Specific size

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Specific quantities are physical quantities that are usually related to the mass of a substance or body or to the spatial dimensions of a system ( volume , surface area , length ).

According to the DIN standard, the term is specifically reserved for mass reference only; its umbrella term is related size .


A state variable or process variable is converted into a specific or related variable by dividing it by the associated reference . The purpose of this method is that the size value is then independent of certain scalings of the system and can be handled more easily in calculations.

In thermodynamics, this particularly indicates the difference between an extensive state variable or process variable and a (related) intensive variable .

In contrast to those of extensive quantities, the formula symbols for intensive quantities usually consist of small letters or they are specially indexed.

Standardized size designations

Logo of the German Institute for Standardization DIN 5485
Area physics
title Naming principles for physical quantities; Word compositions with adjectives and basic words
Latest edition 1986-08

In the German standard DIN 5485 naming principles for physical quantities; Word compositions with adjectives and basic words are recommended:

  • mass per unit quantities should specifically be mentioned
  • volume- related variables are density , -volumendichte or -raumdichte called
  • For area-related sizes, the designation -area density or -cover is recommended
  • For length -related, length density , covering or curtain is recommended
  • Quotient of mass, volume, molar or particles of a substance and the volume of the mixing phase , the material is in the should - concentration are called

These recommendations largely correspond to the international standard EN ISO 80000-1 : 2013 Sizes and units - General , Chapter A.6.

In fact, there are many terms in everyday scientific and technical life that do not follow these criteria. It is therefore not easy to deduce the reference from the name of the size. A number of sizes are specified in relation to mass or volume, depending on the context, without the designation making this clear. Figures in masses - and volume are non-standard.

In addition, the above DIN also recommends:

Every quantity related to a quantity of its own quantity type is a quantity of the dimension number . If the unit of measurement in the numerator and denominator is the same, it is called the ratio ; however, the dimension number is also, for example, an indication in mm / m.

In addition to these physical quantities, there are also numerous other special “specific” values, such as in the calculation of probability, or in the economy the energy or material inputs or costs related to certain consumption sectors that are used as parameters .

Examples of related sizes

Mass-related specific sizes

designation Definition equation unit Art
specific volume v State variable
specific work w Process variable
specific enthalpy h State variable
specific entropy s State variable
gas constant R
1 Material constant
 is the molar or general gas constant with the unit J / (kmol K) and is the molar mass of the special gas with the unit kg / kmol.

Further, mostly mass-related quantities:
Specific heat capacity c, specific energy e / gravimetric energy density w (e.g. enthalpy h see above, specific latent heat : heat of fusion Δ Q s , heat of vaporization Δ Q v , heat of crystallization Δ Q k , calorific value h s and calorific value h i for liquid and solid fuels ), specific impulse , I sp , specific charge , specific power , specific humidity s .

Dimensional specific sizes, densities

Dimensional specific variables:
density (specific gravity) γ, volumetric energy density w ( calorific value H s and calorific value H i for gaseous fuels), specific surface area s m / s v , electrical resistivity ρ, emittance M ° / M ° ν , specific Rotation angle , specific heat resistance R λ , charge density (space charge density ρ, ( surface) surface charge density σ, line charge density λ), population density (number per area), traffic density (number per length), thread density of a fabric (number per area)

Specific quantities related to density

Density-related quantities in materials science that allow scaling:
Relative density (specific density) d , specific stiffness R sp , specific strength Φ, specific adhesion .