# Questions tagged [entropy]

An important extensive property of all systems in thermodynamics, statistical mechanics, and information theory, quantifying their disorder (randomness), i.e., our lack of information about them. It characterizes the degree to which the energy of the system is *not* available to do useful work.

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### Estimate correlation function by information in the subsystems

Motivation $\newcommand{\expect}[1]{\langle \hat{#1}\rangle}$ Let me start with thoughts I had so far; there is a TLDR with my question at the bottom. Imagine a pure system $\rho$ and with a bipartite ...
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### Is $C _v=T\left(\frac{\partial S}{\partial T}\right)_{V}$ true irrespective of reversibility of the process?

Without specifying if the process is reversible or not my textbook uses the relation $C_{V}=\left(\frac{\partial Q}{\partial T}\right)_{V}=T\left(\frac{\partial S}{\partial T}\right)_{V}$ Is this ...
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### Could a Szilard engine power a car?

Is it possible to scale a Szilard engine up to the point of being able to power everyday things like cars, and if so what might a practical version look like? My understanding is that Szilard engine ...
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### What does “entropy” really mean?

I'm currently taking a course of introduction to biophysics in college and we currently covered a subject called entropy (S). We did some maths based on the Carnot cycle and came up with the following ...
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### Why $C_p=C_v$ for incompressible substances?

I'm learning entropy change for incompressible substances and on the way I saw this equality of $C_p=C_v$. I can't seem to understand this equality and I'm not able to visualize any such example. My ...
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### How to calculate the overall entropy when continuous 2D positions are known?

If I have the positions of particles in continuous 2D space, how do I calculate the entropy of the system? The particles are invariant, I just need some measure of how organised they are, but I'm not ...
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### Definition of quantum information, please!

I've been using the phrase "quantum information" for some time, but am beginning to wonder if the phrase means the same thing to me as to the QM community. Specifically, my understanding is ...
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I think of entropy as energy spatial spreading, but i find that in adiabatic expansion there is energy spreading but entropy does not increase. Is the internal energy decrease a way of energy ...
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