Heat can never pass spontaneously from a colder to a hotter body. As a result of this fact, natural processes that involve energy transfer must have one direction, and all natural processes are irreversible. This law also predicts that the entropy of an isolated system always increases with time. Entropy is the measure of the disorder or randomness of energy and matter in a system. The entropy of the 1 kg of water at 0 ^oC is the same if we obtained the water from ice, or if we cooled the water from room temperature down to 0 ^oC.  When a small amount of heat `DeltaQ` is added to a substance at temperature T, without changing its temperature appreciably, the entropy of the substance changes by `DeltaS = DeltaQ/T`.  When heat is removed, the entropy decreases, when heat is added the entropy increases.

Entropy Law is also referred to as the Law of Entropy or the Second law of Thermodynamics.

`DeltaS =  DeltaQ/T`

• S is the change in entropy
• Q is the change in heat content of the system
• T is the temperature of the system

Notes

The second law of thermodynamics indicates that, while many physical processes that satisfy the first law are possible, the only processes that occur in nature are those for which the entropy of the system either remains constant or increases.Entropy, like temperature and pressure, can be explained on both a macro scale and a micro scale. Since thermodynamics deals only with the macro scale, the change in entropy delta S is defined here to be the heat transfer delta Q into the system divided by the temperature T.

In fact physical, chemical, and electrical energy can be completely changed into heat. But the reverse (heat into physical energy, for example) cannot be fully accomplished without outside help or without an inevitable loss of energy in the form of irretrievable heat. This does not mean that the energy is destroyed; it means that it becomes unavailable for producing work. The irreversible increase
 of this non-disposable energy in the universe is measured by the abstract dimension that Clausius in 1865 called entropy.

Entropy in the universe is the total entropy of the universe increase whenever an irreversible process occurs and unchanged when a reversible process occurs.