SI base unit
From Wikipedia, the free encyclopedia.
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.
| SI Base units [{{localurl:Template:{ | ||||
|---|---|---|---|---|
| Name | Symbol | Quantity | Definition | |
| metre | m | Length | The unit of length is equal to the length of the path traveled by light in a vacuum during the time interval of 1/299 792 458 of a second (17th CGPM (1983) Resolution 1, CR 97). This number is exact; the metre is defined this way. | |
| 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 1/1 000 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 (but 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). | |
| 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 newton 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 | Amount of substance | The unit of amount of substance is the amount of substance which contains as many elementary entities 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). | |
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2 See also 3 External links |
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.
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