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Classical thermodynamics

Classical thermodynamics accounts for the adventures of thermodynamic systems in terms, either of their time-invariant equilibrium states, or else of their continually repeated cyclic processes, but, formally, not both in the same account. It uses only time-invariant, or equilibrium, macroscopic quantities measurable in the laboratory, counting as time-invariant a long-term time-average of a quantity, such as a flow, generated by a continually repetitive process.[41][42] Classical thermodynamics does not admit change over time as a fundamental factor in its account of processes. An equilibrium state stands endlessly without change over time, while a continually repeated cyclic process runs endlessly without change over time.

In the account in terms of equilibrium states of a system, a state of thermodynamic equilibrium in a simple system (as defined below in this article), with no externally imposed force field, is spatially homogeneous.

In the classical account strictly and purely in terms of cyclic processes, the spatial interior of the 'working body' of a cyclic process is not considered; the 'working body' thus does not have a defined internal thermodynamic state of its own because no assumption is made that it should be in thermodynamic equilibrium; only its inputs and outputs of energy as heat and work are considered.[43] It is of course possible, and indeed common, for the account in terms of equilibrium states of a system to describe cycles composed of indefinitely many equilibrium states.

Classical thermodynamics was originally concerned with the transformation of energy in cyclic processes, and the exchange of energy between closed systems defined only by their equilibrium states. For these, the distinction between transfers of energy as heat and as work was central.

As classical thermodynamics developed, the distinction between heat and work became less central. This was because there was more interest in open systems, for which the distinction between heat and work is not simple, and is beyond the scope of the present article. Alongside amount of heat transferred as a fundamental quantity, entropy, considered below, was gradually found to be a more generally applicable concept, especially when chemical reactions are of interest. Massieu in 1869 considered entropy as the basic dependent thermodynamic variable, with energy potentials and the reciprocal of thermodynamic temperature as fundamental independent variables. Massieu functions can be useful in present-day non-equilibrium thermodynamics. In 1875, in the work of Josiah Willard Gibbs, the basic thermodynamic quantities were energy potentials, such as internal energy, as dependent variables, and entropy, considered as a fundamental independent variable.

All actual physical processes are to some degree irreversible. Classical thermodynamics can consider irreversible processes, but its account in exact terms is restricted to variables that refer only to initial and final states of thermodynamic equilibrium, or to rates of input and output that do not change with time. For example, classical thermodynamics can consider long-time-average rates of flows generated by continually repeated irreversible cyclic processes. Also it can consider irreversible changes between equilibrium states of systems consisting of several phases (as defined below in this article), or with removable or replaceable partitions. But for systems that are described in terms of equilibrium states, it considers neither flows, nor spatial inhomogeneities in simple systems with no externally imposed force field such as gravity. In the account in terms of equilibrium states of a system, descriptions of irreversible processes refer only to initial and final static equilibrium states; rates of progress are not considered.[45][46]




Date: 2015-12-11; view: 806


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