# SI units

A physical quantity is something that can be measured. For any measurement, the unit being used must be stated to give an understanding of the scale of the measurement.

For example, distance can be measured in kilometres or in miles. They are similar, but not the same and it is important to identify which was used for the measurement, to know how far the distance actually is.

## Système Internationale d'Unités

The units that scientists use all over the world are standardised in the Système Internationale d'Unités - . It is important to remember these six fundamental (or 'base') units of measurement:

• metre (m) - unit of length
• kilograms (kg) - unit of mass
• second (s) - unit of time
• ampere (A) - unit of electrical current
• kelvin (K) - unit of temperature
• mole (mol) - unit of the amount of substance

## Derived units

There are many quantities scientists measure that come from the base units. These derived units are very useful to quote as measurements, but they are not fundamental as they come from fundamental units.

For example, frequency is the number of times something happens per unit of time. This is a useful quantity, but it is a division into the time unit. The standard unit for frequency is considering the number 'per second', which is called 'hertz, Hz', but this comes from the fundamental unit 'second'.

Derived units include:

NameUnitAbbreviation
FrequencyhertzHz
ForcenewtonN
EnergyjouleJ
PowerwattW
PressurepascalPa
Electric chargecoulombC
Electric potential differencevoltV
Electric resistanceohmΩ
Magnetic flux densityteslaT