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SI units
Physics
Système International d’Unités: the international system of units now recommended for most scientific purposes. A coherent and rationalized system of units derived from the m.k.s. units, SI units have now replaced c.g.s. units and Imperial units for many purposes. The system has seven base units and two dimensionless units (formerly called supplementary units) from which all other units are derived. There are eighteen derived units with special names. Each unit has an agreed symbol (a capital letter or an initial capital letter if it is named after a scientist, otherwise the symbol consists of one or two lower-case letters). Decimal multiples of the units are indicated by a set of prefixes; whenever possible a prefix representing 10 raised to a power that is a multiple of three should be used. See Appendix 3 for a full list of SI units.
Mathematics
The units used for measuring physical quantities in the internationally agreed system Système International d’Unités are known as SI units. There are seven base units and other supplementary units and prefixes (such as kilo-) to create further units. For further details, see appendix 4.
Astronomy
Système International d'Unités, or International System of Units, the generally recognized set of units for scientific and technical measurement. The SI system is based on seven units: the metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), and candela (cd). Other units, such as metres per second for velocity, are derived from these base units.
Chemistry
Système International d’Unités: the international system of units now recommended for all scientific purposes. A coherent and rationalized system of units derived from the m.k.s. units, SI units have now replaced c.g.s. units and Imperial units. The system has seven base units and two dimensionless units (formerly called supplementary units), all other units being derived from these nine units. There are 18 derived units with special names. Each unit has an agreed symbol (a capital letter or an initial capital letter if it is named after a scientist, otherwise the symbol consists of one or two lower-case letters). Decimal multiples of the units are indicated by a set of prefixes; whenever possible a prefix representing 10 raised to a power that is a multiple of three should be used. See Appendix 3.
Chemical Engineering
The system of base units of the international metric system. The numbers used express the ratio of a measured quantity to some fixed standard for which the unit is the name or symbol for the standard. There are three classes of units: base units, derived units and supplementary units. The seven base units are metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), candela (Cd) and mole (mol). Derived units are formed by combining base units such as Newton (N), joule (j), pascal (Pa) and watt (W). Two supplementary units are the radian and steradian (sr), which are units for plane and solid angles, respectively. Prefixes are used for the basic SI unit with the exception of weight, where the prefix is used with the unit gram (g), not the basic SI unit kilogram (kg). Prefixes are also not used for units of angular measurement (degrees, radians), time (seconds) or temperature (oC or K). The prefixes are used in a way that the numerical value of a unit lies between 0.1 and 1000. For example, 56 kN rather than 5.6x104 N, 11.2 kPa rather than 11,200 Pa and 6.2 mm rather than 0.0062 m. Since May 2019, the kilogram, ampere, kelvin and mole have been defined by the exact value for the Planck constant, the elementary electric charge, the Boltzmann constant and the Avogadro number, respectively. http://physics.nist.gov/cuu/Units/history.htmlOfficial website of National Institute of Standards and Technology (NIST).
Computer
The system of units of measurement adopted internationally for scientific and technical use under the Système International d’Unités. There are seven fundamental units—the metre, second, kilogram, ampere, kelvin, candela, and mole—of seven dimensionally independent physical quantities. Other units are derived algebraically from these base units without the use of numerical factors, or are dimensionless (like the radian). The symbols for all units are standardized, as are the prefixes (and their symbols) that represent decimal multiples (e.g. nano-, micro-, milli-, kilo-, mega-, giga-) of the units.
Electronics and Electrical Engineering
The internationally agreed system of units intended for all scientific and technical purposes. The system is based on the MKS system and replaces the CGS and Imperial systems of units. Units in the SI system can be base units, derived units, or dimensionless units. The base units are an arbitrarily defined set of dimensionally independent physical quantities. In any purely mechanical system of units only three base units – of mass, length, and time – are required. In a consistent electric and magnetic system four base units are needed. In the SI system there are seven base units: the metre, kilogram, second, ampere, kelvin, candela (the unit of luminous intensity), and mole (the unit of amount of substance). Each base unit has a special symbol. The derived units are formed by the combination, by multiplication and/or division, of two or more base units without the use of numerical factors. The unit of speed, for example, is formed from the combination of one metre divided by one second; this can be expressed in terms of the symbols of the units: m s–1 or m/s. Some derived units have special names, such as coulomb, volt, hertz, or joule. The coulomb, which is the derived unit of charge, is formed from a combination of one ampere times one second (A s in symbol form). The named derived units have special symbols; for example, C is the symbol for coulomb. There are two named dimensionless units, the radian and steradian, which are the units of plane and solid angle, respectively. A set of fourteen prefixes, including micro- and kilo-, are used with the SI units to form decimal multiples and submultiples of the units. The symbol of a prefix can be combined with the symbol of the unit, as in mA (milliampere). SI units and SI prefixes, together with their symbols, are given in Tables 6–9, in the back matter. When considering electric and magnetic quantities a fourth term is required in addition to the fundamental units of mass, length, and time, for their complete definition. In the MKS system the fourth quantity is the permeability of free space, μ0, which is defined as 4π × 10−7 henry per metre. In the SI system it is the ampere that is the fundamental unit and μ0 then has the value 4π × 10–7 henry per metre. http://www.bipm.org/en/measurement-units/ The SI Brochure, from the Bureau International des Poids et Mesures (BIPM)
Biology
Système International d’Unités: the international system of units now recommended for all scientific purposes. A coherent and rationalized system of units derived from the m.k.s. units (a metric system based on the metre, kilogram, and second), SI units have superseded c.g.s. units and Imperial units. The system has seven base units and two dimensionless units (formerly called supplementary units), all other units being derived from these nine units. There are 22 derived units with special names. Each unit has an agreed symbol (a capital letter or an initial capital letter if it is named after a scientist, otherwise the symbol consists of one or two lower-case letters). Decimal multiples of the units are indicated by a set of prefixes; when possible a prefix representing 10 raised to a power that is a multiple of three should be used. See Appendix 1.
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