Please read Wikipedia founder Jimmy Wales's personal appeal.

SI derived unit

From Wikipedia, the free encyclopedia.

Jump to: navigation, search

SI derived units are part of the SI system of measurement units and are derived from the seven SI base units.

Contents

[hide]

Dimensionless derived units

The following SI units are actually dimensionless ratios, formed by dividing two identical SI units. They are therefore considered by the BIPM to be derived. Formally, their SI unit is simply the number 1, but they are given these special names, for use whenever the lack of a unit might be confusing.


SI dimensionless units

edit

Name Symbol Quantity Definition
radian rad Angle The unit of angle is the angle subtended at the centre of a circle by an arc of the circumference equal in length to the radius of the circle. There are radians in a circle.
steradian sr Solid angle The unit of solid angle is the solid angle subtended at the centre of a sphere of radius r by a portion of the surface of the sphere having an area r2. There are steradians on a sphere.

Derived units with special names

Base units can be put together to derive units of measurement for other quantities. Some have been given names.


SI derived units with special names

edit

Name Symbol Quantity Expression in terms
of other units
Expression in terms
of SI base units
hertz Hz Frequency s−1 s−1
newton N Force, Weight m·kg·s−2 m·kg·s−2
joule J Energy, Work, Heat N·m m2·kg·s−2
watt W Power, Radiant flux J/s m2·kg·s−3
pascal Pa Pressure, Stress N/m2 m−1·kg·s−2
lumen lm Luminous flux cd·sr = m2·m−2·cd cd
lux lx Illuminance lm/m2 = m2·m−4·cd m−2·cd
coulomb C Electric charge or flux s·A s·A
volt V Electrical potential difference, Electromotive force W/A = J/C m2·kg·s−3·A−1
ohm Ω Electric resistance, Impedance, Reactance V/A m2·kg·s−3·A−2
farad F Electric capacitance C/V m−2·kg−1·s4·A2
weber Wb Magnetic flux m2·kg·s−2·A−1 m2·kg·s−2·A−1
tesla T Magnetic flux density, Magnetic inductivity V·s·m−2 = Wb/m2 kg·s−2·A−1
henry H Inductance V·s·A−1 = Wb/A m2·kg·s−2·A−2
siemens S Electric conductance Ω−1 m−2·kg−1 s3·A2
becquerel Bq Radioactivity (decays per unit time) s−1 s−1
gray Gy Absorbed dose (of ionising radiation) J/kg m2·s−2
sievert Sv Equivalent dose (of ionising radiation) J/kg m2·s−2
katal kat Catalytic activity mol/s s−1·mol
degree Celsius °C Thermodynamic temperature t°C = tK - 273.15

Other quantities and units

Other derived SI units

edit

Name Symbol Quantity Expression in terms
of SI base units
square metre area
cubic metre volume
metre per second m·s-1 speed, velocity m·s-1
metre per second squared m·s-2 acceleration m·s-2
radian per second rad·s-1 angular velocity s-1
newton second N·s momentum kg·m·s-1
newton metre second N·m·s angular momentum kg·m²·s-1
newton metre N·m torque, moment of force kg·m²·s-2
reciprocal metre m-1 wavenumber m-1
kilogram per cubic metre kg·m-3 density, mass density kg·m-3
cubic metre per kilogram kg-1·m³ specific volume kg-1·m³
mole per cubic metre m-3·mol amount (-of-substance) concentration m-3·mol
cubic metre per mole m³·mol-1 molar volume m³·mol-1
joule per kelvin J·K-1 heat capacity, entropy kg·m²·s-2·K-1
joule per kelvin mole J·K-1·mol-1 molar heat capacity, molar entropy kg·m²·s-2·K-1·mol-1
joule per kilogram kelvin J·K-1·kg-1 specific heat capacity, specific entropy m²·s-2·K-1
joule per mole J·mol-1 molar energy kg·m²·s-2·mol-1
joule per kilogram J·kg-1 specific energy m²·s-2
joule per cubic metre J·m-3 energy density kg·m-1·s-2
newton per metre N·m-1 = J·m-2 surface tension kg·s-2
watt per square metre W·m-2 heat flux density, irradiance kg·s-3
watt per metre kelvin W·m-1·K-1 thermal conductivity kg·m·s-3·K-1
square metre per second m²·s-1 kinematic viscosity, diffusion coefficient m²·s-1
pascal second Pa·s = N·s·m-2 dynamic viscosity kg·m-1·s-1
coulomb per cubic metre C·m-3 electric charge density m-3·s·A
ampere per square metre A·m-2 electric current density A·m-2
siemens per metre S·m-1 conductivity kg-1·m-3·s³·A²
siemens square metre per mole S·m²·mol-1 molar conductivity kg-1·s³·mol-1·A²
farad per metre F·m-1 permittivity kg-1·m-3·s4·A²
henry per metre H·m-1 permeability kg·m·s-2·A-2
volt per metre V·m-1 electric field strength kg·m·s-3·A-1
ampere per metre A·m-1 magnetic field strength A·m-1
candela per square metre cd·m-2 luminance cd·m-2
coulomb per kilogram C·kg-1 exposure (X and gamma rays) kg-1·s·A
gray per second Gy·s-1 absorbed dose rate m²·s-3

Conversion between kelvins and degrees Celsius

It has been suggested that this article or section be merged into Degree Celsius#Conversions. (Discuss)

A change in temperature of 1°C is equal to a change in temperature of 1K.

Temperature in °C = Temperature in kelvins - 273.15

Thus, one could think of the Kelvin scale as the same as the Celsius scale, with its zero point moved down to absolute zero. This perspecitive is historically accurate; however, it has become more convenient to fix the standard for the kelvin, and thus the Celsius scale is derived from that standard (i.e., it now depends on absolute zero and the triple point of water with a 0.01 K offset — the boiling point of water no longer has anything to do with the official definition of degrees Celsius).

Temperature differences are often measured in degrees Celsius; however, it doesn't matter: differences in temperature are equivalent whether kelvins or degrees Celsius are used.

Therefore, a change in temperature (ΔT), when expressed in an equation, can be calculated using either kelvins or degrees celsius so long as one is consistent.

See also

References

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