SI Base Units

24 Jul 2010


The SI unit system consists of seven base units, with a number of other units derived from those foundations. Below are the base SI units, along with their precise definitions, showing why it took so long to define some of them.

* meter (m) - The base unit of length; determined by the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second.

* kilogram (kg) - The base unit of mass; equal to the mass of the international prototype of the kilogram (commissioned by the CGPM in 1889).

* second (s) - The base unit of time; duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state in the cesium 133 atom.

* ampere (A) - The base unit of electrical current; constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circuit cross-section, and placed 1 meter apart in vacuum, would produce between those conductors a force equal to 2 x 10-7 newtons per meter of length.

* Kelvin(degrees K) - The base unit of thermodynamic temperature; the fraction 1/273.16 of the thermodynamic temperature of the triple point of water (the triple point is the point in a phase diagram where three phases coexist in equilibrium).

* mole (mol) - The base unit of substance; the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilograms of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

* candela (cd) - The base unit of luminous intensity; the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

SI Derived Units
From these base units, many other units are derived. For example, the SI unit for velocity is m / s (meter per second), using the base unit of length and the base unit of time to determine the length traveled over a given period of time.

Listing all of the derived units here would be unrealistic, but in general when a term is defined, as in the Physics Glossary, the relevant SI units will be introduced along with them. If looking for a unit that isn’t defined, check out the National Institute of Standards & Technology’s SI Units page.


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