Specific quantity

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In the natural sciences, including physiology and engineering, a specific quantity generally refers to an intensive quantity obtained by the ratio of an extensive quantity of interest by another extensive quantity (usually mass or volume). If mass is the divisor quantity, the specific quantity is a massic quantity.<ref name="IUPACgreen">Cohen, E. R.; et al. (2007). IUPAC Green Book (PDF) (3rd ed.). Cambridge: IUPAC and RSC Publishing. pp. 6 (20 of 250 in PDF file). ISBN 978-0-85404-433-7.</ref> If volume is the divisor quantity, the specific quantity is a volumic quantity.[citation needed] For example, massic leaf area is leaf area divided by leaf mass and volumic leaf area is leaf area divided by leaf volume. Derived SI units involve reciprocal kilogram (kg-1), e.g., square metre per kilogram (m2 · kg−1).

Another kind of specific quantity, termed named specific quantity, is a generalization of the original concept. The divisor quantity is not restricted to mass, and name of the divisor is usually placed before "specific" in the full term (e.g., "thrust-specific fuel consumption").

Named and unnamed specific quantities are given for the terms below.

List

Mass-specific quantities

Per unit of mass (short form of mass-specific):

Geometry specific quantities

Volume-specific quantity, the quotient of a physical quantity and volume ("per unit volume"), also called volumic quantities:<ref name="ISO80000-1">"ISO 80000-1: Quantities and units — Part 1: General". iso.org. Retrieved 2023-10-16.</ref>

Area-specific quantity, the quotient of a physical quantity and area ("per unit area"), also called areic quantities:<ref name="ISO80000-1"/>

Length-specific quantity, the quotient of a physical quantity and length ("per unit length"), also called lineic quantities:<ref name="ISO80000-1"/>

Other specific quantities

In chemistry:

Per unit of other types. The dividing unit is sometimes added before the term "specific", and sometimes omitted.

Usage

Reference tables
Specific properties are often used in reference tables as a means of recording material data in a manner that is independent of size or mass. This allows the data to be broadly applied while keeping the table compact.
Ranking, classifying, and comparing
Specific properties are useful for making comparisons about one attribute while cancelling out the effect of variations in another attribute. For instance, steel alloys are typically stronger than aluminum alloys but are also much denser. Greater strength allows less metal to be used, which makes the choice between the two metals less than obvious.[clarification needed] To simplify the comparison, one would compare the specific strength (strength to weight ratio) of the two metals. A more everyday example relates to grocery shopping: a 2 kg package sells for a higher price than 1 kg package of the same foodstuff, but what matters is the "specific price", commonly called the unit cost (cost in currency units per kilogram).
Mnemonics and qualitative reasoning
In many instances, specific properties are more intuitive or are easier to remember than the original properties, whether in SI or imperial units. For instance, it is easier to conceptualize an acceleration of 2g than an acceleration of 19.6 meters per second squared.

See also

References

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