In thermodynamics, a reversible process
is defined as a process that can be reversed
by inducing infinitesimal changes
to some property of the system, and in so doing leaves no change in either the system or surroundings. During reversible process the entropy
of the system does not increase
and the system is in thermodynamic equilibrium
with its surroundings.
In thermodynamics, an irreversible process
is defined as a process that cannot be reversed, process, that cannot return both the system and the surroundings to their original conditions.
During irreversible process the entropy of the system increases.
A process that eventually returns a system to its initial state is called a cyclic process
. At the conclusion of a cycle, all the properties have the same value they had at the beginning. For such a process, the final state
is the same as the initial state
, and so the total internal energy
change must be zero.
It must be noted, according to the second law of thermodynamics, not all heat provided to a cycle can be transformed into an equal amount of work, some heat rejection must take place. The thermal efficiency, ηth, of any heat engine as the ratio of the work it does, W, to the heat input at the high temperature, QH. ηth = W/QH.