So although energy was conserved in the transfer (the first law), the entropy of the universe has increased in becoming more disordered (the second law) and consequently the availability of energy for doing work has decreased. The hot body loses entropy Q/TH, the cold one gains entropy Q/TC, so the net entropy increase as a result is Q/TCQ/TH, a positive quantity. But the process of increasing the disorder has removed the possibility that the energy that was transferred from A to B can be used for any other purpose - for example, work cannot be extracted from the energy by operating a heat engine between the two reservoirs of different temperatures. So the disorder of the system has increased - and we say the entropy has increased.

When they are put in thermal contact, energy flows from the higher average energy system to the lower average energy system to make the energy of the combined system more uniformly distributed - ie, less ordered.

Q is positive for energy transferred into the system by heat and negative for energy transferred out of the system by heat. The sense of order is associated with the atoms of system A and the atoms of system B being separated by average energy per atom - those of A being the higher energy atoms if system A is at a higher temperature. If we have a process, however, we wish to calculate the change in entropy of that process from an initial state to a final state. The equation for the change in entropy, S, is. The equation for the change in entropy, S, is S Q T, where Q is the heat that transfers energy during a process, and T is the absolute temperature at which the process takes place. When two objects are at different temperatures, the combined systems represent a higher sense of order than when they are in equilibrium with each other. Natural processes will always proceed in the direction that increases the disorder of a system. The second law of thermodynamics is a fundamental principle in physics that deals with the concept of entropy and the direction of natural processes. That disorder can be represented in terms of energy that is not available to be used. Equations relate the entropy change of a system to other properties.

Entropy of the universe always increases Entropy is a measure of the disorder of a system. From the definition of the entropy, it is known that QTdS during a reversible.

Change in entropy formula