Difference between revisions of "International System of Units"

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== {{PAGENAME}} ==
== {{PAGENAME}} ==


The international system of units abbreviated as SI has been universally accepted for international use in all fields of engineering and day to day requirements<ref>Basic Electrical Engineering 4th Ed., C. L. Wadhwa, New Age International (P) Ltd., Publishers</ref>. Therefore, all business and even household transactions are conducted in SI units. SI system offers the following advantages over other system of units.
The international system of units (SI) are divided into three classes<ref>Basic Electrical and Electronics Engineering, R.K. Rajput, University Science Press</ref>:
# Base units
# Derived units
# Supplementary units.
 
The international system of units abbreviated as SI has been universally accepted for international use in all fields of engineering and day-to-day requirements<ref>Basic Electrical Engineering 4th Ed., C. L. Wadhwa, New Age International (P) Ltd., Publishers</ref>. Therefore, all business and even household transactions are conducted in SI units. SI system offers the following advantages over another system of units.


# There is one and only one unit for each physical quantity. Therefore, a table of conversions from one unit to another is not required.
# There is one and only one unit for each physical quantity. Therefore, a table of conversions from one unit to another is not required.
# The system is coherent with the derived units. The conversion factor from the original unit to the derived unit is simply a multiplication or division by 1.
# The system is coherent with the derived units. The conversion factor from the original unit to the derived unit is simply a multiplication or division by 1.
# There are no conversions between electrical and mechanical systems e.g. a motor or an automobile engine is now rated as in kW rather than horse power. Energy is now expressed in watt-sec rather than in Joule etc.
# There are no conversions between electrical and mechanical systems e.g. a motor or an automobile engine is now rated as in kW rather than horsepower. Energy is now expressed in watt-sec rather than in Joule etc.


There are a large number of quantities (more than thirty) which an electrical engineer deals in. However it is not necessary to assign a standard unit to each quantity as these quantities are functionally related through experiments, mathematical derivation or definitions. The minimum number of quantities required to express the units of all other quantities are known as fundamental quantities. The following are the considerations for selection of fundamental quantities.
There are a large number of quantities (more than thirty) which an electrical engineer deals in. However, it is not necessary to assign a standard unit to each quantity as these quantities are functionally related through experiments, mathematical derivation, or definitions. The minimum number of quantities required to express the units of all other quantities is known as fundamental quantities. The following are the considerations for the selection of fundamental quantities.


<ol style="list-style-type:lower-roman">
<ol style="list-style-type:lower-roman">
<li>A minimum number of constant should be required to establish relationship between the various quantities involved in the study of the given discipline.</li>
<li>A minimum number of constants should be required to establish a relationship between the various quantities involved in the study of the given discipline.</li>
<li>The measuring units shall he of a practical size. There are seven fundamental units which are listed below with their name, quantity symbol and unit symbol.</li>
<li>The measuring units shall be of a practical size. There are seven fundamental units which are listed below with their name, quantity symbol, and unit symbol.</li>
</ol>
</ol>


 
== Unit Definitions ==
; Length-metre, 1,  m
; Length-metre, 1,  m
: It is defined in terms of wavelengths of a particular radiation from krypton 86.
: It is defined in terms of wavelengths of a particular radiation from krypton 86.
Line 26: Line 31:
; Current-ampere,  I, A
; Current-ampere,  I, A
: It is defined as the constant electric current in two infinite parallel conductors separated from each other by 1 m, produce a force of  2 x  10<sup>-9</sup>  N/m.
: It is defined as the constant electric current in two infinite parallel conductors separated from each other by 1 m, produce a force of  2 x  10<sup>-9</sup>  N/m.
== Base Units ==
{| class="wikitable table60"
|+ SI Base Units
! Quantity
! Name
! Symbol
|-
|length
|metre
|m
|-
|mass
|kilogram
|kg
|-
|time
|second
|s
|-
|electric current
|ampere
|A
|-
|thermodynamic temperature
|kelvin
|K
|-
|luminous intensity
|candela
|cd
|-
|amount of substance
|mole
|mol
|-
|}
== Derived Units ==
{| class="wikitable table60"
|+ SI Derived Units
! Name
! Unit
! Quantity Symbol
! Unit Symbol
! Other Units
|-
|Electric Capacitance
|farad
|C
|F
|C/V
|-
|Electric Conductance
|siemens
|G
|S
|A/V
|-
|Electric Inductance
|henry
|L
|H
|Wb/A
|-
|Electric Potential Difference
|volt
|V or E
|V
|W/A
|-
|Electric Resistance
|ohm
|R
|<math>\Omega</math>
|V/A
|-
|Energy
|joule
|W
|J
|N.m
|-
|Force
|newton
|F
|N
|kg.m/s<sup>2</sup>
|-
|Frequency
|hertz
|f
|Hz
|s<sup>-1</sup>
|-
|}
== Supplementary Units ==
The SI units assigned to a third class called “Supplementary units” may be regarded either as base units or as derived units.
{| class="wikitable table60"
|+ SI Supplementary Units
! Quantity
! Name
! Symbol
|-
|plane angle
|radian
|rad
|-
|solid angle
|steradian
|sr
|-
|}


== References ==
== References ==
<references />
<references />

Latest revision as of 07:07, 23 January 2022

International System of Units

The international system of units (SI) are divided into three classes[1]:

  1. Base units
  2. Derived units
  3. Supplementary units.

The international system of units abbreviated as SI has been universally accepted for international use in all fields of engineering and day-to-day requirements[2]. Therefore, all business and even household transactions are conducted in SI units. SI system offers the following advantages over another system of units.

  1. There is one and only one unit for each physical quantity. Therefore, a table of conversions from one unit to another is not required.
  2. The system is coherent with the derived units. The conversion factor from the original unit to the derived unit is simply a multiplication or division by 1.
  3. There are no conversions between electrical and mechanical systems e.g. a motor or an automobile engine is now rated as in kW rather than horsepower. Energy is now expressed in watt-sec rather than in Joule etc.

There are a large number of quantities (more than thirty) which an electrical engineer deals in. However, it is not necessary to assign a standard unit to each quantity as these quantities are functionally related through experiments, mathematical derivation, or definitions. The minimum number of quantities required to express the units of all other quantities is known as fundamental quantities. The following are the considerations for the selection of fundamental quantities.

  1. A minimum number of constants should be required to establish a relationship between the various quantities involved in the study of the given discipline.
  2. The measuring units shall be of a practical size. There are seven fundamental units which are listed below with their name, quantity symbol, and unit symbol.

Unit Definitions

Length-metre, 1, m
It is defined in terms of wavelengths of a particular radiation from krypton 86.
Mass-kilogram, m kg
It is defined equal to the mass of the international prototype kept in Sevres, France.
Time-seconds, t, s
It is defined in terms of the duration of a specific number of periods of a particular radiation from the cesium-133 atom.
Current-ampere, I, A
It is defined as the constant electric current in two infinite parallel conductors separated from each other by 1 m, produce a force of 2 x 10-9 N/m.

Base Units

SI Base Units
Quantity Name Symbol
length metre m
mass kilogram kg
time second s
electric current ampere A
thermodynamic temperature kelvin K
luminous intensity candela cd
amount of substance mole mol

Derived Units

SI Derived Units
Name Unit Quantity Symbol Unit Symbol Other Units
Electric Capacitance farad C F C/V
Electric Conductance siemens G S A/V
Electric Inductance henry L H Wb/A
Electric Potential Difference volt V or E V W/A
Electric Resistance ohm R V/A
Energy joule W J N.m
Force newton F N kg.m/s2
Frequency hertz f Hz s-1

Supplementary Units

The SI units assigned to a third class called “Supplementary units” may be regarded either as base units or as derived units.

SI Supplementary Units
Quantity Name Symbol
plane angle radian rad
solid angle steradian sr

References

  1. ↑ Basic Electrical and Electronics Engineering, R.K. Rajput, University Science Press
  2. ↑ Basic Electrical Engineering 4th Ed., C. L. Wadhwa, New Age International (P) Ltd., Publishers
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