System of Units: MKS, CGS, FPS and SI-Units

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Units and MeasurementReference Notes – System of Units
Unit: Units and Measurement
Science | Physics Class 11

Different systems of units are used to perform measurement. The different systems of units are explained below:

  1. MKS-System: In this system, length is measured in Meter (M), mass is measured in Kilogram (K) and time is measured in Second (S).
  2. CGS-System: In this system, length is measured in centimeter (C), mass in gram (G) and time in second (S).
  3. FPS-System: In this system, length is measured in foot (F), mass in pound (P) and time in second (S).

SI-Units:
In this system, seven basic and two supplementary units are taken. The fundamental quantities adopted by the SI-units, their units and symbols are given below:

Units of Fundamental Quantities:

Fundamental QuantityUnitSymbol
LengthMeterM
MassKilogramKg
TimeSeconds
TemperatureKelvinK
CurrentampereA
Luminous intensitycandelacd
Amount of substancesmolemol

In addition to the above seven fundamental units in SI-units, two supplementary units are also defined. These supplementary units are given in the table below:

QuantityUnitSymbol
Plane Angleradianrad
Solid anglesteradiansr

Definitions of SI Base Units:

  1. Metre: The meter is defined as the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second.
  2. Second: The second is the duration of 9,162,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atoms.
  3. Kilogram: The kilogram is the mass equal to the mass of standard platinum-iridium alloy cylinder (90% platinum and 10% iridium) kept at the International Bureau of Weights and Measures in Paris. This cylinder is about 4 cm high and 4 cm in diameter.

Advantages of SI-Units:
The advantages of SI-Units are given below:

  1. It is a rational system of units.
  2. It is a coherent system of units.
  3. It is a decimal system.
  4. It is highly useful in practical and theoretical work.

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