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[a] While processes in isolated systems are never reversible, [3] cyclical processes can be reversible or irreversible. [4] Reversible processes are hypothetical or idealized but central to the second law of thermodynamics. [3] Melting or freezing of ice in water is an example of a realistic process that is nearly reversible.
While reversible processes are a useful and convenient theoretical limiting case, all natural processes are irreversible. A prime example of this irreversibility is the transfer of heat by conduction or radiation.
An efficiency for a process or collection of processes that compares it to the reversible ideal may also be found (see Exergy efficiency). This approach to the second law is widely utilized in engineering practice, environmental accounting , systems ecology , and other disciplines.
The equal sign refers to a reversible process, which is an imagined idealized theoretical limit, never actually occurring in physical reality, with essentially equal temperatures of system and surroundings. [10] [11] For an isentropic process, if also reversible, there is no transfer of energy as heat because the process is adiabatic; δQ = 0 ...
A and B can react to form C and D or, in the reverse reaction, C and D can react to form A and B. This is distinct from a reversible process in thermodynamics. Weak acids and bases undergo reversible reactions. For example, carbonic acid: H 2 CO 3 (l) + H 2 O (l) ⇌ HCO 3 − (aq) + H 3 O + (aq).
Example of a real system modelled by an idealized process: PV and TS diagrams of a Brayton cycle mapped to actual processes of a gas turbine engine Thermodynamic cycles may be used to model real devices and systems, typically by making a series of assumptions to reduce the problem to a more manageable form. [ 2 ]
For a particular reversible process in general, the work done reversibly on the system, ,, and the heat transferred reversibly to the system, , are not required to occur respectively adiabatically or adynamically, but they must belong to the same particular process defined by its particular reversible path, , through the space of thermodynamic ...
An example of a cycle of idealized thermodynamic processes which make up the Stirling cycle. A quasi-static thermodynamic process can be visualized by graphically plotting the path of idealized changes to the system's state variables. In the example, a cycle consisting of four quasi-static processes is shown.