Second Law of Thermodynamics
All processes obey the 1st Law of Thermodynamics. However, some 1st law processes never occur. For example, heat transfer from cold reservoir to hot reservoir or flow from low pressure to high pressure.
The 2nd Law of Thermodynamics defines:
• direction of change for processes
• final equilibrium for spontaneous processes
• criterion for theoretical performance limits of cycles
• quality of energy
Energy changes and transfer involves both conservation principle and degradation in quality. Therefore, the thermal efficiency of all heat engines must be less than 100% due to dissipative effects. Processes occurring in a system such as heat engine are irreversible since either the system or its surroundings cannot be returned to their initial states. A reversible process is an idealization.
Heat engines (heat pumps) are closed systems, which operates continuously, or cyclically, and produce (use) work while exchanging heat across its boundaries.