Edexcel A Level Physics:复习笔记1.1 SI Units

SI Units

  • Everytime a quantity is measured or calculate, it must be quoted with its units
  • All units in Physics can be reduced to seven base units from which every other unit can be derived
    • These other quantities are called derived units
  • These seven units (of which you need to know six) 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


Derived Units

  • Derived units are derived from the SI Base units mathematically
  • The base units of physical quantities can be deduced, such as:
    • Newtons, N
    • Joules, J
    • Pascals, Pa


  • 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


Exam Tip

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