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International System of Units

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  • SI Base Quantities and Units
  • Systems of Units and SI Units

notes

The International System Units

Internationally accepted system of units is Système Internationale d’ Unites (French for International system of Units) or SI. It was developed and recommended by General Conference on Weights and Measures in 1971. In earlier times, scientists of different countries were using different systems of units for measurements.

Systems of Units:-
A system of units is the complete set of units, both fundamental and derived, for all kinds of physical quantities. The common system of units which is used in mechanics are given below:

  1. CGS System: In this system, the unit of length is centimetre, the unit of mass is gram and the unit of time is second.

  2. FPS System: In this system, the unit of length is foot, the unit of mass is pound and the unit of time is second.

  3. MKS System: In this system, the unit of length is metre, the unit of mass is kilogram and the unit of time is second.

  4. SI System: This system contain seven fundamental units and two supplementary fundamental units.

Base Quantity Name Symbol
Length metre m
Mass kilogram kg
Time second s
Electric Current ampere A
Thermodynamic Temperature kelvin K
Amount of Substance mole mol
Luminous intensity candela cd


Supplementary Fundamental Units
Radian and steradian are two supplementary fundamental units. It measures plane angle and solid angle respectively.

Some units retained for general use

Name Symbol Value in SI Unit
minute min 60 s
hour h 60 min = 3600 s
day d 24 h = 86400 s
year y 365.25 d = 3.156 × 107
degree o 1o = (`pi`/180) rad
litre L 1 dm3 = 10-3 m3
tonne t 103 kg
carat e 200mg
bar bar 0.1 MPa = 105 Pa
curie Ci 3.7 × 1010 s-1
roentgan R 2.58 × 10-4 C/kg
quintal q 100kg
barn b 100 fm2 = 10-28 m2
are a 1 dam2 = 102 m2
hectare ha 1 hm2 = 104 m2
standard atmospheric temperature atm 101325 Pa = 1.013  105 Pa

 

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