In physics, an extensive quantity is a state quantity that changes with the size of the system under consideration . Examples of this are mass , amount of substance , volume , entropy and the thermodynamic potentials ( internal energy , free energy , enthalpy and free enthalpy ). The counterpart of extensive size is intensive size .
The dependency of a variable on the system under consideration can be understood, for example, using two identical systems that are separated by a partition. If one removes this separation and extends the consideration to the entire system, the difference between intensive and extensive quantities becomes clear: All quantities which now have the same value as before the removal of the partition are intensive quantities; on the other hand, all quantities which now have a different value are extensive quantities.
If one forms the quotient from two extensive quantities of the same system, one obtains an intensive quantity:
- If one relates an extensive quantity to the volume , one obtains a density as an intensive quantity. Example: The intensive particle density belongs to the extensive number of particles .
- If one relates an extensive quantity to the mass, then one gets a specific quantity as an intensive quantity . Example: The specific heat capacity belongs to the extensive heat capacity .
- If one relates an extensive quantity to the amount of substance , the intensive quantity is a molar quantity . Example: The intensive molar mass belongs to the extensive mass .
Many - but not all - extensive quantities are set- like quantities .
The change of an extensive quantity does not have to be followed by a change in the thermodynamic equilibrium .
|Characteristic||Sizes depend on the amount of fabric||Sizes do not depend on the amount of fabric|
|property||Changes with the size of the system under consideration. Extensive sizes are additive. If the amount of substance is multiplied, all extensive quantities multiply.||Is not affected by the size of the system. Intense sizes are not additive.|
|Examples||Amount of substance n , volume V , internal energy U , free energy F , free enthalpy G , mass m , electrical charge Q||Temperature T , density ρ , pressure p , concentration c = n / V , viscosity, refractive index, electrical voltage U , dielectric constant ε , dipole moment|
|Specialty||A state variable is also extensive if it is proportional to all other state variables known to be extensive. The proportionality only applies as long as all non-extensive state variables remain constant.|
|constitutive properties||colligative properties|
|context||The product of an extensive and an intensive quantity is an extensive quantity|
- extensive size. In: Spectrum Lexicon of Physics.