What does the Second Law of Thermodynamics state regarding a system operating in a complete cycle?

The Second Law of Thermodynamics fundamentally establishes that in any thermodynamic process, particularly in a complete cycle, heat cannot spontaneously flow from a colder body to a hotter body. This principle is derived from observations of natural processes and has profound implications for the efficiency and operation of heat engines and refrigerators.

When a system operates within a complete cycle, it undergoes different thermodynamic states, and the Second Law indicates that to transfer heat from a lower temperature to a higher temperature, an external work input is required. This is evident in refrigeration cycles, where work must be done to remove heat from a colder area and reject it to a warmer area, effectively reversing the natural flow of heat.

This concept is foundational for understanding energy conversions in thermal systems. The inability to create a perpetual motion machine, which is suggested in the choices referring to eternal motion without energy loss, aligns with the Second Law, which asserts that energy transformations are inherently inefficient due to entropy production. Therefore, the principle that heat cannot be transferred spontaneously from cold to hot without work encapsulates the essence of the Second Law.