CIE A Level Physics (9702) 2019-2021

Revision Notes

1.1.2 SI Units

SI Base Quantities

  • There are a seemingly endless number of units in Physics
  • These can all be reduced to six base units from which every other unit can be derived
  • These seven units are referred to as the SI Base Units; this is the only system of measurement that is officially used in almost every country around the world

SI Base Quantities Table

SI Base Quantities Table, downloadable AS & A Level Physics revision notes

Exam Tip

You will only be required to use the first five SI base units in this course, so make sure you know them!

Derived Units

  • Derived units are derived from the seven SI Base units
  • The base units of physical quantities such as:
    • Newtons, N
    • Joules, J
    • Pascals, Pa, can be deduced
  • To deduce the base units, it is necessary to use the definition of the quantity
  • The Newton (N), the unit of force, is defined by the equation:
    • Force = mass × acceleration
    • N = kg × m s–2 = kg m s–2
    • Therefore, the Newton (N) in SI base units is kg m s–2
  • The Joule (J), the unit of energy, is defined by the equation:
    • Energy = ½ × mass × velocity2
    • J = kg × (m s–1)2 = kg m2 s–2
    • Therefore, the Joule (J) in SI base units is kg m2 s–2
  • The Pascal (Pa), the unit of pressure, is defined by the equation:
    • Pressure = force ÷ area
    • Pa = N ÷ m2 = (kg m s–2) ÷ m2 = kg m–1 s–2
    • Therefore, the Pascal (Pa) in SI base units is kg m–1 s–2

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