The Thermodynamics Table lists the entropies of some substances at 25 вњ¬. Continue this process until you reach the temperature for which you want to know the entropy of a substance (25 вњ¬ is a common temperature for reporting the entropy of a substance). Then you can use equation (1) to calculate the entropy changes. Even though equation (1) only works when the temperature is constant, it is approximately correct when the temperature change is small. Now start introducing small amounts of heat and measuring the temperature change. Since there is no disorder in this state, the entropy can be defined as zero. Imagine cooling the substance to absolute zero and forming a perfect crystal (no holes, all the atoms in their exact place in the crystal lattice). The absolute entropy of any substance can be calculated using equation (1) in the following way. At absolute 0 (0 K), all atomic motion ceases and the disorder in a substance is zero. On this scale, zero is the theoretically lowest possible temperature that any substance can reach.

The temperature in this equation must be measured on the absolute, or Kelvin temperature scale. Using this equation it is possible to measure entropy changes using a calorimeter. Where S represents entropy, DS represents the change in entropy, q represents heat transfer, and T is the temperature. This causes irreversiblities inside the system and an increase in its entropy.One useful way of measuring entropy is by the following equation: Internal changes in the system: Due to internal changes in the movements of the molecules of the system there is further disturbance inside the system. Any change in the heat content of the system leads to disturbance in the system, which tends to increase the entropy of the system. Here are the various causes of the increase in entropy of the closed system are:Дue to external interaction: In closed system the mass of the system remains constant but it can exchange the heat with surroundings. In actual practice whenever there is change in the state of the system the entropy of the system increases. In actual practice the reversible isentropic process never really occurs, it is only an ideal process. Causes of the Increase in Entropy of the System This also implies that the process which is reversible and adiabatic is always an isentropic process. For the isentropic process to be reversible, it is essential that it is adiabatic process also, if it is not adiabatic, then the isentropic process cannot be reversible. The isentropic process can be reversible or irreversible. Thus during isentropic process the value of в€†S=0. During isentropic process the value of entropy of the system at initial and final state remains constant. The process during which the entropy of the system remains constant is called as isentropic process. If at all it is necessary to measure the value of the entropy at a particular state of the system, then zero value of entropy is assigned to the previous chosen state of the system. Thus the entropy is always measured as the change in entropy of the system denoted by в€†S and not merely S. The entropy of the system is measured in terms of the changes the system has undergone from the previous state to the final state. The entropy of the system is not measured in absolute terms rather it is measured in relative terms.

The subject matter of entropy has been developed due to invaluable contributions from Sadi Carnot, James Prescott Joule, William Thomson (Lord Kelvin), Rudolf Clausius, Max Planck and others. The property вЂentropyвЂ™ plays central role in the study of thermodynamics and it has been introduced via the concept of the working of the heat engine. Entropy is denoted by вЂSвЂ™, while specific entropy is denoted by вЂsвЂ™ in all mathematical calculations. Entropy is heat or energy change per degree Kelvin temperature. Entropy is the extensive property of the system (depends on the mass of the system) and its unit of measurement is J/K (Joule per degree Kelvin). It is taken from the Greek word вЂtropeeвЂ™ which means transformation. The word entropy was first used by Rudolf Clausius. There are three important EвЂ™s in the study of the thermodynamics: energy, equilibrium and entropy.