SI Unit

physics

Definition

SI Unit

SI units are the units of the International System, the modern metric system of measurement. They are built from seven dimensionally independent base units, with all other derived units expressed as products of powers of these bases.

Formally, every SI unit $u$ is a dimension

$$[u]=\prod _{i=1}^7{b}_i^{{\alpha }_i},{\alpha }_i\in \mathbb{Q},$$

over the base units $b_i$. The seven bases are:

Base unit	Symbol	Quantity measured
second	$\mathrm{s}$	time
metre	$\mathrm{m}$	length
kilogram	$\mathrm{kg}$	mass
ampere	$\mathrm{A}$	electric current
kelvin	$\mathrm{K}$	thermodynamic temperature
mole	$\mathrm{mol}$	amount of substance
candela	$\mathrm{cd}$	luminous intensity

Examples of derived units include the joule of energy ($\mathrm{J}=\mathrm{kg}{\mathrm{m}}^2{\mathrm{s}}^{-2}$), the newton of force ($\mathrm{N}=\mathrm{kg}\mathrm{m}{\mathrm{s}}^{-2}$), and the watt of power ($\mathrm{W}=\mathrm{kg}{\mathrm{m}}^2{\mathrm{s}}^{-3}$).