International System of Units
The International System of Units, or SI (from its French name Système international d'unités) is a system of measurement units. It comprises, base and derived units.
Seven base units form the core of the SI:
- the metre (m) for length;
- the kilogram (kg) for mass;
- the second (s) for time;
- the ampere (A) for electric current;
- the kelvin (K) for thermodynamic temperature;
- the mole (mol) for amount of substance;
- the candela (cd) for luminous intensity.
Derived units are obtained in terms of the base units according to a system of equations relating the corresponding quantities. The SI derived units are obtained from the SI base units according to the International System of Quantities, or ISQ. For example, the SI derived unit of velocity is metre per second (m/s), because the derived quantity velocity in the ISQ is defined as length divided by time.
22 SI derived units have been given special names and symbols, such as newton (N = kg · m/s2) as the unit for force, including:
- units for electricity: coulomb (C, for electric charge), volt (V, for electric potential and electric tension), farad (F, for capacitance), ohm (Ω, for resistance) and siemens (S, for conductance);
- units for magnetism: weber (W, for magnetic flux), tesla (T, for magnetic flux density), and henry (H, for inductance);
- units for light: lumen (lm, for luminous flux) and lux (lx, for illuminance).
In addition to the SI units, some additional units have also been adopted for international use, including:
- units of time: minute (min), hour (h), and day (d);
- metric units used in everyday life: litre (l, for volume) and tonne (metric ton, t, for mass).
In the SI decimal prefixes for multiples and submultiples, ranging from yotta (Y) 1024 to yocto (y) 10–24 are used.
IEC has also standardized prefixes for binary multiples (where kibi (Ki) means 210 = 1 024 instead of kilo (k) which means 103 = 1 000) in its International Standard IEC 60027-2, Letter symbols to be used in electrical technology — Part 2: Telecommunications and electronics.