Units & Prefixes (Edexcel GCSE Physics: Combined Science)

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SI Units

  • 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

  • These base units are then used to derive other common units
  • These units have special names, for example:
    • Newtons, [kg m / s2]
    • Joules, [N m]
    • Pascals, Pa [kg / m s2]

Common Units Table

Common Units Table, downloadable IGCSE & GCSE Physics revision notes

Examiner Tip

Sometimes marks in an exam question are given for the unit, so make sure you remember which is the correct one for the quantity in your answer eg. If the answer is a force, it must have the units of Newtons (N)

Prefixes

  • Physical quantities can span a huge range of values
  • For example, the diameter of an atom is about 10–10 m (0.0000000001 m), whereas the width of a galaxy may be about 1021 m (1,000,000,000,000,000,000,000 m)
    • This is a difference of 31 powers of ten

  • Powers of ten are numbers that can be achieved by multiplying 10 times itself
  • These come under two categories of units:
    • Multiples eg. 102, 103
    • Sub-multiples eg. 10-1, 10-2

  • Each power of ten is defined by a prefix, these are listed in the table below:

Prefixes Table

Prefixes Table, downloadable IGCSE & GCSE Physics revision notes

Example Conversions

  • 12 GPa = 12 gigapascals = 12 × 109 Pa (12 000 000 000 Pa)
  • 5 kN = 5 kilonewtons = 5 × 103 N (5000 N)
  • 0.1 μA = 0.1 microamps = 1 × 10–7 A (0.0000001 A)
  • 7 nC = 7 nanocoulombs = 7 × 10–9 C (0.000000007 C)

Examiner Tip

You will often see very large or very small numbers categorised by powers of ten, so it is very important you become familiar with these as getting these prefixes wrong is a very common exam mistake!

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.