In
thermodynamics
Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of th ...
, the reduced properties of a fluid are a set of
state variables
A state variable is one of the set of variables that are used to describe the mathematical "state" of a dynamical system. Intuitively, the state of a system describes enough about the system to determine its future behaviour in the absence of a ...
scaled by the fluid's state properties at its
critical point. These dimensionless thermodynamic coordinates, taken together with a substance's
compressibility factor
In thermodynamics, the compressibility factor (Z), also known as the compression factor or the gas deviation factor, describes the deviation of a real gas from ideal gas behaviour. It is simply defined as the ratio of the molar volume of a gas to ...
, provide the basis for the simplest form of the
theorem of corresponding states
According to van der Waals, the theorem of corresponding states (or principle/law of corresponding states) indicates that all fluids, when compared at the same reduced temperature and reduced pressure, have approximately the same compressibility fa ...
.
[
Reduced properties are also used to define the ]Peng–Robinson equation of state Cubic equations of state are a specific class of thermodynamic models for modeling the pressure of a gas as a function of temperature and density and which can be rewritten as a cubic function of the molar volume.
Equations of state are generally ...
, a model designed to provide reasonable accuracy near the critical point. They are also used to critical exponents
Critical or Critically may refer to:
*Critical, or critical but stable, medical states
**Critical, or intensive care medicine
* Critical juncture, a discontinuous change studied in the social sciences.
* Critical Software, a company specializing i ...
, which describe the behaviour of physical quantities near continuous phase transitions.Hagen Kleinert
Hagen Kleinert (born 15 June 1941) is professor of theoretical physics at the Free University of Berlin, Germany (since 1968)Honorary Doctorat the
West University of Timișoaraandat thin Bishkek. He is alsHonorary Memberof th For his contributio ...
and Verena Schulte-Frohlinde, ''Critical Properties of φ4-Theories'', pp.8
World Scientific (Singapore, 2001)
''(Read online a
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Reduced pressure
The reduced pressure is defined as its actual pressure divided by its critical pressure
In thermodynamics, a critical point (or critical state) is the end point of a phase equilibrium curve. The most prominent example is the liquid–vapor critical point, the end point of the pressure–temperature curve that designates conditions ...
:
Reduced temperature
The reduced temperature of a fluid is its actual temperature, divided by its critical temperature:[
:
where the actual temperature and critical temperature are expressed in absolute temperature scales (either ]Kelvin
The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phy ...
or Rankine). Both the reduced temperature and the reduced pressure are often used in thermodynamical formulas like the Peng–Robinson equation of state.
Reduced specific volume
The reduced specific volume (or "pseudo-reduced specific volume") of a fluid is computed from the ideal gas law
The ideal gas law, also called the general gas equation, is the equation of state of a hypothetical ideal gas. It is a good approximation of the behavior of many gases under many conditions, although it has several limitations. It was first stat ...
at the substance's critical pressure and temperature:[
:
This property is useful when the specific volume and either temperature or pressure are known, in which case the missing third property can be computed directly.
]
See also
*Departure function
In thermodynamics, a departure function is defined for any thermodynamic property as the difference between the property as computed for an ideal gas and the property of the species as it exists in the real world, for a specified temperature ''T'' ...
References
{{reflist
Thermodynamic properties