Entropy

Entropy is a thermodynamic quantity whose value depends on the physical state or condition of a system. It is useful in physics as a means of expressing the Second Law of Thermodynamics. That is, while the law may be stated in terms of it being impossible to extract heat from a reservoir and convert it totally to usable work, in terms of entropy the law states that any changes occurring in a system that is thermally isolated from its surroundings are such that its entropy never decreases.

This behavior corresponds to the fact that entropy is a measure of the disorder of a system. On average all of nature proceeds to a greater state of disorder. Examples of irreversible progression to disorder are pervasive in the world and in everyday experience. Bread crumbs will never gather back into the loaf. Helium atoms that escape from a balloon never return. A drop of ink placed in a glass of water will uniformly color the entire glass and never assemble into its original shape.

Entropy as a measure of disorder can be shown to depend on the probability that the particles of a system are in a given state of order. The tendency for entropy to increase occurs because the number of possible states of disorder that a system can assume is greater than the number of more ordered states, making a state of disorder more probable. For example, the entropy of the ordered state of the water molecules in ice crystal is less than it is when the crystal is melted to liquid water. The entropy difference involved corresponds to the transfer of heat to the crystal in order to melt it.

It may appear that there are exceptions to the general rule of ultimate progression to disorder; the growth of crystals, plants, animals, and humans are all remarkable examples of order or organization. However, these are open systems that exchange matter and energy with their surroundings for their growth and sustenance. If a composite of the system plus its environment is considered, then it can always be shown that its entropy will never decrease, as long as the composite system is isolated.

Entropy is defined in physics as the ratio of the heat absorbed by a system to its absolute temperature (i.e., temperature based on the Kelvin scale). When a certain amount of heat passes to a system from one at a higher temperature, the entropy of the two systems combined increases. This is an irreversible process characterizing the general tendency of matter to seek temperature equilibrium, a state of maximum entropy or disorder.

This progressive tendency of nature toward disorder has been considered by many scholars as one of the primal natural processes that serve as a gauge for the irreversible nature of time. Accordingly, a considerable number have identified the relentless increase of entropy with what they term the thermodynamic arrow of time. In addition, the degradation associated with the increase of entropy has been discussed by some scholars of science and religion as a meaningful metaphor for evil.

See also DISORDER; THERMODYNAMICS, SECOND LAW OF

LAWRENCE W. FAGG