A thermodynamic cycle is a series of processes that involve the transfer of heat and work between a system and its surroundings in a way that the system returns to its initial state at the end of the cycle. In other words, a thermodynamic cycle is a closed loop in which a working substance—typically a gas—undergoes various transformations (such as expansion, compression, heating, and cooling) that cause it to absorb and release energy.
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A thermodynamic cycle returns a working substance to its initial state. Its net internal-energy change is zero, so the net work output equals net heat input.