Specific quantity

Last updated January 20, 2025

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.^{[ 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).

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:

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^{[ clarification needed ]} 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.^{[ 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.

Related Research Articles

Enthalpy is the sum of a thermodynamic system's internal energy and the product of its pressure and volume. It is a state function in thermodynamics used in many measurements in chemical, biological, and physical systems at a constant external pressure, which is conveniently provided by the large ambient atmosphere. The pressure–volume term expresses the work $that was done against constant external pressure to establish the system's physical dimensions from to some final volume, i.e. to make room for it by displacing its surroundings. The pressure-volume term is very small for solids and liquids at common conditions, and fairly small for gases. Therefore, enthalpy is a stand-in for energy in chemical systems; bond, lattice, solvation, and other chemical "energies" are actually enthalpy differences. As a state function, enthalpy depends only on the final configuration of internal energy, pressure, and volume, not on the path taken to achieve it.$

In thermodynamics, the specific heat capacity of a substance is the amount of heat that must be added to one unit of mass of the substance in order to cause an increase of one unit in temperature. It is also referred to as Massic heat capacity or as the Specific heat. More formally it is the heat capacity of a sample of the substance divided by the mass of the sample. The SI unit of specific heat capacity is joule per kelvin per kilogram, J⋅kg⁻¹⋅K⁻¹. For example, the heat required to raise the temperature of 1 kg of water by 1 K is 4184 joules, so the specific heat capacity of water is 4184 J⋅kg⁻¹⋅K⁻¹.

Specific impulse is a measure of how efficiently a reaction mass engine, such as a rocket using propellant or a jet engine using fuel, generates thrust. In general, this is a ratio of the impulse, i.e. change in momentum, per mass of propellant. This is equivalent to "thrust per massflow". The resulting unit is equivalent to velocity, although it doesn't represent any physical velocity ; it is more properly thought of in terms of momentum per mass, since this represents a physical momentum and physical mass.

Thermodynamic temperature is a quantity defined in thermodynamics as distinct from kinetic theory or statistical mechanics.

The volumetric heat capacity of a material is the heat capacity of a sample of the substance divided by the volume of the sample. It is the amount of energy that must be added, in the form of heat, to one unit of volume of the material in order to cause an increase of one unit in its temperature. The SI unit of volumetric heat capacity is joule per kelvin per cubic meter, J⋅K⁻¹⋅m⁻³.

Latent heat is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process—usually a first-order phase transition, like melting or condensation.

Thrust-specific fuel consumption (TSFC) is the fuel efficiency of an engine design with respect to thrust output. TSFC may also be thought of as fuel consumption (grams/second) per unit of thrust, hence thrust-specific. This figure is inversely proportional to specific impulse, which is the amount of thrust produced per unit fuel consumed.

Physical or chemical properties of materials and systems can often be categorized as being either intensive or extensive, according to how the property changes when the size of the system changes. The terms "intensive and extensive quantities" were introduced into physics by German mathematician Georg Helm in 1898, and by American physicist and chemist Richard C. Tolman in 1917.

Psychrometrics is the field of engineering concerned with the physical and thermodynamic properties of gas-vapor mixtures.

Density and dense usually refer to a measure of how much of some entity is within a fixed amount of space. Types of density include:

The heating value of a substance, usually a fuel or food, is the amount of heat released during the combustion of a specified amount of it.

The molar heat capacity of a chemical substance is the amount of energy that must be added, in the form of heat, to one mole of the substance in order to cause an increase of one unit in its temperature. Alternatively, it is the heat capacity of a sample of the substance divided by the amount of substance of the sample; or also the specific heat capacity of the substance times its molar mass. The SI unit of molar heat capacity is joule per kelvin per mole, J⋅K⁻¹⋅mol⁻¹.

Thermodynamics is expressed by a mathematical framework of thermodynamic equations which relate various thermodynamic quantities and physical properties measured in a laboratory or production process. Thermodynamics is based on a fundamental set of postulates, that became the laws of thermodynamics.

In physics, energy density is the quotient between the amount of energy stored in a given system or contained in a given region of space and the volume of the system or region considered. Often only the useful or extractable energy is measured. It is sometimes confused with stored energy per unit mass, which is called specific energy or gravimetric energy density.

Specific energy or massic energy is energy per unit mass. It is also sometimes called gravimetric energy density, which is not to be confused with energy density, which is defined as energy per unit volume. It is used to quantify, for example, stored heat and other thermodynamic properties of substances such as specific internal energy, specific enthalpy, specific Gibbs free energy, and specific Helmholtz free energy. It may also be used for the kinetic energy or potential energy of a body. Specific energy is an intensive property, whereas energy and mass are extensive properties.

The thermodynamic properties of materials are intensive thermodynamic parameters which are specific to a given material. Each is directly related to a second order differential of a thermodynamic potential. Examples for a simple 1-component system are:

The Glossary of fuel cell terms lists the definitions of many terms used within the fuel cell industry. The terms in this fuel cell glossary may be used by fuel cell industry associations, in education material and fuel cell codes and standards to name but a few.

Space Engine Systems Inc. (SES) is a Canadian aerospace company and is located in Edmonton, Alberta, Canada. The main focus of the company is the development of a light multi-fuel propulsion system to power a reusable spaceplane and hypersonic cruise vehicle. Pumps, compressors, gear boxes, and other related technologies being developed are integrated into SES's major R&D projects. SES has collaborated with the University of Calgary to study and develop technologies in key technical areas of nanotechnology and high-speed aerodynamics.

References

↑ 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.
1 2 3 4 5 "ISO 80000-1: Quantities and units — Part 1: General". iso.org. Retrieved 2023-10-16.

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[IUPACgreen-1] 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.

[ISO80000-1-2] 1 2 3 4 5 "ISO 80000-1: Quantities and units — Part 1: General". iso.org. Retrieved 2023-10-16.

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