In reversible process things happen very slowly, without any resisting force, without any space limitation, everything happens in a highly organized way (it is not physically possible; it is an idealization). Internally reversible process—a system undergoes through a series of equilibrium states, and when the process is reversed, the system passes through exactly the same equilibrium states while returning to its initial state. Externally reversible process—heat transfer between a reservoir and a system is an externally reversible process if the surface of contact between the system and reservoir is at the same temperature.
A process is said to be reversible if it is possible for its effects to be eradicated in the sense that there is some way by which both the system and its surroundings can be exactly restored to their respective initial states. A process is irreversible if there is no way to undo it. That is, there is no means by which the system and its surroundings can be exactly restored to their respective initial states. A system that has undergone an irreversible process is not necessarily precluded from being restored to its initial state. There are many effects whose presence during a process renders it irreversible. These include the following: heat transfer through a finite temperature difference, unrestrained expansion of a gas or liquid to a lower pressure, spontaneous chemical reaction, mixing of matter at different compositions or states, friction, electric current flow through a resistance, magnetization or polarization with hysteresis, inelastic deformation, etc. Irreversibilities can be divided into two classes—internal and external. Internal irreversibilities are those that occur within the system, while external irreversibilities are those that occur within the surroundings, normally the immediate surroundings. For a gas as the system, the work of expansion arises from the force exerted by the system to move the boundary against the resistance offered by the surroundings:
W = ∫ FdX = ∫ PAdX = ∫ PdV
where the force is the product of the moving area and the pressure exerted by the system there. Adx is the change in total volume of the system.
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