momentum: eV/c2, M eV/c2 or GeV/c2. (e.g. an electron accelerated by 10MeV has momentum of 10MeV/c.)
