Answer:
In thermodynamics, a reversible process is a process whose direction can be reversed to return the system to its original state by inducing infinitesimal changes to some property of the system's surroundings. Throughout the entire reversible process, the system is in thermodynamic equilibrium with its surroundings. Having been reversed, it leaves no change in either the system or the surroundings. Since it would take an infinite amount of time for the reversible process to finish, perfectly reversible processes are impossible. However, if the system changing responds much faster than the applied change, the deviation from reversibility may be negligible. In a reversible cycle, a cyclical reversible process, the system, and its surroundings will be returned to their original states if one-half cycle is followed by the other half cycle.
Explanation:
Thermodynamic processes can be carried out in one of two ways: reversibly or irreversibly. Reversibility means the reaction operates continuously at quasi-equilibrium. In an ideal thermodynamically reversible process, the energy from work performed by or on the system would be maximized, and that from the heat would be zero. However, heat cannot fully be converted to work and will always be lost to some degree (to the surroundings). (This is true only in the case of a cycle. In the case of an ideal process, heat can be completely converted into work, e.g., isothermal expansion of an ideal gas in a piston-cylinder arrangement.) The phenomenon of maximized work and minimized heat can be visualized on a pressure-volume graph as the area beneath the equilibrium curve, representing work done. To maximize work, one must follow the equilibrium curve precisely.
Irreversible processes, on the other hand, are a result of straying away from the curve, therefore decreasing the amount of overall work done; an irreversible process can be described as a thermodynamic process that departs from equilibrium. Irreversibility is defined as the difference between the reversible work and the actual work for a process. When described in terms of pressure and volume, it occurs when the pressure (or the volume) of a system changes so dramatically and instantaneously that the volume (or the pressure) does not have time to reach equilibrium. A classic example of irreversibility is allowing a certain volume of gas to be released into a vacuum. By releasing pressure on a sample and thus allowing it to occupy a large space, the system and surroundings are not in equilibrium during the expansion process and there is little work done. However, significant work will be required, with a corresponding amount of energy dissipated as heat flow to the environment, to reverse the process (compressing the gas back to its original volume and temperature).
An alternative definition of a reversible process is a process that, after it has taken place, can be reversed and, when reversed, returns the system and its surroundings to their initial states. In thermodynamic terms, a process "taking place" would refer to its transition from one state to another.
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