SI Units
SI Units.
Extracted from: Weisstein, Eric W. "Torque." Eric Weisstein's World of Physics.
"SI" stands for "System International" and is the set of physical units agreed upon by international convention. The SI units are sometimes also known as MKS units, where MKS stands for "meter, kilogram, and second." In 1939, the CCE recommended the adoption of a system of units based on the meter, kilogram, second, and ampere. This proposal was approved by the Comité International des Poids et Mesures (CIPM) in 1946. Following an international inquiry by the Bureau International des Poids et Mesures (BIPM), which began in 1948, in 1954 the 10th Conférence Générale des Poids et Mesures (CGPM) approved the introduction of the ampere, kelvin, and candela as base units for electric current, thermodynamic temperature, and luminous intensity, respectively. However, the ampere is scheduled to be phased out as a base unit in the near future in favour of the ohm, which can be measured extremely accurately using the quantum Hall effect. In turn, the volt will probably replace the ohm further in the future when measurements using Josephson junctions increase in precision.
The name International System of Units (SI) was given to the system by the 11th CGPM in 1960. At the 14th CGPM in 1971, the current version of the SI was completed by adding the mole as base unit for amount of substance, bringing the total number of base units to seven. The seven fundamental units are summarized in the following table.
|physical quantity |symbol |unit abbreviation |unit name |
|length |l |m |meter |
|mass |m |kg |kilogram |
|time |t |s |second |
|current |I |A |Ampere |
|temperature |T |K |Kelvin |
|luminous intensity | |cd |candela |
|amount of substance |n |mol |mole |
The derived SI units consist of combinations of the seven base units, and are summarized in the following table.
|quantity |symbol |SI symbol |SI unit |
|area |A |[pic] |square meter |
|volume |V |[pic] |cubic meter |
|plane angle |[pic] |rad |radian |
|solid angle |[pic] |sterrad |steradian |
|frequency |f |Hz |Hertz |
|velocity |v |[pic] |meters per second |
|acceleration |a |[pic] |meters per second squared |
|force |F |N |Newton |
|pressure |P or p |Pa |Pascal |
|power |P |W |Watt |
|energy |E |J |Joule |
|voltage |V |V |Volt |
|resistance |R |[pic] |Ohm |
|conductance |G |S |Siemens |
|charge |Q |C |Coulomb |
|capacitance |C |F |Farad |
|magnetic flux |[pic] |Wb |Weber |
|magnetic flux density |B |T |Tesla |
|inductance |L |H |Henry |
|luminous flux |F |lm |lumen |
|illumination |E |lx |lux |
|activity |A |Bq |Becquerel |
|energy dose | |Gy |Gray |
|equivalent dose | |Sv |Sievert |
In 1960, the 11th CGPM adopted a first series of prefixes and symbols of prefixes to form the names and symbols of decimal multiples and submultiples of SI units. Over the years, the list has been extended as summarized in the following table.
|factor |prefix |symbol |
|1024 |yotta- |Y |
|1021 |zetta- |Z |
|1018 |exa- |E |
|1015 |peta- |P |
|1012 |tera- |T |
|109 |giga- |G |
|106 |mega- |M |
|103 |kilo- |k |
|102 |hecto- |h |
|101 |deca- |da |
|[pic] |deci- |d |
|[pic] |centi- |c |
|[pic] |milli- |m |
|[pic] |micro- |[pic] |
|[pic] |nano- |n |
|[pic] |pico- |p |
|[pic] |femto- |f |
|[pic] |atto- |a |
|[pic] |zepto- |z |
|[pic] |yocto- |y |
REFERENCES.
ASTM. Standard for Metric Practice. Publication E380-79. ASTM, 1979. Superseded by IEEE/ASTM SI10-1997.
Bureau International des Poids et Mesures. "The International System of Units (SI)." .
Bureau International des Poids et Mesures. "The International System of Units (SI), 7th ed." 1998. .
Bureau International des Poids et Mesures. "SI Prefixes." .
IEEE/ASTM. Standard for the Use of the International System of Units (SI): The Modern Metric System. Publication SI10-1997 (STD 268). 1997.
Jackson, J. D. "Conversion of Equations and Amounts between SI Units and Gaussian Units." Appendix §4 in Classical Electrodynamics, 3rd ed. New York: Wiley, pp. 783-784, 1998.
National Institute of Standards and Technology. "International System of Units." .
Page, C. and Vigoureux, P. (Eds.). "The International Bureau of Weights and Measures 1875-1975." NBS Special Publication 420, 1975 Edition.
Taylor, B. N. "Guide for the Use of the International System of Units (SI)." NIST Special Publication 811, 1995 Edition. .
Taylor, B. N. (Ed.). "The International System of Units (SI)." NIST Special Publication 330, 1991 Edition. .
See also:
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