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 the information content of a single-particle system?

I'm trying to better understand how information is quantified in the case of a closed system containing a single (fundamental) particle located at position $\vec{r}$ and traveling with a velocity $\...
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Adiabatic piston: why is Callen's argument flawed?

This question is related to this one. I would ask you to read that question and my answer to the question itself before answering this one. The problem is the following. In his book Thermodynamics, ...
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Is this definition of information valid?

First following, "that entropy, is information: indeed,in the micro-canonical language entropy is determined by the number of microstates compatible with a given macrostate." Carlo Rovelli;...
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Is there a difference between a pattern's results and its processes? [closed]

Is there a difference between a pattern's results and its processes? The following link will send you to Wikipedia site where they define Pattern
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An isolated Earth

It is known the fact that there is no way to extract energy (in any form) from any system without introducing some energy. The Earth for example, gets energy from the Sun, from nuclear fusion of ...
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Thermodynamics entropy physics [closed]

At 1C temperature, for ice to change in liquid the change in entropy for system is less negative, then the change in entropy for sorrounding which is more positive, so why is that? Although the bond ...
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Isn't energy absolute according to Thermodynamics?

I was taught that Internal Energy $U$ is a relative quantity: only changes in $U$ are meaningful. It doesn't have an absolute value, since it always comes with an arbitrary constant (for example $U = ...
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Can an actual physical system have zero entropy? [duplicate]

Obviously, a pure quantum state has zero entropy. It appears that in theory, this is correct; but in practice, it is not, because there is always some mixing with the environment. So, zero entropy is ...
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Example of entropy and information

I was trying to write up a basic, intuitive explanation of entropy, and I wanted to see if this example I made up makes sense. Imagine a cell membrane separating our two compartments. It can contain ...
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Is there a minimum energy content of information, other than 0 Joules?

Lets say I want to send the bit string 010110 to someone. Is there a theoretical lower bound on the energy needed to do this?
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Why do we need an explanation for the arrow of time?

I'm reading Sabine Hossenfelder's new book Existential Physics, where she explains that because fundamental laws are time-reversible, we need an explanation for the arrow of time. Why is that exactly? ...
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Entropy change during adiabatic expansion of ideal gas

I was told that the entropy change is $0$ during adiabatic expansion. However, according to $\mathrm dS=\frac{C}{T}\,\mathrm dT$, $\delta S$ is not zero because temperature is not constant during ...
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Why doesn't the entropy increase when two similar gases mix with each other?

Entropy increases when two substances mix with each other. For example, the entropy of mixing of two different gases are given by $$\Delta S= 2Nk\ln\frac{V_f}{V_i}\;.$$ But, the entropy doesn't ...
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What does the no-hair theorem state about the entropy of black holes?

So does the no hair theorem say that all classical black holes will have zero entropy? If yes, why?
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Vapor Compression Refrigeration Cycle

My question here is, why do we need point 2 to be on the saturated liquid line, i.e. why do we need a saturated liquid for the throttling process? Why can we not use it in the liquid region or ...
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Maximum Efficiency of Thermodynamic Cycle [closed]

I am a non-engineering student about to start an MSc on Sustainable Energy and I'm trying to get a head start so I don't fall behind. I would really appreciate some guidance on the following past ...
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Is metaphysics a subject that should replace physics and philosophy? [closed]

I'm really seeking some insight and the following may seem opinionated, but it's really my observations and I could be wrong and I'm completely open to hear thoughts and correction from anyone. If ...
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Boltzmann or Gibbs entropy for the canonical ensemble?

For the microcanonical ensemble the entropy, is given by the Boltzmann entropy which equals: $$S = k_\mathrm{B} \ln(\Omega(E))$$ where $k_\mathrm{B}$ is Boltzmann's constant and $\Omega(E)$ the number ...
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Convert ambient heat into electricity without violating laws?

This is an idea I've been playing around with for a while, even though many people tell me it's impossible. However, as far as I can tell, it does not actually violate the laws of physics. I am ...
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How to understand the difference between standard entropy and generated entropy

Coming from the definition of entropy variation given as follows: $$\Delta S=\int ^{2}_{1}\left( \dfrac {\delta Q}{T}\right) _{r}+\sigma$$ I don't understand the difference between the two parts. ...
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What's wrong with my argument about entanglement entropy in QFT being time-independent?

Let's say we need to compute the entanglement entropy (EE) of a subsystem $A$ ($A=[0,L]$, $L>0$) in a 2D CFT. The density matrix of the total system (i.e., the real axis) is given by $$ \rho(t)=\...
6 votes
10 answers
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Simplest possible thought experiment that illustrates the difference between the past and the future?

We are led to believe that the "arrow of time" is determined by the direction in which entropy increases. Entropy is a measure of disorder, but disorder is very much in the eye of the ...
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Confusion regarding Gibbs' paradox

I am trying to understand the following commentary I found in Wikipedia about this paradox: Now a door in the container wall is opened to allow the gas particles to mix between the containers. No ...
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Are complexity and disorder correlated in entropy?

I am coming from the musical field, but I am looking into the topic of entropy. In many articles from the field of physics, I keep finding what I consider a sort of misunderstanding, but I may be ...
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What are some examples of microscopic quantities?

Mass, volume, energy, entropy, temperature, pressure are some macroscopic quantities. Which means we can think of them even without considering the molecular nature of matter. What are some examples ...
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Do irreversible thermodynamic processes CONSTITUTE time or do they MOVE IN time? [closed]

Time can be associated with irreversibility. A broken egg can't reassemble. Most, if not all, processes are irreversible and this is associated with time going in one direction. Ŕemarkably, living ...
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If in principle all processes are $TPC$ symmetric then what causes the direction of time?

Let's assume all processes are TPC symmetric. Be it the breaking of an egg (just reverse all momenta of all particles involved), the evolution or collapse of the wavefunction (a big if but let's ...
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Maximum entropy: an assumption or a fact

From what I gather (and please correct me if I'm wrong), Jaynes argues that thermodynamic and information entropy are the same since the assumption in statistical thermodynamics that the energy ...
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Entropy and neutral charge

If you have a system $a$ with $N$ particles with charge $+q$ and you put it in contact with a system $b$ that has $N$ particles with charge $-q$, i think the system will evolve to a neutral charged ...
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Isolation vs the Minimum Energy Principle

The Second Law of Thermodynamics says, that in equilibrium the Entropy S of an isolated system is maximized. The First Law of Thermodynamics says that an isolated system has fixed internal energy U. ...
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Conceptually, why is the entropy of black hole related to Planck length?

I was watching a lecture on the holographic principle, and it presented the equation for the entropy of a black hole as $$S_{BH} = \frac{A}{4l_p^2}$$ My question is why, conceptually, the entropy of a ...
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It seems that enthalpy is maximized at equilibrium

For a closed system at constant pressure $dH=TdS+VdP=TdS$. On the other side we have that $dS \ge \delta Q/T$ that is $TdS \ge \delta Q$. So we have $dH \ge \delta Q$ and if the transformation is ...
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In-principle, can we precisely simulate physics in reverse?

A naïve question, I'll admit, about entropy and the various arrows of time. If we have as input the position or momentum (the wave function, I suppose) of every particle, would it then be possible to ...
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Entanglement entropy of the infinite transverse field Ising chain at the critical point

Consider the $1$D transverse-field Ising model, $$H = -\sum_i \sigma_i^z \sigma_{i+1}^z-h\sum_i \sigma_{i}^x$$ at the critical point $h=1$ with periodic boundary conditions. Question: What is the (von ...
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Would Maxwell's demon having infinite memory storage break the second law of thermodynamics?

Apologies if my understanding is wrong, I am literally a child. Maxwell's demon is meant to break the second law of thermodynamics by making a disorderly system orderly (see image below). But the ...
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Conservation of information in expanding universe

As the observable universe expands the amount of information is increasing, at least locally. How is this compatible with the conservation of information in quantum mechanics?
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What would "break" in reality if I had a perpetual motion device?

Let's say aliens came and gave me a small box that outputs more energy than is input. As one example, the box can take current and voltage $I$ and $V$ as input, and output $I$ and $2\cdot V$. Or I can ...
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Entropy of a deterministic reversible system

Suppose a deterministic reversible system evolving from state A of gas located in a small bottle in an otherwise empty room, to state B where the gas is dispersed throughout the room. Why is the ...
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Computation of two-site entanglement entropy of the critical transverse-field Ising chain

The paper Osborne and Nielsen, Phys. Rev. A 66, 032110 gives an exact solution for the two-site reduced density matrix for the transverse-field Ising model with periodic boundary conditions (Eq. 26): $...
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Iterative calculation of Entropy increase when Bottle A charges Bottle B [closed]

I’m doing a classical problem in an iterative manner on a spreadsheet. This is a trial for a more complicated study. As shown in Figure 1, at each opening of the valve, 0.0784 Kg of air (denoted as ...
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Could the passage of time be sinusoidal? [closed]

As we know the universe is moving towards an equilibrium, or high state of entropy. From what I understand, we call this the flow of time. When google announced their time crystal, they call it "...
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Weinberg's proof of Gibbs' $H$-theorem

I'm trying to understand Weinberg's explanation of a general $H$-theorem he attributes to Gibbs (Foundations of Modern Physics, p. 35), but I'm having trouble with the mathematical modeling. The goal ...
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Interpretation of thermodynamics applied to whole universe gets me confused… [closed]

While thinking about thermodynamics, I was confused about the interpretation of its fundamental concept. If we assume the whole universe as a thermal system, and describe all energy transfer as a ...
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Entropy of a non isolated system [closed]

I'm trying to self study thermodynamics. I've stumbled upon a problem on my textbook (which is italian by the way): a real thermal machine based on a Carnot's cycle works between two sources of ...
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Does the entropy of the universe change as expansion exceeds the speed of light?

The potential encoded information in a photon that is at the edge of the observable universe would seem to be lost as the universe expands. Does that loss of information contribute to the overall ...
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Difference between reversible and irreversible heat transfer

Reference books define reversible heat transfer as heat transfer occurs across a infinitesimal thermal gradient i.e. dT and irreversible heat transfer as heat transfer across finite thermal gradient i....
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How can Entropy be maximal when it is undefined everywhere else?

This question is about classical thermodynamics. I learned that when an isolated system is not in equilibrium, its thermodynamic variables such as Entropy are undefined. I also learned that when an ...
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2 answers
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What is entropy change due to mass transfer?

Since entropy is an extensive property, it can change via mass transfer. But entropy change for internally reversible process is defined as $\int\frac{\delta Q}{T}$, which does not account for mass ...
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How much information can be stored in a system with synthetic dimensions?

Okay this is a completely serious question and keep in mind I have a PhD in theoretical condensed matter physics, in which I have somewhat of a specialization in Floquet physics. So as the title says, ...
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Gibbs entropy, Clausius' entropy and irreversibility

I have a bunch of doubts and confusions on the concept of entropy which have been bothering me for a while now. The most important ones are of a more technical nature, arisen from the reading of this ...

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