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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What is "order" and "disorder" in entropy?

What is "disorder" in entropy? Entropy is measurement of "disorder". (Some says it's not "disorder") I had read "disorder" and "order" of entropy in ...
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Understanding entropy [duplicate]

I am currently doing some research on entropy and I am trying to get my head around the concept. One thing that I am getting right now is that entropy is just an application of probability and ...
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What makes procesess internally reversible?

So I read that processes where there is no thermal gradients in the system are internally reversible, but for other cases, do we have to consider the temperature of boundary to calculate entropy ...
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Is there any physical example of a real closed system?

Is there any physical example of a real closed system? I am aware that the whole universe can be considered as a closed system, but I am looking for a smaller example.
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Finding energy of a rubber band

I'm solving a thermodynamics exercise and at one point I'm asked to find the energy if a rubber band. The rubber band has temperaturte $T$ lenght $L$ and tension $J$. I'm given the following ...
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Entropy production rate due to heat transfer

I don't understand how you can derive a formula for the rate of entropy generation due to heat transfer between two sources with temperatures $T_2$ and $T_1$ without varying the temperature themselves....
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Entropy: Disorder or energy dispersal?

The first definition of entropy given by Clausius is I believe this $$S=Q/T$$ It is as I understand a common fact to understand entropy and maybe often teach it as a measure of disorder through the ...
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Thermal noise in suspensions of mesoscopic particles

By considering the steady state mean-squared-displacement of a particle undergoing Langevin dynamics in a harmonic potential well, Einstein was able to relate the diffusion constant of a colloidal ...
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Paradoxical definition of equilibrium using entropy

I've read that equilibrium is the state at which entropy has a maximum. However I found this definition paradoxical because on books entropy is usually defined only for equilibrium states. How is it ...
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Making sense of Entropy being energy over temperature

Over the internet, I found many resources that explain very well what entropy represents. However, the dimensions still appear hard for me to make sense of: energy, simply put, is the ability to do ...
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Must the entropy change produced within a system always be greater than 0?

In McQuarrie's Physical Chemistry text the following is claimed in regard to a system's change in entropy: $$dS=dS_{exch}+dS_{prod}$$ That is, the system's change in entropy can be separated into a ...
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Does $W_{out}$ in gas turbines refer to the shaft or the blades?

I was studying the Bryton cycle for a gas turbine and a question came to my mind about the turbine work output. That is, are we talking about the work output for the turbine blades or about the work ...
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Irreversibility of heat transfer between two reservoirs

In a closed system the entropy change is given by $\Delta s=s_{gen}+\int \delta Q/T$. If $s_{gen}=0$, the state change is called reversible. Why is a heat flux between a hot and a cold reservoirs ...
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Is falling in vacuum reversible or irreversible process?

Process is reversible if you can return to initial state through the same intermediate equillibrium states and cause infinitesimaly small changes in surrounding area. Does that mean that falling in ...
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How is a irreversible process (conventionally) represented on a $T-S$ plane and why cannot it be (really) represented?

A reversible process can be represented on a $T-S$ plane, and the area under the curve is the heat exchanged by the system. On $P-V$ plane a irreversible process is conventionally represented with a ...
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Boltzmann distribution as an explanation for the 2nd law of thermodynamics

The Wikipedia page for Boltzmann distribution describes it as a distribution of microstates as a function of energy and temperature. From my understanding, the 2nd law of thermodynamics is based on ...
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Why doesn't the formula for the entropy change of an isobaric process include a work term?

I tried to derive the entropy change of an isobaric process and arrived at: $\Delta S = C_pln(T_2/T_1)+nRln(V_2/V_1)$ However, it appears the answer is just: $\Delta S = C_pln(T_2/T_1)$ It seems like ...
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2-qubit unitary time evolution entropy

So I have a 2 qubit system with the Hamiltonian $H=J(a^\dagger_1 a_2+a_2^\dagger a_1)$ . The time evolution of the density matrix is with the unitary operator $U(t)=e^{-iHt}$: $\rho(t) = U(t) \rho(0) ...
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How practical is the Landauer limit to actual computing (including distant future)?

Landauer's Principle is often presented as a fundamental limit of efficiency for classical computing. It states that in order to erase one bit of information, at least the following amount of heat has ...
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Clausius inequality leading to absurd result

Background: After deriving Clausius inequality, the author of this book derives the following relation: Consider the cycle shown in the figure in which leg $A \rightarrow B$ is irreversible. In the ...
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How does an increase in entropy increase stability, despite an increase in energy

My understanding of the basics of entropy are as follows: there are more ways for a system to be arranged in a disordered state, than an ordered state. I am happy with that explanation. When it comes ...
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Why is work entropy-free?

These notes I found online state that "work is entropy‐free, and no entropy is transferred with work." I take this to mean that entropy is not generated in a work process. Why is this? Why ...
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Why is the second law of thermodynamics not symmetric with respect to time reversal?

The question might have some misconceptions/ sloppy intuition sorry if that's the case (I'm not a physicist). I seem to have the intuition that given a system of $N$ charged particles in 3D space ...
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On writing the entropy of a system as function of other parameters; Reif exercise 5.14

I am having trouble understanding the proposed solution for a problem taken from "Fundamentals of Statistical And Thermal Physics" by F. Reif. The problem is taken from the chapter 5 - ...
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Entropy - can we express number of microsates as a function parameterized by degrees of freedom?

In some of the answers and comments from this question people contended (not in so many words) that because entropy is parameterized by number of microstates $\Omega$, and the definition of $\Omega$ ...
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When will a physical system have infinite entropy? [duplicate]

What physical system will have infinite entropy? I myself can give an example of a system that has infinitely many elements, an infinite universe will have infinite entropy, or a system that can have ...
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Maximal entropy distribution in a finite doman with random endpoints

I am trying to solve for the distribution of a random variable $x$, which will maximise my entropy in a finite domain, let's say $[0, R]$. $$ S = -\int_0^Rdxp(x)\ln p(x) $$ The distribution that ...
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How do we define entropy? [duplicate]

A few dumb questions about entropy. (I apologize if the answers are already available.) We usually associate entropy with the disorder/ randomness of a body but I don't understand what is this "...
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Is it possible to peform thermodynamic work by harvesting entanglement from the vacuum?

I have already asked two questions on this topic: 1. What happens to the vacuum when entanglement is harvested? [closed] and 2. Does entanglement harvesting require cooling the environment to ...
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Why is the variation of powers of reduced density matrices of an interval given by integrals of the Stress tensor around that interval

I have a questions which seems trivial but I don't know how to handle it properly.. In Cardy and Calabreses derivation of the entanglement entropy of a line (https://arxiv.org/abs/hep-th/0405152) in ...
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An entropy of the Wigner function

Is there an entropy that one can use for the Wigner quasi-probability distribution? (In the sense of a phase-space probability distribution, not - just von Neumann entropy.) One cannot simply use $\...
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What is the entropy change due to work?

The equation $dS=\frac{dQ}{T}$ tells us the entropy change due to heat transfer. Likewise, the equation $dS=\frac{P}{T}dV$ tells us the entropy change due to a volume change. However, I am not sure ...
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Why is heat transfer reversible when temperature difference is infinitesimal?

I don't understand why heat transfer from hot reservoir to the system is considered reversible in this case: $T_{reservoir}$ = $T_{system}$ + dT but it's considered irreversible in this case: $T_{...
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Demonstration of Clausius theorem for irreversible cycles

If we have a generic reversible cycle, we can approximate it with $n$ reversible Carnot cycles like in the pic, and we obtain: $$\sum_{i=1}^n\frac{Q_{i}}{T_{i}}=0$$ When $n \rightarrow \infty$: $$\...
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Entropy: What can(not) be done to a system without changing the rest of the universe?

I listened to this lecture by Elliott Lieb at 4:06 about entropy and I thought about this definition from the slide: "In Thermodynamics it (Entropy) measures what can or cannot be done to a body ...
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Entropy change speed with induced heat flux density and thermally isolated system volume equation

I have a thermal conduction problem. The text reads as follows (i apologize, if my english is not good, I'm not a native speaker) A temperature gradient is generated in a thermally isolated system. ...
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Has a reduction in entropy ever been observed?

On the whole, the entropy of the universe is always increasing. There are far more possible states of "high" entropy than there are of "low" entropy. The example I've seen most ...
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Is it possible for the entropy in an isolated system to decrease?

As far as I can tell, the concept of entropy is a purely statistical one. In my engineering thermodynamics course we were told that the second law of Thermodynamics states that "the entropy of an ...
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An apparent contradiction in the definition of density matrix

Let $A$ and $B$ be two subsystems of the system AB which is in a pure or isolated bipartite state and $$|\psi_{AB}\rangle=\sum_{i}\sqrt{p_i}|\psi^{i}_{AB}\rangle$$, then by definition \begin{equation} ...
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Formula of Entropy

Please, clear my confusion as to why this formula in an exam paper is written this way? Also, is/are there any way to calculate the difference of entropy? If I want to use the property tables of ...
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How do I use the property table for thermodynamics for water?

When to use the saturated water table and when to use the superheated water table? Also, please mention when to use the pressure table and when the temperature table?
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The second law and the advantage of measurement?

Background To the best of my knowledge, almost all living systems make use of information obtained from sensory organs. If this result is a reflection of natural selection, then this fact suggests ...
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Why do reversible processes have to be quasi-static?

I have already read two posts on this topic (here and here) and was still confused on what it is about a quasi-static process that makes it reversible. The answer that has gotten me closest to ...
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How to ascertain entanglement in Heisenberg picture?

It appears to me that the definition of entanglement explicitly refers to the state of the system in the Schrodinger picture, i.e., if a system $\psi\in\mathcal{\otimes_i\mathcal{H}_i}$ is such that $\...
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Calculating the entropy. Where are the borders of a heat engine? (Feynman Lectures)

Quote from Feynman Lectures. Lecture 44. Ch.44-6 Entropy: $$ S_{b}-S_{a}=\int _{a}^{b}{\frac {dQ}{T}}$$ The question is, does the entropy difference depend upon the path taken? There is more than one ...
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Entropy and large-scale structure of the universe

In their article Disturbing Implications of a Cosmological Constant, Leonard Susskind and others write: Certainly, given enough time and a suitable inflaton, recurrences will eventually bring the box ...
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Where is the flaw in this brownian ratchet design?

As I understand, Brownian ratchets do not work because the paddle, which is supposed to keep the gear spinning in a single direction, also oscillates with thermal noise. A the paddle can dissipate the ...
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Informational content of a single symbol

In the communication context, every book i've started to read says: if you want to send a datum about a random experiment $X$, let it be a fair coin toss, that is $$p(tails)=p=0.5; p(heads)=q=0.5$$ ...
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Wien on Temperature and Entropy

As far as I understand, Wien defines entropy, which he uses in his derivation, as $$ S = \text{v} \int\limits_0^{\infty} \varphi(\rho, \nu) d \nu, $$ where $\text{v}$ is the volume occupied by ...
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Why did people think that black holes have zero entropy before the works of Bekenstein and Hawking?

To the extent I know, before Bekenstein and Hawking, the general consensus among physicists was that the black holes have zero temperature and zero entropy. I can possibly understand why people would ...

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