Entropy is a thermodynamic property that is closely associated with the physical quantities of thermal energy and temperature. It is a measure of how much the energy of atoms and molecules become more spread out in a process and can be defined in terms of statistical probabilities of a system or in terms of the other thermodynamic quantities. Changes in the internal energy are closely related to changes in the enthalpy, which is a measure of the heat flow between a system and its surroundings at constant pressure
The Second Law of Thermodynamics and Entropy
The second law of thermodynamics is a physical law based on universal experience concerning heat and energy interconversions. One simple statement of the law is that heat always moves from hotter objects to colder objects (or “downhill”), unless energy in some form is supplied to reverse the direction of heat flow. Another definition is: “Not all heat energy can be converted into work in a cyclic process.”
The second law of thermodynamics in other versions establishes the concept of entropy as a physical property of a thermodynamic system. It can be used to predict whether processes are forbidden despite obeying the requirement of conservation of energy as expressed in the first law of thermodynamics and provides necessary criteria for spontaneous processes. The second law may be formulated by the observation that the entropy of isolated systems left to spontaneous evolution cannot decrease, as they always arrive at a state of thermodynamic equilibrium where the entropy is highest at the given internal energy An increase in the combined entropy of system and surroundings accounts for the irreversibility of natural processes, often referred to in the concept of the arrow of time
Entropy and Information Theory
Entropy has relevance to other areas of mathematics such as combinatorics and machine learning. The definition can be derived from a set of axioms establishing that entropy should be a measure of how informative the average outcome of a variable is. For a continuous random variable, differential entropy is analogous to entropy
Entropy also bears a close relationship to the concept of information entropy (H), which is defined as the average amount of information produced by a stochastic source of data. Information entropy can be seen as a measure of uncertainty or unpredictability in a random variable, and it quantifies the amount of information needed to describe it. Information entropy can also be interpreted as the minimum number of bits required to encode or compress a message without losing any information
Entropy is closely related to something known as Kolmogorov Complexity, which essentially relates to how easily a string can be compressed or described. The Kolmogorov complexity of a string is the length of the shortest possible description or program that produces that string. The Kolmogorov complexity is related to information entropy by the following inequality: H(X) ≤ K(X) + c, where H(X) is the information entropy, K(X) is the Kolmogorov complexity, and c is a constant that depends on the choice of description language.
Conclusion
Entropy is closely related to the second law of thermodynamics, which states that heat flows from hot to cold and that not all heat can be converted into work. Entropy measures how much the energy of a system becomes more spread out or disordered in a process, and it determines whether a process is spontaneous or reversible. Entropy also has applications in information theory, where it quantifies the uncertainty or information content of a random variable, and it is related to the Kolmogorov complexity, which measures how compressible or describable a string is.
