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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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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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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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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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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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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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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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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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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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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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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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 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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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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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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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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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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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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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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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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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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Entropy as a multiplicative measure of disorder as opposed to an additive one. Probability distribution?

Entropy $S,$ is usually defined as an additive measure because mathematically and physically speaking it's usually easier to work with. I'm wondering how to write entropy as a multiplicative measure......
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Normalization of the partition function

I am used to think that multiplying the partition function by a constant is meaningless, and shouldn't change any calculation, as under $Z' = A Z = A \sum_n \exp(-\beta E_n)$ for an arbitrary $A$, the ...
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A 'better' definition of time [closed]

I have been annoyed by official definitions of time from many Google searches and decided after listening to Sean Carroll's podcast to write my own. Here is what I did so far: What is time? A state ...
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Does "very non-extensive" quantum matter exist?

This is a two-fold question: I have read that the entropy of a black hole is proportional to its surface area. It left me wondering if it is possible to create some (non-trivial) quantum matter with ...
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Entropy of perfect gas function of internal energy and number of particles [closed]

In a statistical physics lecture, I found an equation describing the entropy $S$ of an ideal gas and it is said that equation can be obtained thanks to elementary thermodynamics formulae. The equation ...
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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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How can free expansion be truly irreversible if particles have a small chance of returning to their original state?

According to Halliday-Resnick, a free expansion of a gas is an irreversible process. However, the text continues that in a system of particles in a box, it is possible (though very unlikely) for a ...
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Black hole horizon : entropy or surface tension?

Usually (from Hawking's calculations), a black hole has entropy defined by its horizon's area: $$S = \frac{A}{4G}, \tag{1}$$ where $A = 4 \pi r_H^2$ is the area. From thermodynamics: $$dE \equiv dM = ...
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What's wrong with the Maxwell demon? Seems very trivial consequence of the second law of Thermodynamics...?

Maxwell Demon's Paradom: The entropy production of the system $X$ becomes negative: $$\Delta S(X) + Q_X \leq 0$$ Exorcism of the demon: I believe that this paradox has been resolved by an inequalities ...
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Why is quantum entanglement associated with increase rather then decrease of a system's entropy? [duplicate]

As far as i understand quantum entanglement (and I'm far from being an expert on it), it is a phenomena involving two or more subatomic particles that reflects itself in immediate correlation between ...
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Does an irreversible thermodynamic process always increase the entropy?

I know that if over the a given thermodynamic process the entropy of the universe increases, then that process is irreversible. But does the opposite also hold true, i.e. do all irreversible processes ...
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Reaction entropy in internal combustion engines

In air-standard analysis of internal combustion engines, the combustion phase is modeled as a heat input providing an energy corresponding to the reaction enthalpy. The entropy variation is calculated ...
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Quantum Entanglement and Informational Bonding [duplicate]

We know that information has a time to follow its trajectory to the other medium in question, gravitational waves travel exactly at the speed of light, radio waves and radiation propagate at the same ...
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How does a system with two black bodies connected via a circulator avoid violating the second law of thermodynamics

Consider two different temperature black bodies connected by an ideal circulator (or similar passive and directional device) such that thermal radiation emitted from the cooler black body is able to ...
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Gibbs energy and compression ratio

I came across a very interesting Gibbs energy-based interpretation about the importance of compressing the fuel before combustion in an engine (Kennesaw State University, [1]). However, while the ...
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What is the correct entropy density of the cosmic vacuum?

Which of the following two arguments is correct? (1) The total entropy of the cosmic vacuum should be the same as the entropy of the cosmological horizon (with radius $R$). The horizon entropy $S$ is ...
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How can the entropy of an isolated system possibly change (if we consider the fundamental equation)?

By the definition of an isolated system, $dE = dN = dV = 0$ (energy, no. of particles, and volume). If we assume that a macrostate has a unique ($E$, $V$, $N$) then we can write $$dS = \frac1T\cdot dE ...
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Possibility of Violating Second Law of Thermodynamics

A common climate change denier argument is that back-radiation proposed by climate-change theory violates the second law of thermodynamics because it implies that a cold body warms a hot body. My ...
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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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