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What is entropy?



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Photo: Erich Westendarp /

The term entropy is used in many contexts and is typically defined in different ways in the individual disciplines. Entropy is a statistically defined factor that is not directly measurable – like temperature or pressure, for instance. The term is best known in physics. Classical thermodynamics defines entropy in terms of heat. Heat is a very “disordered” form of energy with high entropy. If heat is added to a system, the degree of disorder in the system – its entropy – will increase. Temperature matters as well: at low temperatures there is a much greater increase in the degree of disorder than at high temperatures. Disordered conditions, rich in entropy, are more probable in nature than ordered, low entropy ones. An ordered system will revert at some point to a disordered state, while a disordered system never spontaneously transitions to an ordered one. For example, stones in a wall are in a highly ordered, low entropy state. This state could only be achieved by the application of energy. Left to itself, such a wall would deteriorate over the course of time. At some point, a disordered, entropic state will prevail. As another example, if perfume is sprayed on a single spot, the smell (due to molecules in a gaseous state) will be concentrated at that spot for only a short time. It soon becomes weak or even unnoticeable. The molecules have distributed themselves evenly in the room, with an accompanying increase in entropy.

Saskia Blank of the Ideas 2020 Team answered this question.