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In thermodynamics, the Joule–Thomson effect (also known as the Joule–Kelvin effect or Kelvin–Joule effect) describes the temperature change of a real gas or liquid (as differentiated from an ideal gas) when it is expanding; typically caused by the pressure loss from flow through a valve or porous plug while keeping it insulated so that no heat is exchanged with the environment.
This temperature change is known as the Joule–Thomson effect, and is exploited in the liquefaction of gases. Inversion temperature depends on the nature of the gas. For a van der Waals gas we can calculate the enthalpy using statistical mechanics as
For real gasses, the molecules do interact via attraction or repulsion depending on temperature and pressure, and heating or cooling does occur. This is known as the Joule–Thomson effect. For reference, the Joule–Thomson coefficient μ JT for air at room temperature and sea level is 0.22 °C/bar. [7]
It yields an analytic analysis of the Joule–Thomson coefficient and associated inversion curve, which were instrumental in the development of the commercial liquefaction of gases. It shows that the specific heat at constant volume c v {\displaystyle c_{v}} is a function of T {\displaystyle T} only.
The Joule–Thomson effect, the temperature change of a gas when it is forced through a valve or porous plug while keeping it insulated so that no heat is exchanged with the environment. The Gough–Joule effect or the Gow–Joule effect, which is the tendency of elastomers to contract if heated while they are under tension.
The Thomson effect is an extension of the Peltier–Seebeck model and is credited to Lord Kelvin. Joule heating, the heat that is generated whenever a current is passed through a conductive material, is not generally termed a thermoelectric effect.
At temperatures below their inversion temperature gases will cool during Joule expansion, while at higher temperatures they will heat up. [ 5 ] [ 6 ] The inversion temperature of a gas is typically much higher than room temperature; exceptions are helium, with an inversion temperature of about 40 K, and hydrogen, with an inversion temperature ...
This is the definition declared in the modern International System of Units in 1960. [14] The definition of the joule as J = kg⋅m 2 ⋅s −2 has remained unchanged since 1946, but the joule as a derived unit has inherited changes in the definitions of the second (in 1960 and 1967), the metre (in 1983) and the kilogram . [15]