# 2.01 SI Units

As previously stated the actual property we are measuring (e.g. length, temperature etc) is called a quantity and in order to measure quantities we need to use clearly
defined units such as metres, kilograms etc.

The units we use for measurement in science are called SI units. These units are specified by the International Bureau of Weights and Measures who are based at Sevres in
France. The SI units of measurements for each measured quantity are listed in the following sections.

Note. You should not try to learn all of these units from the outset. The relevant units will be introduced as you progress through your studies.

## Objectives

Your objectives should be to
- Understand what is meant by the terms base unit and base quantity
- Understand the basic principles of how base units are defined using standards set by the International Bureau of Weights and Measures
- Be able to recall the first seven base quantities and their units
- Understand the difference between base units and quantities and derived units and quantities
- Understand that all derived units can be expressed in terms of their equivalent base units

## The seven base SI units

**Base unit** |
**Symbol** |
**Physical quantity** |

metre |
m |
Length |

second |
s |
Time |

kilogram |
kg |
Mass |

ampere |
A |
Electric current |

Kelvin |
K |
Temperature |

candela |
cd |
Luminous intensity |

mole |
mol |
Amount of substance |

The size of each of these base units is set by the experimental measurement of a physical quantity under specific conditions at which it always has the same
constant value. In addition to the quantities measured directly by each base unit, all other physical quantities can be measured using different combinations
of these base units.