Specific quantity explained

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.^[1] If volume is the divisor quantity, the specific quantity is a volumic quantity. 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 (m²kg⁻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):

• Specific absorption rate, power absorbed per unit mass of tissue at a given frequency
• Specific activity, radioactivity in becquerels per unit mass
• Specific energy, defined as energy per unit mass
  • Specific internal energy, internal energy per unit mass
  • Specific kinetic energy, kinetic energy of an object per unit of mass
• Specific enthalpy, enthalpy per unit mass
• Specific enzyme activity, activity per milligram of total protein
• Specific force, defined as the non-gravitational force per unit mass
• Specific growth rate, increase in cell mass per unit cell mass per unit time
• Specific heat capacity, heat capacity per unit mass, unless another unit is named, such as mole-specific heat capacity, or volume-specific heat capacity
• Specific latent heat, latent heat per unit mass
• Specific leaf area, leaf area per unit dry leaf mass
• Specific modulus, a materials property consisting of the elastic modulus per mass density of a material
• Specific orbital energy, orbital energy per unit mass
• Specific power, per unit of mass (or volume or area)
• Specific relative angular momentum, of two orbiting bodies is angular momentum per unit reduced mass, or the vector product of the relative position and the relative velocity
• Specific surface area, per unit of mass, volume, or cross-sectional area
• Specific volume, volume per unit mass, i.e. the reciprocal of density

Geometry specific quantities

Volume-specific quantity, the quotient of a physical quantity and volume ("per unit volume"), also called volumic quantities:^[2]

• Specific mass, actually meaning volume-specific mass, or mass per unit volume; same as density.
• Specific weight, weight per unit volume
• Charge density, the electric charge per volume
• Energy density, potential energy per unit volume
• Force density, force per unit volume
• Power density, power per unit volume
• Particle density (particle count), number of particles per unit volume

Area-specific quantity, the quotient of a physical quantity and area ("per unit area"), also called areic quantities:^[2]

• Current density, the ratio of electric current to area
• Surface power density, power per unit area
• Specific surface energy, free energy per unit surface area

Length-specific quantity, the quotient of a physical quantity and length ("per unit length"), also called lineic quantities:^[2]

• Linear charge density, charge per unit length
• Linear mass density, mass per unit length
• Linear number density, number of entities per unit length
• reciprocal length quantities

Other specific quantities

In chemistry:

• Molar quantities^[2]
• Concentration^[2]

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

• Brake-specific fuel consumption, fuel consumption per unit of braking power
• Thrust-specific fuel consumption, fuel consumption per unit of thrust
• Specific acid catalysis, in which the reaction rate is proportional to the concentration of the protonated solvent molecules
• Specific acoustic impedance, ratio of sound pressure p to particle speed at a single frequency
• Specific capacity of a water well, quantity of water produced per (length) unit of drawdown
• Specific conductance, conductance per meter. Identical to electrical conductivity
• Specific detectivity of a photodetector
• Specific fuel consumption (disambiguation). Fuel consumption per unit thrust, or per unit power. Type defined as above.
• Specific gas constant, per molar mass
• Specific heat of vaporization, enthalpy of vaporization, vaporizing heat per mole
• Specific humidity, mass of water vapor per unit mass dry air
• Specific impulse, impulse (momentum change) per unit of propellant (either per unit of propellant mass, or per unit of propellant by Earth-weight)
• Specific melting heat, enthalpy of fusion; melting heat per mole
• Specific modulus, elastic modulus per mass density
• Specific resistance (disambiguation), several scientific meanings
• Specific rotation of a chemical, angle of optical rotation α of plane-polarized light per standard sample with a path length of one decimeter and a sample concentration of one gram per millilitre
• Specific speed, unitless figure of merit used to classify pump impellers (pump-specific) and turbines (turbine-specific). Ratio of performance against reference pump that needs one unit of speed to pump one unit volume per one unit hydraulic head pressure. For a turbine, it is performance measured against a reference turbine that develops one unit of power per one unit speed per one unit of hydraulic head.
• Specific storage, specific yield, and specific capacity, quantify the capacity of an aquifer to release groundwater from storage per unit decline in hydraulic head pressure
• Specific strength, material strength (pressure required at failure) per unit material density
• Specific surface area, per unit of mass, volume, or cross-sectional area
• Specific thrust, thrust per unit air intake rate

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. 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

• Intensive_and_extensive_properties#Specific_properties

Notes and References

1. Book: IUPAC Green Book . IUPAC and RSC Publishing . Cohen, E. R. . Quantities, Units and Symbols in Physical Chemistry . 2007 . Cambridge . 6 (20 of 250 in PDF file) . 978-0-85404-433-7 . 3rd. etal.
2. Web site: ISO 80000-1: Quantities and units — Part 1: General . iso.org . . 2023-10-16.