Measure of a system's energy that is unavailable for work , or of the degree of a system's disorder.
When heat is added to a system held at constant temperature, the change in entropy is related to the change in energy, the pressure , the temperature, and the change in volume. Its magnitude varies from zero to the total amount of energy in a system. The concept, first proposed in 1850 by the German physicist Rudolf Clausius (18221888), is sometimes presented as the second law of thermodynamics , which states that entropy increases during irreversible processes such as spontaneous mixing of hot and cold gases, uncontrolled expansion of a gas into a vacuum, and combustion of fuel. In popular, nontechnical use, entropy is regarded as a measure of the chaos or randomness of a system.