SI derived unit

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Short description: Measurement unit derived from base metric units


SI derived units are units of measurement derived from the seven base units specified by the International System of Units (SI). They are either dimensionless or can be expressed as a product of one or more of the base units, possibly scaled by an appropriate power of exponentiation.

The SI has special names for 22 of these derived units (for example, hertz, the SI unit of measurement of frequency), but the rest merely reflect their derivation: for example, the square metre (m2), the SI derived unit of area; and the kilogram per cubic metre (kg/m3 or kg⋅m−3), the SI derived unit of density.

The names of SI derived units, when written in full, are always in lowercase. However, the symbols for units named after persons are written with an uppercase initial letter. For example, the symbol for hertz is "Hz", but the symbol for metre is "m".[1]

Derived units with special names

The International System of Units assigns special names to 22 derived units, which includes two dimensionless derived units, the radian (rad) and the steradian (sr).

Named units derived from SI base units[2]
Name Symbol Quantity Equivalents SI base unit
Equivalents
hertz Hz frequency 1/s s−1
radian rad angle m/m 1
steradian sr solid angle m2/m2 1
newton N force, weight kg⋅m/s2 kg⋅m⋅s−2
pascal Pa pressure, stress N/m2 kg⋅m−1⋅s−2
joule J energy, work, heat m⋅N, C⋅V, W⋅s kg⋅m2⋅s−2
watt W power, radiant flux J/s, V⋅A kg⋅m2⋅s−3
coulomb C electric charge or quantity of electricity s⋅A, F⋅V s⋅A
volt V voltage, electrical potential difference, electromotive force W/A, J/C kg⋅m2⋅s−3⋅A−1
farad F electrical capacitance C/V, s/Ω kg−1⋅m−2⋅s4⋅A2
ohm Ω electrical resistance, impedance, reactance 1/S, V/A kg⋅m2⋅s−3⋅A−2
siemens S electrical conductance 1/Ω, A/V kg−1⋅m−2⋅s3⋅A2
weber Wb magnetic flux J/A, T⋅m2,V⋅s kg⋅m2⋅s−2⋅A−1
tesla T magnetic induction, magnetic flux density V⋅s/m2, Wb/m2, N/(A⋅m) kg⋅s−2⋅A−1
henry H electrical inductance V⋅s/A, Ω⋅s, Wb/A kg⋅m2⋅s−2⋅A−2
degree Celsius °C temperature relative to 273.15 K K K
lumen lm luminous flux cd⋅sr cd
lux lx illuminance lm/m2 cd⋅m−2
becquerel Bq radioactivity (decays per unit time) 1/s s−1
gray Gy absorbed dose (of ionizing radiation) J/kg m2⋅s−2
sievert Sv equivalent dose (of ionizing radiation) J/kg m2⋅s−2
katal kat catalytic activity mol/s s−1⋅mol.

Examples of derived quantities and units

Other units used with SI

Some other units such as the hour, litre, tonne, bar, and electronvolt are not SI units, but are widely used in conjunction with SI units.

Supplementary units

Until 1995, the SI classified the radian and the steradian as supplementary units, but this designation was abandoned and the units were grouped as derived units.[3]

See also

References

Bibliography

  • I. Mills, Tomislav Cvitas, Klaus Homann, Nikola Kallay, IUPAC (June 1993). Quantities, Units and Symbols in Physical Chemistry (2nd ed.). Blackwell Science Inc. p. 72. 

External links

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