What Is Entropy and How to Calculate It

Entropy is defined  as  the quantitative measure of disorder or randomness in a system. The concept comes out of thermodynamics, which deals with the transfer of heat energy within a system. Instead of talking about some form of absolute entropy, physicists generally discuss the change in entropy that takes place in a specific thermodynamic process. Key Takeaways: Calculating Entropy Entropy is a measure of probability and the molecular disorder of a macroscopic system.If each configuration is equally probable, then the entropy is the natural logarithm of the number of configurations, multiplied by Boltzmanns constant: S kB  ln WFor entropy to decrease, you must transfer energy from somewhere outside the system. How to Calculate Entropy In an isothermal process, the change in entropy (delta-S) is the change in heat (Q) divided by the absolute temperature (T): delta-S  Ã‚  Q/T In any reversible thermodynamic process, it can be represented in calculus as the integral from a processs initial state to its  final state of dQ/T. In a more general sense, entropy is a measure of probability and the molecular disorder of a macroscopic system. In a system that can be described by variables, those variables may assume a certain number of configurations. If each configuration is equally probable, then the entropy is the natural logarithm of the number of configurations, multiplied by Boltzmanns constant: S kB  ln W where S is entropy, kB is Boltzmanns constant, ln is the natural logarithm, and W represents the number of possible states. Boltzmanns constant is  equal to 1.38065 Ãâ€" 10−23  J/K. Units of Entropy Entropy is considered to be an extensive property of matter that is  expressed in terms of energy divided by temperature. The SI units of entropy are J/K (joules/degrees Kelvin). Entropy and the Second Law of Thermodynamics One way of stating the second law of thermodynamics is as follows: in any  closed system, the entropy of the system will either remain constant or increase. You can view this as follows: adding heat to a system causes the molecules and atoms to speed up. It may be possible (though tricky) to reverse the process in a closed system without drawing any energy from or releasing energy somewhere else to reach the initial state. You can never get the entire system less energetic than when it started. The energy doesnt have any place to go. For irreversible processes, the combined entropy of the system and its environment always increases. Misconceptions About Entropy This view of the second law of thermodynamics is very popular, and it has been misused. Some argue that the second law of thermodynamics means that a system can never become more orderly. This is untrue. It just means that to become more orderly (for entropy to decrease), you must transfer energy from somewhere outside the system, such as when a pregnant woman draws energy from food to cause the fertilized egg to form into a baby. This is completely in line with the second laws provisions. Entropy is also known as disorder, chaos, and randomness, though all three synonyms are imprecise. Absolute Entropy A related term is absolute entropy, which is denoted by S rather than ΔS. Absolute entropy is defined according to the third law of thermodynamics. Here a constant is applied that makes it so that the entropy at absolute zero is defined to be zero.