a thermodynamic cycle is a series of thermodynamic processes which returns a system to its initial state. properties depend only on the thermodynamic state and thus do not change over a cycle. variables such as heat and work are not zero over a cycle, but rather depend on the process. the first law of thermodynamics dictates that the net heat input is equal to the net work output over any cycle. the repeating nature of the process path allows for continuous operation, making the cycle an important concept in thermodynamics.