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The systems formalised the concept of a collection of related units called a coherent system of units. In a coherent system, base units combine to define derived units without extra factors. [4]: 2 For example, using metre per second is coherent in a system that uses metre for length and second for time, but kilometre per hour is not coherent.
The current international standard for the metric system is the International System of Units (Système international d'unités or SI). It is a system in which all units can be expressed in terms of seven units. The units that serve as the SI base units are the metre, kilogram, second, ampere, kelvin, mole, and candela.
A unit of time is any particular time interval, used as a standard way of measuring or expressing duration. The base unit of time in the International System of Units (SI), and by extension most of the Western world, is the second, defined as about 9 billion oscillations of the caesium atom.
The SI system after 1983, but before the 2019 revision: Dependence of base unit definitions on other base units (for example, the metre is defined as the distance travelled by light in a specific fraction of a second), with the constants of nature and artefacts used to define them (such as the mass of the IPK for the kilogram).
Stored charge per unit electric potential farad (F = C/V) L −2 M −1 T 4 I 2: scalar Catalytic activity concentration: Change in reaction rate due to presence of a catalyst per unit volume of the system kat⋅m −3: L −3 T −1 N: intensive Chemical potential: μ: Energy per unit change in amount of substance J/mol L 2 M T −2 N −1 ...
SI derived units are units of measurement derived from the seven SI base units specified by the International System of Units (SI). They can be expressed as a product (or ratio) of one or more of the base units , possibly scaled by an appropriate power of exponentiation (see: Buckingham π theorem ).
The first group of metric units are those that are at present defined as units within the International System of Units (SI). In its most restrictive interpretation, this is what may be meant when the term metric unit is used. The unit one (1) is the unit of a quantity of dimension one. It is the neutral element of any system of units. [2]
[a] This system underlies the International System of Units (SI) [b] but does not itself determine the units of measurement used for the quantities. The system is formally described in a multi-part ISO standard ISO/IEC 80000 (which also defines many other quantities used in science and technology), first completed in 2009 and subsequently ...