Unit Conversions for Energy & Mass (Edexcel International A Level (IAL) Physics): Revision Note

Unit Conversions for Energy & Mass

Units of Energy

• The electronvolt is a unit of energy

• It is equivalent to the amount of energy transferred to an electron accelerated across a potential difference of 1 V:

1 eV = 1.6 × 10^–19 J

• In order to convert between electronvolts and joules:

  • Multiply electronvolts by 1.6 × 10^–19 to get the equivalent energy in joules

  • Divide joules by 1.6 × 10^–19 to get the equivalent energy in electronvolts

Converting between electronvolts and joules

• Sometimes, units of MeV or GeV are used

  • These are given by: 

1 MeV = 1 × 10⁶ eV = 1.6 × 10^–13 J

1 GeV = 1 × 10⁹ eV = 1.6 × 10^–10 J

Units of Mass

• Energy and mass are related by Einstein's energy-mass relation

$\Delta E=c^2\Delta m$

$\Delta m=\frac{\Delta E}{c^2}$

• Therefore, units of mass can be related to units of energy by division of c²

  • This provides particle physicists convenient units of calculation to work with

  • This is especially useful in experiments involving particle collisions, where annihilation and creation is common

• Possible units of mass are therefore:

$\frac{MeV}{c^2}$, or, $\frac{GeV}{c^2}$

• The following conversions are used to convert into S.I. units: 

$1\frac{MeV}{c^2}$= 1.78 × 10^–30 kg

$1\frac{GeV}{c^2}$= 1.78 × 10^–27 kg