A important property of all systems in thermodynamics and statistical mechanics. Entropy characterizes the degree to which the energy of the system is *not* available to do useful work

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Entropy increase vs Conservation of information (QM)

Unitarity of quantum mechanics prohibits information destruction. On the other hand, the second law of thermodynamics claims entropy to be increasing. If entropy is to be thought of as a measure of ...
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Can the energy/power required to mix two fluids together be calculated?

Given two fluids; say for example oxygen and nitrogen gases. By simply introducing, again for example 1 liter of each gas into a closed container, the process of diffusion alone will eventually cause ...
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Is amount of entropy subjective?

From all sources I have seen it follows that the proof you can't decrease the amount of entropy in the Universe is given only statistically - the order is just one of the many ways how things can be ...
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Entropy change in Heisenberg picture

If we stick with Heisenberg picture where density matrix $\rho$ is constant, how do we account for entropy increase? I've read the answer to State collapse in the Heisenberg picture but I don't see ...
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Probability of spontaneous Boltzmann brain formation

I was reading through Scott Aaronson's notes (pdf), but I can't make sense of his discussion about Boltzmann brains on p61. Specifically the fact that it says: But the problem is worse. Since in ...
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do relativistic velocities change the apparent entropy content of the moving object?

Imagine a box of hot gas. It has a certain (large) amount of entropy, which we can relate to the amount of information needed to completely specify the position and velocity of every gas particle in ...
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Intuitively, why does removing solutes cost $k_B T$ of free energy per molecule?

I can calculate that if you want to, for example, desalinate water, you will have to pay a free energy cost of $k_B T$ for each ion you remove. In other words, removing an ion from a volume of water ...
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Does a venturi injector contradict the 2nd law of thermodynamics?

The picture below shows a venturi injector. When the water flow enters the narrower cross section its velocity increases and its pressure decreases. Subsequently, fluid is sucked (or injected) into ...
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Does Noether's theorem apply to entropy?

Entropy appears to have a translation symmetry - adding some constant value to it doesn't appear to my fairly rudimentary understanding of physics alter the actual physics. Is this correct? Now ...
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How does the heat death looks like from inside the system?

As this answer points out, any human would first freeze rather than experience the heat death. However, assuming hypothetically that we could make some robot live at such low temperature (or even ...
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Find an expression for S(T,x) from tension and specific heat

I'm working on a problem from a Statistical Mechanics lecture series online, and on the homework, I hit a bump in this problem. Here is the problem set, and I'm asking about #1.c. Short version, we ...
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Is there a definition of relative Renyi entropy?

Is there a Renyi entropy analogue of ``$H(X \vert Y)$" ? If yes then is there any known meaning to that? Googling around I found a few different notions, equation 18 here, ...
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Can I measure the volume of a locked room by pumping gas through keyhole and measuring its entropy?

Suppose that I have a locked room and a keyhole in the door and I want to measure the room's volume. Suppose also that I have some "magical" "artificial" inert gas A that doesn't interact with ...
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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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Entropy and the principle of least action

Is there any link between the law of maximum entropy and the principle of least action. Is it possible to derive one from the other ?
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Can the entropy of a part of a closed system become less than the initial entropy of the entire system?

I am reading Sir Roger Penrose's book "What came before the Big Bang?". Here, I came across a chapter where he talks about how sunlight provides us energy in low entropy and how through various ...
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Relation between Von Neumann entropy (and other entanglement measures) and thermodynamical entropy

Suppose I have a bipartite system (with Hilbert space $H = H_a \times H_b$) and the following state: $$\sigma = \sum_{n} \frac{e^{-\beta E_n}}{Z} \rho_n$$ where $Z = \sum_n e^{- \beta E_n}$ and ...
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Gough-Joule effect in rubber band

When a stretched rubber band is heated, surprisingly, its tension will increase. What is the physics behind this?! Another thing. The Entropic spring model says, for a good specimen, that the ...
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Calculation of entropy change in irreversible cycles, meaning of $\delta Q/T$ in irreversible processes

Lets take the two cycles in the pictures working with an ideal gas. We perform one, and than perform the other. The cycle is made reversible by making the gas exchange heat with a heat bath having ...
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What is known about Renyi entropy of a probability density function?

I see most discussions about Renyi entropy to be using either of these two kinds of definitions, for $\alpha > 0, \alpha \neq 1$ $H_{\alpha}(p_i)=\frac{1}{1-\alpha}\log \sum p_i^{\alpha}$ for a ...
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Entropy of the Sun

Is it possible to measure or calculate the total entropy of the Sun? Assuming it changes over time, what are its current first and second derivatives w.r.t. time? What is our prediction on its ...
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What is reversible and irreversible process? Can process below even be irreversible?

I heard the definitions, but I have much trouble understanding them. My teacher told me that in an example with two chambers and gas particles (look below) entropy increases, but I cannot understand ...
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Unmixing of gases: What is the relevant temperature for my Entropy?

This answer to a question about unmixing gases states: However, let us now assume we have to hand a source of mechanical work, and a large heat reservoir at temperature $T$. I'll assume that ...
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How can there be entropy change in this system?

How can there be an entropy change in this system? Suppose if I have a system consisting of liquid water, $1\, \mathrm{kg}$ at $290\,\mathrm{K}$, I stir it, and do say, $10\, \mathrm{J}$ of work on ...
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What is the entropy of a pure state?

Well, zero of course. Because $S = -\text{tr}(\rho \ln \rho)$ and $\rho$ for a pure state gives zero entropy. But... all quantum states are really pure states right? A mixed state just describes ...
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Entropy change in a reversible carnot cycle with 2 beakers

Consider 2 beakers each containing 1kg of water. One beaker has initial temperature of 25 degrees celsius and the other at 100 degrees celsius. The beaker are now mixed. Assuming no heat exchanged is ...
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Deriving the partition function in MaxEnt

I'm trying to understand this paper on Maximum Entropy by Jaynes, and am stuck on something which should be rather simple. We're attempting to maximize the entropy $-\sum_i p_i \ln(p_i)$ subject to ...
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Is there a relationship between force and entropy?

Is there any relation between force and entropy? If any, I would like to know what the equation of this relation will be. What is the entropy of strong and weak nuclear force? Can we determine it? If ...
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What is the entropy of strong and weak nuclear forces? [duplicate]

Can we determine the entropy of strong and weak nuclear forces? If so, what will be the formula to determine the strong and weak force entropy? Thank you :)
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Are reversible adiabatic processes always isentropic?

If my understanding is correct, neither reversible nor adiabatic processes are necessarily isentropic. But are reversible adiabatic processes always isentropic?
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Resonance in Benzene violating Second Law of Thermodynamics

In chemistry a few months ago we were taught the resonant structure of benzene, that states the double bonds upon the six carbon atoms flicker back and forth between the two possible states it can be ...
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What perpetuates change? [duplicate]

People say that our notion of "time" comes from entropy which in turn is just a large scale effect of the laws of physics, but what is it that perpetuates the laws of physics from one state into the ...
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Link between Hawking-Bekenstein Black hole entropy and entanglement entropy

I'm currently doing a project on two sided Ads-Schwarzschild black holes in the context of Ads/CFT. I want to show that the entanglement entropy between the two CFTs corresponds approximately to the ...
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Entropy of radiation emitted into space

In several papers I see something equivalent to the following expression for the entropy of radiation given by an astronomical object such as the Sun (assuming the object can be approximated as a ...
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Entropy Change in an irreversible process

I have just started learning thermodynamics and the concept of entropy confuses me. Suppose I have a gas in a cylindrical container fitted with a piston. I take it through an adiabatic irreversible ...
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How can a reversible adiabatic expansion not increase entropy?

In the second stage of the Carnot cycle, a gas is thermally insulated and allowed to expand and do work on the piston. I understand the reason people give is that because entropy is $\,dS = \,dQ/T$ ...
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clausious inquality

Take a reservoir which gains a heat Q irreversibly at temp T from a surrounding at temp T0 then the entropy change of reservoir is given by Q/t and that of the surrounding is -Q/T0...How is this ...
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entropy change during irreversible heat loss [duplicate]

When a thermodynamic system gains heat if the heat gain is irreversible then there is a entropy generation along with the entropy transfer..so is there a similar entropy generation associated with ...
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How does the introduction of living things into a closed system affect the rate of change of entropy?

Does the introduction of living things into a closed system increase or decrease the overall rate of change of entropy of a system?
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How to measure Entropy?

While $$\delta Q=T\ dS$$ allows for obtaining the entropy change of an isolated system in equilibrium by measuring the heat exchange with the environment, I was wondering whether there are ways to ...
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What is the resolution to Gibb's paradox?

This question is essentially a duplicate of Gibbs Paradox - why should the change in entropy be zero?. The question concerns the following situation: I have some gas of identical particles and they ...
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Gibbs Paradox - why should the change in entropy be zero?

The Gibbs paradox deals with the fact that for an ideal gas with $N$ molecules in a volume $V$ seperated by a diaphragm into two subvolumes $V_1,V_2$ with $N_1,N_2$ particles in each subvolume, ...
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Why does maximal entropy imply equilibrium?

From a purely thermodynamical point of view, why does that entropy have to be a maximum at equilibrium? Say there is equilibrium, i.e. no net heat flow, why can the entropy not be sitting at a ...
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Transfer of heat through a finite temprature difference

Consider the following derivation There are two reservoirs at temp A & B at temp T1 & T2 .They are connected by a metal rod such that T1 > T2. Q amonut of heat is transfered ...
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How does dark matter collapse?: Entropy considerations

Inspired by this question. I believe that the usual explanation that preserves the second law of thermodynamics as an astrophysical gas cloud collapses under gravity is that the gas must heat and ...
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Entropy Change for a Thermally Isolated System

Clausius' Theorem states that $\int\frac{dQ}{T}\leq0$ for a closed cycle, with equality for a reversible cycle. Suppose we wish to take our system around a closed cycle such that the path from A to B ...
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2answers
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How does gravity increase (or, at least, not-decrease) entropy?

I'm a total physics n00b (i.e. I only know the physics as taught in IT grades, and don't remember much of it), and was talking about entropy (initially, not with the physical implications). My friend ...
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Do some of the laws of thermodynamics break down in quantum mechanics?

I do not know if this is a stupid question as I am not an expert in thermodynamics and certainly no expert in QM. So, we know that the laws of thermodynamics are laws based on statistics. They ...
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Second Law of Thermodynamics and the Arrow of Time: Why isn't time considered fundamental?

I've come across this explanation that the "arrow of time" is a consequence of the second law of thermodynamics, which says that the entropy of an isolated system is always increasing. The argument is ...