Final answer:
The Second Law of Thermodynamics states that heat naturally flows from hotter to colder objects and cannot spontaneously do the opposite, due to the nature of entropy increase. This law also limits the complete conversion of heat into work without any other effect in a cyclical process.
Step-by-step explanation:
The Second Law of Thermodynamics is a fundamental principle of physics that describes the nature of heat transfer and the limitations of possible energy transformations. According to the Clausius statement of the Second Law, heat never flows spontaneously from a colder object to a hotter object. This principle emphasizes the directionality of heat transfer, which always occurs spontaneously from higher-temperature bodies to lower-temperature bodies, in alignment with increasing entropy. Specifically, entropy tends to increase in a system, as heat transfers from the hot body, with higher entropy change at lower temperatures, to the colder body, leading to an overall increase in entropy for the system.
In practical terms, this law implies that heat engines or refrigerators require external work to transfer heat from cold to hot bodies and cannot fully convert heat into work in a cyclical process without any other effect, as indicated by the Kelvin statement of the Second Law.