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The smaller the scale, the less the second law applies. Thermodynamics is a theory of macroscopic systems and therefore the second law applies only to macroscopic systems with well-defined temperatures. Thus, a heat engine with 100% efficiency is thermodynamically impossible. At least some of the energy must be passed on to heat a low-temperature energy sink. That is, it is impossible to extract energy by heat from a high-temperature energy source and then convert all of the energy into work. The mathematics involved in such an event are described by fluctuation theorem.Ī third formulation of the second law, by Lord Kelvin, is the heat engine formulation, or Kelvin statement: It is impossible to convert heat completely into work. Such events have been observed at a small enough scale where the likelyhood of such a thing happening is large enough. The exception to this is in statistically unlikely events where hot particles will "steal" the energy of cold particles enough that the cold side gets colder and the hot side gets hotter, for an instant. For example, the electrical energy going into a refrigerator is converted to heat and goes out the back, representing a net increase in entropy. This can happen in a non-isolated system if entropy is created elsewhere, such that the "total" entropy is constant or increasing, as required by the second law. Note that from the mathematical definition of entropy, a process in which heat flows from cold to hot has decreasing entropy. 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 obviously true from everyday experience. If a system is at equilibrium, by definition no spontaneous processes occur, and therefore the system is at maximum entropy.Īlso, due to Rudolf Clausius, is the simplest formulation of the second law, the heat formulation or Clausius statement: Heat generally cannot spontaneously flow from a material at lower temperature to a material at higher temperature. (An exception to this rule is a reversible or "isentropic" process, such as frictionless adiabatic compression.) Processes that decrease total entropy of the universe are impossible. Thus, while a system can undergo some physical process that decreases its own entropy, the entropy of the universe (which includes the system and its surroundings) must increase overall. The formulation of the second law that refers to entropy directly is as follows: In a system, a process that occurs will tend to increase the total entropy of the universe. 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. There are many versions of the second law, but they all have the same effect, which is to explain the phenomenon of irreversibility in nature. Entropy is a measure of how far along this evening-out process has progressed. In simple terms, the second law is an expression of the fact that over time, ignoring the effects of self-gravity, differences in temperature, pressure, and density tend to even out in a physical system that is isolated from the outside world. The second law traces its origin to French physicist Sadi Carnot's 1824 paper " Reflections on the Motive Power of Fire", which presented the view that motive power ( work) is due to the fall of caloric ( heat) from a hot to cold body ( working substance). The second law of thermodynamics is an expression of the universal law of increasing entropy, stating that the entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium.