Thermodynamic cycle is based on 2nd law of thermodynamics.
There are three laws of thermodynamics, not inlcluding the understood fourth law. Thermodynamics is the study of how energy is transferred in a system.
zeroth law forms the basis for first law of thermodynamics
relationship between the thermodynamic quantity entropy
1st law
Empirical scales are based on the measurement of physical parameters that express the property of interest to be measured trough some formal, most commonly a simple linear, functional relationship. For the measurement of temperature, the formal definition of thermal equilibrium in terms of the thermodynamic coordinate spaces of thermodynamic systems, expressed in the zeroth law of thermodynamics, provides the framework to measure temperature.
* First law of thermodynamics, about the conservation of energy: : :: The change in the internal energy of a closed thermodynamic system is equal to the sum of the amount of heat energy supplied to the system and the work done on the system. --from Wikipedia
The second law of thermodynamics, generally stated, is that the entropy of an isolated system always increases in any natural process where change occurs. In a system at equilibrium, of course, the entropy remains constant.
heat being completelyconverted into mechanical energy
There were several mathematical ideas which interacted with thermodynamics. By far the most important of these is statistics.Some physicists believe that the Second Law of Thermodynamics is the one physical law of whose validity we can be totally certain, because it is based on statistics.
It forbids heat to move from a cold region to a hot regions spontaneously (you have to "pump" it there - meaning you have to do work to get it to move that direction). Alternatively - it forbids any natural/spontaneous process to DECREASE the entropy of the universe.
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The 1st Law of thermodynamics is a restatement of the law of conservation of energy.