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The SI-unit system comprises the SI base units and the SI derived units. The SI base units are the fundamental building block of the SI unit system. All the other units get derived from these seven fundamental units of physical quantities. The seven SI units are second, meter, ampere, kilogram, kelvin, mole, and candela.
Now, the question comes: what are derived units? The derived units are formed through some mathematical combinations from the seven SI units. Using the SI units’ products, quotients, and powers, these units are formed, and the number of the derived units is potentially unlimited. These units get associated with the derived quantities, the quantities derived from the base quantities. The dimensions for these units are expressed in terms of the dimensions of the base units.
Some Standard International Derived Units
The units of measuring physical quantities derived from the seven base units of the International System of Units (SI) are the SI derived units. These units can be either dimensionless, or their dimensions can be expressed in terms of the products of one or more base units. There are 22 derived units for which the SI has unique names, but the remaining reflect their derivation.
The 22 derived SI units with unique names
Hertz: Hertz is the SI unit for the physical quantity frequency. This unit is represented by the symbol Hz and is expressed as s-1 in the SI base units.
Newton: The SI unit for the physical quantity force is named Newton and is represented by the symbol N. Its expression in SI base units is m kg s-2.
Joule: For the physical quantities work, heat, and energy, the SI unit is termed, Joule. It is represented with the symbol J and is expressed as N m= m2 kg s-2.
Pascal: It is the unique name given to the SI unit of pressure and stress. The expression is N m-2= m-1 kg s-2, and the symbol for Pascal is Pa.
Watt: Watt is the name given to the SI unit of power and radiant flux. The symbol for watt is W and is expressed as J s-1= m2 kg s-3 in the SI base units.
Coulomb: It is the name for the SI unit of electrical charge and is represented as C. The SI base unit expression for Coulomb is As.
Ohm: Ohm is the SI unit for electrical resistance, and its symbol is Ω. Its expression in SI base units is V A-1= m2 kg s-3 A-2.
Volt: For electrical potential and electromotive force, the SI unit is Volt. It has a symbol v and is expressed as J C–= m2 kg s-3 A-1.
Siemens: Represented as S, Siemens is the SI unit of electric conductance. It is expressed as ohm-1= m-2 kg-1 s–3 A2.
Farad: Electrical capacitance has its SI unit named Farad, represented by F. The expression for Farad is C V-1= m-2 kg-1 s4 A2.
Tesla: Tesla is the unique name for the SI unit of Magnetic flux density. Represented by symbol T, it is expressed as V s m-2= kg s-2 A-1.
Henry: Expressed as V A-1 s= m2 kg s-2 A-2 and represented by H, Henry is the SI unit for inductance.
Weber: Weber is the SI unit for magnetic flux. Its symbol is Wb and is expressed as V s= m2 kg s-2 A-1.
Lumen: For luminous flux, lumen is the SI unit. It is represented as lm and expressed as cd sr.
Degree celsius: The symbol for this is °C and is the derived SI unit for temperature.
Lux: For illuminance, the SI unit is lux, and it is represented as lx and expressed as cd sr m-2.
Gray: Gray is the SI unit for absorbed dose of radiation. It is represented by Gy and expressed as J kg-1= m2 s-2.
Sievert: It is the SI unit for dose equivalent of radiation. The symbol for Sievert is Sv and is expressed as J kg-1= m2 s-2.
Katal: The SI unit for catalytic activity is Katal, represented as kat and expressed as mol/s= s-1 mol.
Becquerel: This is the SI unit for radioactive activity and is represented with the symbol bq. The SI base unit expression for it is s-1.
Radian: It is the SI unit for plane angle. The expression for radian is 1 m m-1. The symbol for radian is rad.
Steradian: It is the SI unit for solid angle and is represented sr and is expressed as 1 m2 m-2.
Conclusion
The SI unit system consists of base units and the derived SI units. This unit system makes it easy to universalize the measurements and avoid confusion. The derived units are formed from the seven base units. Hence, the base units form the building blocks of the SI unit system. Also, some derived SI units are given unique names while the others are expressed merely with their derivation from the SI base units.
