Entropy
Entropy, a measure of the disorder in a system containing energy or information. The less ordered a system is, the greater is its entropy. The concept of entropy is useful in the physical sciences, particularly in thermodynamics (the study of heat processes). It is also useful in communications and other fields.
In thermodynamics, entropy is given in terms of the heat transferred to or from a substance at a given temperature. In communications theory, entropy is a measure of the information that is lost in the transmission of signals; messages tend to become distorted or degraded because of random errors called noise.
A general characteristic of natural processes is that they tend to lead to an increase in entropythat is, to greater disorder. This tendency occurs because there are many more random arrangements than ordered arrangements possible for the parts of any system. For example, when a box containing coins arranged in rows is shaken, the coins will come to rest in a disorganized pattern since so many random positions for the coins are possible.
In a physical system containing a warm substance and a cool one, heat will flow naturally from the warmer to the cooler substance. This principle, known as the Second Law of Thermodynamics, can be explained in terms of entropy. As a substance is heated, its entropy increases because the motion of its molecules increases and becomes more random. Heat flows naturally from the warmer to the cooler substance because the heating of the cooler substance increases the total entropy of the system.
In the conversion of energy into different forms or in the use of energy to produce work, some of the energy is wasted as heat produced by friction or some other means. This waste heat causes an increase in entropy. A system's entropy, therefore, indicates the amount of heat energy not available for doing work.
Although the entropy of a given system can decrease, it can do so only by increasing the entropy of a related system by the same or a greater amount. For example, as a refrigerator cools the air inside it, the air's entropy decreases; however, the heat released by the refrigerator in the process increases the entropy of the outside air by a greater amount. The universe is a self-contained system whose entropy is continually increasing. For this reason, scientists believe that the universe may reach a state of maximum entropy in which all matter has a uniform temperature.