Edexcel A Level Physics:复习笔记8.9 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

• 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