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

• 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:

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

Step 1: Write the known quantities

• • Rest mass of a proton,?mp?= 1.67 × 10–27?kg
  • Speed of light?c?= 3 × 108?m s–1

Step 2: Substitute quantities into Einstein's energy-mass relation

E?=?mpc2

E?= (1.67 × 10–27) × (3 × 108)2?= 1.50 × 10–10?J

Step 3: Convert joules to electronvolts

• • To convert a quantity of energy in joules to electronvolts, divide by 1.6 × 10–19

= 9.4 × 108?eV = 0.94 GeV

Step 4: Convert electronvolts to GeV/c2

• • 0.94 GeV is equivalent to a mass of 0.94 GeV/c2, which is roughly 1 GeV/c2