The fact that the absolute value of specific entropy is unknown is not a problem because it is the change in specific entropy (Δs) and not the absolute value that is important in practical problems. Given a discrete random variable, which takes values in the alphabet and is distributed according to : where denotes the sum over the variables possible values. For example, the specific entropy of water or steam is given using the reference that the specific entropy of water is zero at 32☏. In information theory, the entropy of a random variable is the average level of 'information', 'surprise', or 'uncertainty' inherent to the variables possible outcomes. Also, like enthalpy, the entropy of a substance is given with respect to some reference value. Further confusions are produced by some attempts to generalize entropy with similar but not the same concepts in other disciplines. Like enthalpy, entropy cannot be measured directly. Entropy is the most used and often abused concept in science, but also in philosophy and society.

Entropy is represented by the letter S and can be defined as ΔS in the following relationships. Entropy is sometimes referred to as a measure of the inability to do work for a given heat transferred. Because entropy tells so much about the usefulness of an amount of heat transferred in performing work, the steam tables include values of specific entropy (s = S/m) as part of the information tabulated. Entropy quantifies the energy of a substance that is no longer available to perform useful work. Because entropy is a property, changes in it can be determined by knowing the initial and final conditions of a substance. If we add the same quantity of heat at a higher temperature and a lower temperature, randomness will be maximum at a lower temperature. The entropy formula is given as follows: S q rev,iso /T. > Thermodynamics Directory | Heat Transfer DirectoryĮntropy Definition - Thermodynamic PropertiesĮntropy (S) is a property of a substance, as are pressure, temperature, volume, and enthalpy. During entropy change, a process is defined as the amount of heat emitted or absorbed isothermally and reversibly divided by the absolute temperature. Entropy Definition and Equation Thermodynamics

Absolute entropy definition