SI Units & Prefixes (SQA National 5 Physics): Revision Note

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

• These base units are then used to derive other common units

• These units have special names, for example:

  • Newtons, N [kg m s^-2]

  • Joules, J [N m]

  • Pascals, Pa [kg m s^-2]

Common units table

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)

  • 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

• Certain powers of ten are defined by a prefix, these are listed in the table below:

Prefixes Table

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)