SI Units (OCR GCSE Physics A (Gateway))

Revision Note

Ashika

Written by: Ashika

Reviewed by: Caroline Carroll

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

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 Tips and Tricks

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, e.g.. if the answer is a force, it must have the units of Newtons (N).

Prefixes are very commonly forgotten in exams too, especially on the axes of graphs! Always double check the units given and whether there are any prefixes, e.g., if points are plotted in units of kN, then every value is multiplied by a factor of 1000!

Last updated:

You've read 0 of your 5 free revision notes this week

Sign up now. It’s free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

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.

Caroline Carroll

Author: Caroline Carroll

Expertise: Physics Subject Lead

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.