# Tagged Questions

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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### Mixed quantum states and “complete knowledge of the system”

I ran across this statement in a professor's notes and I think it's just a typo, but I wanted to take the opportunity to check my understanding. So in his notes it says: even if we have complete ...
1answer
190 views

### Can exergy and exergy destruction be understood through thermodynamical and/or statistical-mechanical principles?

My textbook Fundamentals of engineering thermodynamics, Moran and Shapiro states: The exergy is the maximum theoretical work obtainable for an overall system consisting of a system and the ...
1answer
319 views

### Entropy with infinite baths

I'm struggling with the following problem: (Stephen J. Blundell, Concepts in Thermal Physics S. 154 Problem 14.5): A block of lead of heat capacity 1kJ/K is cooled from 200K to 100K in two ...
1answer
383 views

### How certain is the heat death of the universe?

According to our current scientific knowledge, how certain is it that heat death shall be the ultimate fate of our universe, and why? Are there any serious hypotheses competing with heat death, and if ...
1answer
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### Why the dissolution of hydrophobic compounds decreases the entropy of water molecules in the vicinity of the solute?

The following is a quote from Lehninger's Principles of Biochemistry, 4th edition, pg.52: (...) dissolving hydrophobic compounds in water produces a measurable decrease in entropy. Water ...
1answer
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### Can anyone explain the idea behind dS ∝ dV/V?

In a lecture on entropy, one of the equations $dS ∝ \frac {dV}{V}$ was explained as "a fractional change in volume as a measure of the increase in randomness" (related to $\frac{dQ}{T}$) How does ...
3answers
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### Is the flatness of space a measure of entropy?

This is a bit quirky: For a very long time I've found Stephen Hawking's evaporating small black holes a lot more reasonable and intuitive than large black holes. The main reason is that gravity is ...
0answers
158 views

### Gauge fixing the Einstein's gravity action

This is in reference to this paper, arXiv:1204.4061. I was wondering if someone can give me a reference which explains this gravitational gauge fixing that they have done in $2.10$ and how that ...
2answers
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### How irreversible processes are possible?

Susskind says that all laws of mechanics are reversible and any valid mechanic law most be reversible: you can always determine the previous state of any physically valid system. However, the simplest ...
1answer
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### In calculating entropy, why can the partitioning of an ensemble into microstates be chosen “somewhat arbitrarily”?

I'm confused by statistical entropy. It seems to me like the number of microstates for a given macrostate would increase without bound as finer partitionings of the phase space are chosen. Why is it ...
2answers
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### What is the effect of phase transition on the thermodynamic state variables of a material?

When a material undergoes a phase transition, it releases an amount of heat (under a specific temperature). So the effect of the phase transition on entropy would be equal to: \begin{align} ...
1answer
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### Explain entropy (again)

I think I understand entropy finally. Will you verify for me? $$S = k_B \ln( \Omega)$$ where $\Omega$ (the multiplicity) is the degeneracy of the system at some energy (E)? So if the system is a ...
1answer
303 views

### Will entropy continue to increase even if the universe begins to contract?

If the universe is heading for a big crunch, when the universe starts to collapse will entropy decrease and the arrow of time consequently reverse or not? I'm interested in the explanations, not just ...
1answer
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### Are there anti entropic agents [duplicate]

The entropy of an isolated system always increases, Considering an intelligent actor in the system who can organise different objects in the system, doesnt the measure of disorder reduce albeit for ...
1answer
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### Motivation for the use of Tsallis entropy

Every now and again I hear something about Tsallis entropy, $$S_q(\{p_i\}) = \frac{1}{q-1}\left( 1- \sum_i p_i^q \right), \tag{1}$$ and I decided to finally get around to investigating it. I haven't ...
8answers
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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 ...
3answers
354 views

### How does that Boltzmann distribution interact with entropy?

In an ideal gas, the Boltzmann distribution predicts a distribution of particle energies $E_i$ proportional to $ge^{-E_i/k_bT}$. But, doesn't entropy dictate that the system will always progress ...
2answers
284 views

### Bekenstein entropy black hole v.s Hawking entropy black hole

Historically, Bekenstein estimated the entropy associated with a black hole in 1973, obtaining: $$S_B = \frac{\ln(2)k_Bc^3}{8\pi\hbar G}A.$$ He already acknowledges in his article that his estimates ...
1answer
497 views

### How does Landauer's Principle apply in quantum (and generally reversible) computing

I understand that a reversible computer does not dissipate heat through the Landauer's principle whilst running - the memory state at all times is a bijective function of the state at any other time. ...
3answers
339 views

### Is there really such a thing as an irreversible process?

If an isolated system goes from a state A to B, will it always eventually fluctuate back to state A? If not, give an simple example. Is it right to say that entropy only says that the probability ...
2answers
438 views

### Why doesnt this violate 2nd law of thermodynamics?

Consider an ideal gas in a cylindrical container in a gravitational field, with a piston on top pushing down by gravity. The piston has some locking mechanism that locks it in place if it is displaced ...
1answer
369 views

### Volume quotient in Carnot-cycle

Problem: One kilomole of an ideal, monatomic gas undergoes a reversible Carnot-processes between temperatures 300 °C and 20 °C. The work done during one cycle is 1500 kJ. a) Find the ...
0answers
328 views

### Theoretical or experimental violations of the 2nd Law of Thermodynamics? [closed]

Theoretical challenges to the 2nd Law? What are some the theoretical challenges to the 2nd Law? (cf. Čápek, Vladislav, and Daniel P. Sheehan. Challenges to the Second Law of Thermodynamics: Theory ...
3answers
695 views

### Intuitive understanding of the entropy equation

In thermodynamics, entropy is defined as $d S = \dfrac{\delta q_{\rm }}{T}$. This definition guarantees that heat will transfer from hot to cold, which is the second law of thermodynamics. But, why ...
1answer
584 views

### What is life?'' by a physicist definition [closed]

The question is about defining What is life?'' in the field of Physics. Whether there is any (insightful) way of defining What is life?'' from physicists. There are pioneer works, including ...
3answers
688 views

### Intuitive understanding of the definition of entropy

In Wikipedia, the definition of entropy goes like this: $d S = \dfrac{\delta q_{\rm }}{T}$. The literal interpretation of this equation is that some amount of heat transferred into a system, if the ...
4answers
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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?
5answers
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### Why isn't absolute $0 K$ temperature possible?

So $T$ is defined as $$T = \left(\frac{\partial E}{\partial S}\right)$$ and $S$ is defined as $$S = k_B \ln \Omega$$ where $\Omega$ is the number of accessible states of the system for a given ...
6answers
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### How do you prove the second law of thermodynamics from statistical mechanics?

How do you prove the second law of thermodynamics from statistical mechanics? To prove entropy will only increase with time? How to prove? Please guide.
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### Victorian cosmology after the second law of thermodynamics but before relativity?

In the 19th century, most astronomers adopted an island universe model, in which our galaxy was the only object in an infinite space. They didn't know that the "spiral nebulae" were other galaxies. ...
1answer
959 views

### Equation of state of a rubber band

I have the following question that I attached in png format. I have done part (a), but I am having difficulties in part (b) when I proceed according to the book. I have non zero tension at ...
1answer
365 views

### Second Law of Thermodynamics…confusion over an example

By the second law of thermodynamics, you shouldn't be able to use any amount of mirrors/lenses to focus sunlight onto an object and heat it past the surface temperature of the sun (approximately ...
4answers
1k views