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Derived quantities can be expressed in terms of the base quantities. Note that neither the names nor the symbols used for the physical quantities are international standards. Some quantities are known as several different names such as the magnetic B-field which is known as the magnetic flux density , the magnetic induction or simply as the ...
The SI has special names for 22 of these coherent derived units (for example, hertz, the SI unit of measurement of frequency), but the rest merely reflect their derivation: for example, the square metre (m 2), the SI derived unit of area; and the kilogram per cubic metre (kg/m 3 or kg⋅m −3), the SI derived unit of density.
The constants listed here are known values of physical constants expressed in SI units; that is, physical quantities that are generally believed to be universal in nature and thus are independent of the unit system in which they are measured.
Such a quantity can be regarded as a derived quantity in the form of the ratio of two quantities of the same dimension. The named dimensionless units " radian " (rad) and " steradian " (sr) are acceptable for distinguishing dimensionless quantities of different kind, respectively plane angle and solid angle .
Unit prefixes are common base-10 or base-2 powers multiples and submultiples of units. While a base unit is one that has been explicitly so designated, [2] a derived unit is unit for a derived quantity, involving the combination of quantities with different units; [1] several SI derived units are specially named.
It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 10 12 Hz, K cd, to be 683 when expressed in the unit lm W −1, which is equal to cd sr W −1, or cd sr kg −1 m −2 s 3, where the kilogram, metre and second are defined in terms of h, c and ∆ν Cs."
Two of the base SI units and 17 of the derived units are named after scientists. [2] 28 non-SI units are named after scientists. By this convention, their names are immortalised. As a rule, the SI units are written in lowercase letters, but symbols of units derived from the name of a person begin with a capital letter.
A systems of quantities relates physical quantities, and due to this dependence, a limited number of quantities can serve as a basis in terms of which the dimensions of all the remaining quantities of the system can be defined. A set of mutually independent quantities may be chosen by convention to act as such a set, and are called base quantities.