Thermodynamic operation

A thermodynamic operation is an externally imposed manipulation or change of connection or wall between a thermodynamic system and its surroundings. An externally imposed change in the state of the surroundings of a system can usually be regarded as a virtual thermodynamic operation. It may usually be analyzed as due to changes in the walls that separate the systems in the surroundings. It is assumed in thermodynamics that the operation is conducted in ignorance of any pertinent microscopic information.

A thermodynamic operation requires a contribution from an animate agency, or at least an independent external agency, that does not come from the passive or inanimate properties of the systems. Perhaps the first expression of the distinction between a thermodynamic operation and a thermodynamic process is in Kelvin's statement of the second law of thermodynamics: "It is impossible, by means of inanimate material agency, to derive mechanical effect from any portion of matter by cooling it below the temperature of the surrounding objects." A sequence of events that occurred other than "by means of inanimate material agency" would entail an action by an animate agency, or at least an independent external agency. Such an agency could impose some thermodynamic operations. For example, those operations might create a heat pump, which of course would comply with the second law. A Maxwell's demon conducts an extremely idealized and naturally unrealizable kind of thermodynamic operation.

An ordinary language expression for a thermodynamic operation is used by Edward A. Guggenheim: "tampering" with the bodies.

A typical thermodynamic operation is a removal of an initially separating wall, a manipulation that unites two systems into one undivided system. A typical thermodynamic process consists of a redistribution that spreads a conserved quantity between a system and its surroundings across a previously impermeable but newly unchanging semi-permeable wall between them. More generally, a process can be considered as a transfer of some quantity that is defined by an extensive state variable of the system, so that a transfer balance equation can be written. According to Uffink, "... thermodynamic processes only take place after an external intervention on the system (such as: removing a partition, establishing thermal contact with a heat bath, pushing a piston, etc.). They do not correspond to the autonomous behaviour of a free system."

As a matter of history, the distinction, between a thermodynamic operation and a thermodynamic process, is not found in these terms in nineteenth century accounts. For example, Kelvin spoke of a "thermodynamic operation" when he meant what present-day terminology calls a thermodynamic operation followed by a thermodynamic process. Again, Planck usually spoke of a "process" when our present-day terminology would speak of a thermodynamic operation followed by a thermodynamic process.

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