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 theSI 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 / s2]
- Joules,J[N m]
- Pascals,Pa[kg / m s2]
Common Units Table
Exam 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 ofNewtons(N)
Prefixes
- Physical quantities can span a huge range of values
- For example, the diameter of an atom is about 10–10m (0.0000000001 m), whereas the width of a galaxy may be about 1021m (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:
- Multipleseg. 102, 103
- Sub-multipleseg. 10-1, 10-2
- Each power of ten is defined by aprefix, these are listed in the table below:
Prefixes Table
Example Conversions
- 12GPa = 12gigapascals = 12 × 109Pa (12 000 000 000 Pa)
- 5kN = 5kilonewtons = 5 × 103N (5000)
- 0.1μA = 0.1microamps = 1 × 10–7A (0.0000001 A)
- 7nC = 7nano库仑= 7×10–9C (0.000000007 C)
Exam 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!