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SI Units Of Physics

What is SI unit?

SI unit is an abbreviation of International System of Units which is widely used as a measurement system. The reason why SI units were derived is to make it easier to calculate worldwide. Other reasons why SI units were developed are because of CGS (centimetre-gram-second) and MKS (metre-kilogram-second) systems were insufficient to provide measurements for electrostatic and electromagnetic units. But there are countries which do not follow SI unit system and they are Myanmar, United States and Liberia. SI units are set of base units, derived units and prefixes that are used in Physics.

 SI base units:

These are units which are used in science and technology for dimensional analysis which are independent of dimensions.

There are total seven base units and they are:

Name Symbol Measure Dimension symbol
Metre m Length L
Kilogram kg Mass M
Second s Time T
Ampere A Electric current I
Kelvin K Thermodynamic temperature Ө
Mole mol Amount of substance N
Candela cd Luminous intensity J

SI derived units:

These are units derived from seven base SI units that are either product of one or more base SI units or dimensionless.

There are a total of twenty-two derived SI units and few of them are given below:

Name Symbol Unit used for
Hertz Hz Unit of vibration
Ohm Unit of electrical resistance
Newton N Unit of force
Joule J Unit of work
Weber Wb Unit of magnetic flux

Prefixes

In order to produce multiples and submultiples of the original unit, prefixes are used in unit names. All these prefixes are integers of powers ten and above hundred or below hundredth are having powre of thousand.

Following is the table with few prefixes that are used in International System of Units:

Prefix Symbol Base 1000 Base 10  

English word

Adoption
Short scale Long scale
Yotta Y 10008 1024 Septillion Quadrillion 1991
Zetta Z 10007 1021 Sextillion Trilliard 1991
Hecto h 10002/3 102 Hundred 1795
Deca da 10001/2 101 Ten 1795
10000 100 One NA
Deci d 1000-1/3 10-1 Tenth 1795
Centi c 1000-2/3 10-2 Hundredth 1795
Zepto z 1000-7 10-21 Sextillionth Trilliardth 1991
Yocto y 1000-8 10-24 Septillionth Quadrillionth 1991

Non-SI units accepted for use with SI

There are many non-SI units that are used in commercial, scientific and technical literature. Following is the group of non-SI units:

  • Non-SI units accepted for use with the SI: There are units of time and angle like radian which are used in mathematics, but these are difficult to navigate.
  • Non-SI units obtained by experiments: Units like dalton or atomic mass unit are the units that are used on the basis of natural phenomenon that equivalent to SI unit.
  • Non-SI units associated with CGS and CGS-Gaussian system of units: Units like gauss, gal, stokes are the units used in classical and relativistic electrodynamics.

International System of quantities

International System of quantities (ISQ) are the equations and quantities that provide the context of SI units. In 2009, ISQ was defined in international standard ISO 80000-1. The basic seven base units are used to underlay the quantities of ISQ and quantities like area, electric resistance and pressure are derived from non-contradictory equations.

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