What are the formulations of the second law of thermodynamics?

The second law of thermodynamics can be formulated in several ways. One common formulation states that heat cannot spontaneously flow from a colder body to a hotter body. Another formulation involves entropy, stating that in an isolated system, the total entropy can never decrease over time; instead, it tends to increase, indicating the direction of spontaneous processes. Additionally, it can be expressed in terms of the efficiency of heat engines, where no engine can be 100% efficient in converting heat into work.