SI base unit
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The SI system of units defines seven SI base units: fundamental physical units defined by an operational definition.
All other physical units can be derived from these base units: these are known as SI derived units. Derivation is by dimensional analysis. Use SI prefixes to abbreviate long numbers.
The following are the fundamental units from which all others are derived, they are dimensionally independent. The definitions stated below are widely accepted.
| SI base units | ||||
|---|---|---|---|---|
| Name | Symbol | Measure | Definition | |
| kilogram | kg | Mass | The unit of mass is equal to the mass of the international prototype kilogram (a platinum-iridium cylinder) kept at the Bureau International des Poids et Mesures (BIPM), Sèvres, Paris (1st CGPM (1889), CR 34-38). Note that the kilogram is the only base unit with a prefix; the gram is defined as a derived unit, equal to 1/1000 of a kilogram; prefixes such as mega are applied to the gram, not the kg; e.g. Gg, not Mkg. It is also the only unit still defined by a physical prototype instead of a measurable natural phenomenon (see the kilogram article for an alternate definition). | |
| second | s | Time | The unit of time is the duration of exactly 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom at a temperature of 0 K (13th CGPM (1967-1968) Resolution 1, CR 103). | |
| metre or meter | m | Length | The unit of length is equal to the length of the path travelled by light in a vacuum during the time interval of 1/299 792 458 of a second (17th CGPM (1983) Resolution 1, CR 97). | |
| ampere | A | Electrical current | The unit of electrical current is the constant current which, if maintained in two straight parallel conductors, of infinite length and negligible cross-section, placed 1 metre apart in a vacuum, would produce a force between these conductors equal to 2×10 −7 newtons per metre of length (9th CGPM (1948) Resolution 7, CR 70). | |
| kelvin | K | Thermodynamic temperature | The unit of thermodynamic temperature (or absolute temperature) is the fraction 1/273.16 (exactly) of the thermodynamic temperature at the triple point of water (13th CGPM (1967) Resolution 4, CR 104). | |
| mole | mol | Avogadro's number | The unit of Avogadro's number is the same number particles as there are atoms in 0.012 kilograms of pure carbon-12 (14th CGPM (1971) Resolution 3, CR 78). (Elementary entities may be atoms, molecules, ions, electrons, or particles.) It is approximately equal to 6.02214199×1023 units. | |
| candela | cd | Luminous intensity | The unit of luminous intensity is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian (16th CGPM (1979) Resolution 3, CR 100). | |
No circular references
Note that there are two base units above which do not appear to be defined purely in terms of other base units.
- The ampere is defined in terms of newtons. However, one newton is 1 kg · m · s-2.
- The candela is defined in terms of hertz, watt and steradians. One hertz is 1 s-1, one watt is 1 J · s-1 = 1 kg · m2 · s-3 and the steradian is the dimensionless solid angle subtended at the centre of a sphere of radius r by a portion of the surface of the sphere having an area r2.
Therefore there are no circular references in the definition of the base units.
