Friday, 9 November 2012

International System of Units

In 1960, an International committee agreed on a set of definitions and standard to describe the Physical Quantities.This System that was established is called the System International (SI).
There are to types of SI Unites which are give below by there name and symbols.

Table 1.  SI base units
SI base unit

Base quantityNameSymbol
masskilogram      kg
electric currentampereA
thermodynamic temperature      kelvinK
amount of substancemolemol
luminous intensitycandelacd


Table 2.  Examples of SI derived units

SI derived unit

Derived quantityNameSymbol
areasquare meterm2
volumecubic meterm3
speed, velocitymeter per secondm/s
accelerationmeter per second squared  m/s2
wave numberreciprocal meterm-1
mass densitykilogram per cubic meterkg/m3
specific volumecubic meter per kilogramm3/kg
current densityampere per square meterA/m2
magnetic field strength  ampere per meterA/m
amount-of-substance concentrationmole per cubic metermol/m3
luminancecandela per square metercd/m2
mass fractionkilogram per kilogram, which may be represented by the number 1kg/kg = 1

Table 3.  SI derived units with special names and symbols

SI derived unit

Derived quantityNameSymbol  Expression
in terms of
other SI units
in terms of
SI base units
plane angleradian (a)rad  -m·m-1 = 1 (b)
solid anglesteradian (a)sr (c)  -m2·m-2 = 1 (b)
frequencyhertzHz  -s-1
forcenewtonN  -m·kg·s-2
pressure, stresspascalPaN/m2m-1·kg·s-2
energy, work, quantity of heat  jouleJN·mm2·kg·s-2
power, radiant fluxwattWJ/sm2·kg·s-3
electric charge, quantity of electricitycoulombC  -s·A
electric potential difference,
electromotive force
electric resistanceohmOmegaV/Am2·kg·s-3·A-2
electric conductancesiemensSA/Vm-2·kg-1·s3·A2
magnetic fluxweberWbV·sm2·kg·s-2·A-1
magnetic flux densityteslaTWb/m2kg·s-2·A-1
Celsius temperaturedegree Celsius°C  -K
luminous fluxlumenlmcd·sr (c)m2·m-2·cd = cd
illuminanceluxlxlm/m2m2·m-4·cd = m-2·cd
activity (of a radionuclide)becquerelBq  -s-1
absorbed dose, specific energy (imparted), kermagrayGyJ/kgm2·s-2
dose equivalent (d)sievertSvJ/kgm2·s-2
catalytic activitykatalkats-1·mol

Table 4.  Examples of SI derived units whose names and symbols include SI derived units with special names and symbols
SI derived unit

Derived quantityNameSymbol
dynamic viscositypascal secondPa·s
moment of forcenewton meterN·m
surface tensionnewton per meterN/m
angular velocityradian per secondrad/s
angular accelerationradian per second squaredrad/s2
heat flux density, irradiancewatt per square meterW/m2
heat capacity, entropyjoule per kelvinJ/K
specific heat capacity, specific entropyjoule per kilogram kelvinJ/(kg·K)
specific energyjoule per kilogramJ/kg
thermal conductivitywatt per meter kelvinW/(m·K)
energy densityjoule per cubic meterJ/m3
electric field strengthvolt per meterV/m
electric charge densitycoulomb per cubic meterC/m3
electric flux densitycoulomb per square meterC/m2
permittivityfarad per meterF/m
permeabilityhenry per meterH/m
molar energyjoule per moleJ/mol
molar entropy, molar heat capacityjoule per mole kelvinJ/(mol·K)
exposure (x and gamma rays)coulomb per kilogramC/kg
absorbed dose rategray per secondGy/s
radiant intensitywatt per steradianW/sr
radiancewatt per square meter steradianW/(m2·sr)
catalytic (activity) concentrationkatal per cubic meterkat/m3

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