SI derived unit
SI derived units are part of the
SI system of measurement
units and are derived from the seven
SI base 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.
Base units can be put together to derive units of measurement for other quantities. Some have been given names.
A change in temperature of 1C° is equal to a change in temperature of 1K.
Temperature in Celsius = Temperature in kelvins â€"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 perspective 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 Celsius degrees; however, it doesn't matter: differences in temperature are equivalent whether kelvins or Celsius degrees are used.
Therefore, a change in temperature (Î
"T), when expressed in an equation, can be calculated using either kelvins or celsius degrees so long as one is consistent.
*
SI*
SI base unit*
SI prefix*
Planck units* 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)
