Revised 24 May 2019
In a low entropy system the energy is free in the sense that it is available for producing mechanical work, whereas in a high entropy system the energy is said to be bound. Lord Kelvin first developed the principle of the degradation of energy which states energy is continually becoming unavailable for work because all natural processes dissipate energy to heat. Within the isolated system of the Universe there is a continuous and irrevocable degradation of free into bound energy.
The change in entropy for reversible and irreversible processes is formalized by the Second Law of Thermodynamics, the law of entropy, which states:
When all systems taking part in a process are included, the entropy S of the total system either remains constant or increases.
In mathematical terms:
ΔS (Universe) 0
An open system may exchange both energy and matter with the outside, whereas a closed system exchanges only energy and not matter with the outside. Although the entropy of an isolated system remains either constant or increases, the entropy of open sub-systems may decrease. Life forms are an example of low entropy systems which, in order to retain the form of low entropy, need to continually take in energy and matter in the form of food and consume more than one unit of food in order to gain one unit in weight. During periods of increase in size and numbers, the entropy of living organisms decreases while the entropy of the combined system, which includes their life support systems, simultaneously increases. Contrary to being a violation to the law of entropy, life forms comprise systems that hasten the increase of entropy in the universe (Schrödinger, 1967, pp. 73-7).