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The SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which all other SI units can be derived.
- Second – Unit of Time. Symbol: s. The second is defined by taking the fixed numerical value of the cesium frequency ∆ν, the unperturbed ground-state hyperfine transition frequency of the cesium-133 atom, to be 9,192,631,770 when expressed in the unit Hz, which is equal to s.
- Meter – Unit of Length. Symbol: m. The meter is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299,792,458 when expressed in the unit m s, where the second is defined in terms of ∆ν.
- Kilogram – Unit of Mass. Symbol: kg. The kilogram is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015 ×10 when expressed in the unit J s, which is equal to kg m2 s, where the meter and the second are defined in terms of c and ∆ν.
- Ampere – Unit of Electric Current. Symbol: A. The ampere is defined by taking the fixed numerical value of the elementary charge e to be 1.602176634 × 10 when expressed in the unit C, which is equal to A s, where the second is defined in terms of ∆ν.
- Length: Meter (m) The meter is the metric unit of length. It's defined as the length of the path light travels in a vacuum during 1/299,792,458 of a second.
- Mass: Kilogram (kg) The kilogram is the metric unit of mass. It's the mass of the international prototype of the kilogram: a standard platinum/iridium 1 kg mass housed near Paris at the International Bureau of Weights and Measures (BIPM).
- Time: Second (s) The basic unit of time is the second. The second is defined as the duration of 9,192,631,770 oscillations of radiation corresponding to the transition between the two hyperfine levels of cesium-133.
- Electric current: Ampere (A) The basic unit of electric current is the ampere. The ampere is defined as the constant current that, if maintained in two infinitely long straight parallel conductors with a negligible circular cross-section and placed 1 m apart in a vacuum, would produce a force between the conductors equal to 2 x 10-7 newtons per meter of length.
The SI comprises a coherent system of units of measurement starting with seven base units, which are the second (symbol s, the unit of time), metre (m, length), kilogram (kg, mass), ampere (A, electric current), kelvin (K, thermodynamic temperature), mole (mol, amount of substance), and candela (cd, luminous intensity).
- The Seven Si Base Units
- Si Unit Definitions – Past and Present
- Derived Units
Here are the seven SI base units and their abbreviations: Note that the names of all of the SI base units are written in lowercase letters, even when they honor a person. The symbols, however, are lowercase letters except when they honor a person. So, the symbol for the kelvin is K because it takes its name from Lord Kelvin and the ampere has the s...
On May 20, 2019, the International Bureau of Weights and Measures (BIPM) officially re-defined the seven base units. The reason is so all of the base unit definitions come from a property of nature (which remain constant) and not some physical object (which could change over time). 1. Second: The second is based on an atomic clock. It is the durati...
There are 22 derived unitswith special names and around 100 other units. The derived units are multiples and combinations of the SI base units.
The seven SI base units are defined by products of powers of the SI defining constants. Prior to 2019, the concept of base units and derived units was used to define the SI. These categories, although no longer essential in the SI, are maintained in view of their convenience and widespread use.
Apr 12, 2010 · The SI is made up of 7 base units that define the 22 derived units with special names and symbols, which are illustrated in NIST SP 1247, SI Base Units Relationship Poster. The SI plays an essential role in international commerce and is commonly used in scientific and technological research and development.
