Volume is a function of state and is interdependent with other thermodynamic properties such as pressure and temperature.
The volume of a thermodynamic system typically refers to the volume of the working fluid, such as, for example, the fluid within a piston.
An isochoric process however operates at a constant-volume, thus no work can be produced.
Many other thermodynamic processes will result in a change in volume.
A polytropic process, in particular, causes changes to the system so that the quantity
In general, compressibility is defined as the relative volume change of a fluid or solid as a response to a pressure, and may be determined for substances in any phase.
Many thermodynamic cycles are made up of varying processes, some which maintain a constant volume and some which do not.
A vapor-compression refrigeration cycle, for example, follows a sequence where the refrigerant fluid transitions between the liquid and vapor states of matter.
Hence, volume is an important parameter in characterizing many thermodynamic processes where an exchange of energy in the form of work is involved.
Volume is one of a pair of conjugate variables, the other being pressure.
As with all conjugate pairs, the product is a form of energy.
is the energy lost to a system due to mechanical work.
In thermodynamic systems where the temperature and volume are held constant, the measure of "useful" work attainable is the Helmholtz free energy; and in systems where the volume is not held constant, the measure of useful work attainable is the Gibbs free energy.
In the case of a constant-volume process, all the heat affects the internal energy of the system (i.e., there is no pV-work, and all the heat affects the temperature).
However, in a process without a constant volume, the heat addition affects both the internal energy and the work (i.e., the enthalpy); thus the temperature changes by a different amount than in the constant-volume case and a different heat capacity value is required.
) is the volume occupied by a unit of mass of a material.
[1] In many cases, the specific volume is a useful quantity to determine because, as an intensive property, it can be used to determine the complete state of a system in conjunction with another independent intensive variable.
The specific volume also allows systems to be studied without reference to an exact operating volume, which may not be known (nor significant) at some stages of analysis.
The specific volume of a substance is equal to the reciprocal of its mass density.
The volume of gas increases proportionally to absolute temperature and decreases inversely proportionally to pressure, approximately according to the ideal gas law:
[2] In contrast to other gas components, water content in air, or humidity, to a higher degree depends on vaporization and condensation from or into water, which, in turn, mainly depends on temperature.
Therefore, when applying more pressure to a gas saturated with water, all components will initially decrease in volume approximately according to the ideal gas law.
However, some of the water will condense until returning to almost the same humidity as before, giving the resulting total volume deviating from what the ideal gas law predicted.
Conversely, decreasing temperature would also make some water condense, again making the final volume deviating from predicted by the ideal gas law.
This fraction more accurately follows the ideal gas law.
On the contrary, Vs (volume saturated) is the volume a gas mixture would have if humidity was added to it until saturation (or 100% relative humidity).
To compare gas volume between two conditions of different temperature or pressure (1 and 2), assuming nR are the same, the following equation uses humidity exclusion in addition to the ideal gas law:
Where, in addition to terms used in the ideal gas law: For example, calculating how much 1 liter of air (a) at 0 °C, 100 kPa, pw = 0 kPa (known as STPD, see below) would fill when breathed into the lungs where it is mixed with water vapor (l), where it quickly becomes 37 °C (99 °F), 100 kPa, pw = 6.2 kPa (BTPS):
Some common expressions of gas volume with defined or variable temperature, pressure and humidity inclusion are: The following conversion factors can be used to convert between expressions for volume of a gas:[3] The partial volume of a particular gas is a fraction of the total volume occupied by the gas mixture, with unchanged pressure and temperature.
It can be approximated both from partial pressure and molar fraction:[4]