Second Law Thermodynamics

The second law of thermodynamics is an expression of the universal principle of 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; and that the entropy change dS of a system undergoing any infinitesimal reversible process is given by dq / T, where dq is the heat supplied to the system and T is the absolute temperature of the system. In classical thermodynamics, the second law is taken to be a basic postulate, while in statistical thermodynamics, the second law is a consequence of applying the fundamental postulate, also known as the equal a priori probability postulate, to the future while empirically accepting that the past was low entropy, for reasons not yet well understood.

The origin of the second law can be traced 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 flow of caloric (heat) from a hot to cold body (working substance). 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 chemical potential tend to even out in a physical system that is isolated from the outside world. Entropy is a measure of how much this evening-out process has progressed.

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.

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, 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. (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. If a system is at equilibrium, by definition no spontaneous processes occur, and therefore the system is at maximum entropy