For example in a refrigerator, heat flows from cold to hot, but only when aided by an external agent (i.e. Informally, "Heat doesn't flow from cold to hot (without work input)", which is true obviously from ordinary experience. Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature. If a system is at equilibrium, by definition no spontaneous processes occur, and therefore the system is at maximum entropy.Ī second formulation, due to Rudolf Clausius, is the simplest formulation of the second law, the heat formulation or Clausius statement: (An exception to this rule is a reversible or "isentropic" process, such as frictionless adiabatic compression.) Processes that decrease the total entropy of the universe are impossible. Thus, while a system can go through some physical process that decreases its own entropy, the entropy of the universe (which includes the system and its surroundings) must increase overall. In a system, a process that occurs will tend to increase the total entropy of the universe. The formulation of the second law that refers to entropy directly is as follows: Thus, the theorems of thermodynamics can be proved using any form of the second law and third law. There are many ways of stating the second law of thermodynamics, but all are equivalent in the sense that each form of the second law logically implies every other form.
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