1.1 use of SI units and their prefixes

• Fundamental SI Base Quantities

  • Endless number of units in physics
  • Can be reduced to six base units from which all other units can be derived
  • Referred to as SI Base Units
  • The only system of measurement officially used in the majority of countries

  
• Derived units

  • Derived units are derived from the seven SI Base Units
  • Newtons, N
  • Joules, J
  • Pascals, Pa
  • To deduce the base units, it is necessary to use the definition of the quantity
  • Newton, N - unit of force
  • Force = mass x acceleration
  • N = kg x ms-2     = kgms-2
  • Newtons in SI Base Units is kgms-2
  • Joules, J - unit of energy
  • Energy = 1/2 x mass x velocity2
  • J = kg (ms-1)2     = kgm2s-2
  • Joules in SI Base Units is kgm2s-2
  • Pascal, Pa - unit of pressure
  • Pressure = force x area
  • Pa = N/m2     = kgms-2 / m2     =kgm-1s-2
  • Pascals in SI Base Units is kgm-1s-2
• Joules to electronvolts

  • Derived from work done equation 
  • W = qV
  • 1eV = 1.6x10-19C x 1V   = 1.6x10-19J
  • J -> eV
  • J / 1.6x10-19 = eV
  • eV -> J
  • eV x 1.6x10-19 = J

  Joules to kilowatt-hours

  • Expand derived units and re-collect terms  
  • 1kWh = 3 600 kWs
  • 3 600 kWs = 3 600 000 Ws
  • 3 600 000 Ws = 3 600 00 J     3.6 MJ
  • J -> kWh
  • J / (3.6x10^6) = kWh
  • kWh -> J
  • kWh x (3.6x10^6) = J
• Prefixes

  • tera
  • giga
  • mega
  • kilo
  • centi
  • milli
  • micro
  • nano
  • pico