All physical quantities consist of a (numerical) magnitude and a unit. A unit of measurement is a standardised quantity of a physical property, used as a factor to express occurring quantities of that property.
Base units
The system of units commonly used by the scientific community in Europe is known as SI, an abbreviation for Système Internationale d’Unités. SI units are a coherent system of units comprising seven base units on which all other units are derived. The seven base units are tabulated below:
| Physical quantity | Unit | Symbol |
| length | metre | m |
| mass | kilogram* | kg |
| time | second | s |
| electric current | ampere | A |
| thermodynamic temperature | kelvin | K |
| amount of substance | mole | mol |
| luminous intensity | candela | cd |
* Note the spelling of gram rather than gramme.
Derived units
The essential features of SI are:
- SI is a metric system. There are seven base units.
- The derived units are directly related to the base units. E.g. the unit of acceleration is 1 m s–2. The unit of force is the Newton, which is the force required to give a body of mass 1 kg an acceleration of 1 m s–2. The unit of energy is the joule, which is the work done when a force of 1 N moves a body a distance of 1 m. Derived units can therefore be expressed as products or quotients of the base units.
- Electrical units are rationalized and are obtained by assigning the value of 4
×10–7 H m–1 to
0, the permeability of a vacuum. This leads to the ampere – a base unit – as the unit of current.
- Multiples and fractions of units are normally restricted to powers of 1000. So the centimetre (cm) and decimetre (dm) are frowned on in conservative SI.
The name for a unit always starts with a lower case letter even though they may have been named after a person e.g. the kelvin is named after Lord Kelvin. The symbol for a unit starts with a capital letter only if it is named after a person e.g. A is the symbol for the ampere. The only exception is the symbol of the litre, L (since a lower case l can be easily confused with the number 1).
Decimal fractions and multiples
Multiples and fractions of units are commonly stated with the following prefixes:
| Fraction | Prefix | Symbol | Multiple | Prefix | Symbol |
| 10-3 | milli | m | 103 | kilo | k |
| 10-6 | micro | 106 | mega | M | |
| 10-9 | nano | n | 109 | giga | G |
| 10-12 | pico | p | 1012 | tera | T |
| 10-15 | femto | f | 1015 | peta | P |
| 10-18 | atto | a | 1018 | exa | E |
| 10-21 | zetto | z | 1021 | zetta | Z |
| 10-24 | yocto | y | 1024 | yotta | Y |
For example, 1 nanojoule = 1 nJ = 1 x 10-9 joule. The nanojoule is written as one whole word, and not as "nano joule".
Notes:
1 millisecond = 1 ms; 1 metre per second = 1 m s–1 i.e. when two or more unit symbols are combined to create a derived unit symbol, they should be separated by a space.
The symbol for the minute is min and NEVER mins, e.g. 5 min, 100 min.
Other units such as the tonne (103 kg) and the day (86 400 s) are acceptable for use within the SI unit system.
You will surely come across the MKS, CGS and MGS systems of units along your physics course. They simply stand for the metre-kilogram-second (MKS), centimetre-gram-second (CGS) and metre-gram-second (MGS) systems of units. For e.g., in the CGS system of units, length is measured in centimetres, mass in grams and time in seconds.
