# SI Units and Derived Units

1 SI Units and Derived Units

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The International System of Units (SI, abbreviated from the French Système international ) is the modern form of the metric system, and is the most widely used system of measurement.

• A system of physical units ( SI units ) based on the metre, kilogram, second, ampere, kelvin, candela, and mole, together with a set of prefixes to indicate multiplication or division by a power of ten.
• It comprises a coherent system of units of measurement built on seven base units (ampere, kelvin, second, metre, kilogram, candela, mole) and a set of twenty decimal prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units.
• The system also specifies names for 22 derived units for other common physical quantities like lumen, watt, etc.
• The SI base units are the building blocks of the system and all the other units are derived from them. When Maxwell first introduced the concept of a coherent system, he identified three quantities that could be used as base units: mass, length and time.
Unit (name)Unit (symbol)Quantity (name)Definition
metremLengthThe distance travelled by light in vacuum in  1/299792458  second.
kilogramkgMassThe mass of a small squat cylinder of ~47 cubic centimetres of platinum-iridium alloy kept in a laboratory in France
secondsTimeThe duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.
ampereAElectric currenta unit of electric current equal to a flow of one coulomb per second.
kelvinKThermodynamic Temperature1/273.16  of the thermodynamic temperature of the triple point of water
molemolAmount of SubstanceThe amount of substance of a system which contains as many elementary entities [n 4]  as there are atoms in 0.012 kilogram of carbon-12.
candelacdLuminous IntensityThe magnitude of an electromagnetic field , in a specified direction, that has a power level of 1/683 watt (1.46 x 10  -3  W) per steradian at a frequency of 540 terahertz (540 THz or 5.40 x 10  14  Hz ).

## SI Derived units

NameSymbolQuantity
hertzHzfrequency
newtonNforce, weight
pascalPapressure, stress
jouleJenergy, work, heat
coulombCelectric charge or quantity of electricity
voltVvoltage (electrical potential), emf
ohmΩresistance, impedance, reactance
siemensSelectrical conductance
weberWbmagnetic flux
teslaTmagnetic flux density
henryHinductance
degree Celsius°Ctemperature relative to 273.15 K
lumenlmluminous flux
luxlxilluminance
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