A 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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Why is there no change in internal energy for an isothermal reversible process?

This concept is used for deriving the relation : Change in Entropy = $2.303 \,\ nR \,\ log_{10} (\frac{V_2}{V_1})$ But I don't understand why change in Internal energy = $0$.
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1answer
2k views

Isentropic processes

I'm having trouble understanding why reversible adiabatic processes are isentropic. I understand that in a reversible adiabatic process there is no heat exchange and so $dQ = TdS = 0.$ However, if ...
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2answers
128 views

How does the second law of thermodynamics forbid the possibility of perpetual machine of the second kind?

Fermi in his lecture asserts: The second law of thermodynamics rules out the possibility of constructing a perpetuum mobile of the second kind. So, this means there can be no machine which just ...
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3answers
276 views

How is the efficiency of a heat engine related to the entropy produced during the process?

I'm reading Schroeder's An Introduction to Thermal Physics. Regarding heat engines, it is stated: Unfortunately, only part of the energy absorbed as heat can be converted to work by a heat engine. ...
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4answers
353 views

Why is entropy's definition useful?

I have somewhat of an understanding for other physical quantities, but as far as entropy goes I only know it to be "disorder". Why is the change in entropy formula an appropriate/useful definition, ...
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0answers
57 views

Why doesn't entropy get decreased in adiabatic expansion process?

I was reading the second step of Carnot cycle in which the system undergoes adiabatic expansion doing work & thus decreasing the internal energy of itself. The entropy didn't change as no further ...
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5answers
927 views

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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2answers
174 views

Why must heat supplied in the definition of entropy be reversible? Can't it be irreversible after all it is a state function?

The definition of entropy contains the term $Q_\text{rev}$ which means the heat supplied or taken out reversibly. I thought yes it can be after all only the initial & final states are important as ...
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1answer
2k views

Sackur-Tetrode equation - clarification required - problem with units

I'm a 2nd year physics undergraduate and recently I've volunteered to give a short presentation on the Sackur-Tetrode equation derivation and its use at removing the Gibbs paradox. I've looked on the ...
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2answers
92 views

Why does heat added to a system cause an increase in entropy that is independent of the amount of particles in the system?

Say we have two gas containers of $N_{2}$ at the same temperature of $300 ~\text{K}$, one containing $10^{23}$ particles and the other containing $10^{13}$ particles. If we add a quantity of heat to ...
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1answer
52 views

Thermodynamics - Entropy Change in an Isolated System and the State Postulate

I was thinking about something. Consider a isolated system consisting of hot and cold water in a rigid tank. The process is to basically to let them reach an equilibrium temperature. Now, because it's ...
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1answer
66 views

Why is the change in entropy greater for processes occurring at lower temperatures? [duplicate]

We have the thermodynamic definition of entropy $\Delta S = q_{rev}/T$. If heat transfer is the same for both processes at different temperatures, this implies that the same process occurring at lower ...
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0answers
90 views

Le Chatelier's principle and energy minimization

In Callen's "Thermodynamics" in section 8.5 he asks us to imagine some perturbation $dX$ of an extensive parameter of a system in contact with a reservoir that acts to hold fixed the intensive ...
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2answers
1k views

The definition of entropy in quantum mechanics

I have seen entropy with several different definitions. Like Von Neumann entropy and Rényi entropy, etc. So I am curious why there are so many different definitions in quantum mechanics while only ...
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0answers
31 views

Is it possible that my 750ml water bottle contains spent fuel from a space shuttle launch

Firstly I live in New Zealand which is very distant from USA which is where the space shuttle was originally launched from. So what is the possibility of molecules from the rocket engines spent ...
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1answer
51 views

How is one process reversible and its reverse non-reversible?

My textbook gives this example of a reversible process: A gas in a piston is expanded over a long period of time, sitting on a hot plate that maintains its temperature. As an infinitesimal amount of ...
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2answers
591 views

Black hole no-hair theorems vs. entropy and surface area

I was revisiting some old popular science books a while ago and two statements struck me as incompatible. No-hair theorems: a black hole is fully-described by just a few numbers (mass, spin etc) ...
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0answers
22 views

Light Sheets and the Holographic Entropy Bound

I'm trying to understand light sheets as related to the holographic entropy bound for information. This bound equates the information on a surface (e.g. the event horizon of a black hole) with the ...
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2answers
60 views

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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1answer
42 views

sudden volume increase piston

I am wondering what happens in the following situation. I have a piston filled with an ideal gas for which I suddenly/instantaneaouly iscrease the volume. In particular I want to know what happens to ...
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1answer
213 views

Why is the second law of thermodynamics indisputable? [duplicate]

Why is the second law of thermodynamics undisputable? On his website Professor Hawking says the following: The cosmologist, Sir Arthur Eddington, once said, 'Don't worry if your theory doesn't ...
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1answer
48 views

Mach Number after Normal Shock

Is there any way that someone can give me more of a conceptual explanation for the fact that the Mach number downstream of a normal shock must be less than or equal to 1? I understand the ...
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0answers
33 views

Best introduction to the link between information and thermodynamics

I have a solid understanding of classical thermodynamics, as well as a reasonable understanding of statistical mechanics and some of the philosophical issues regarding entropy. However, I've never ...
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1answer
88 views

Derive the Boltzmann factor in classical statistical mechanics

In both quantum and classical statistical mechanics, the probability of an NVT system having an energy $E$ is proportional to $$ p(E)\propto e^{-E/T} $$ However, all of the derivations (that I can ...
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22 views

Are the fundamental forces constantly fighting entropy?

If we imagine that the four fundamental forces disappeared, all structures that had a non zero temperature (Kelvin) would quickly disintegrate due to the particles colliding with each other and start ...
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1answer
316 views

Simpler derivation of Sackur-Tetrode equation

Is there a reason the following derivation for the Sackur-Tetrode equation is not common? I am teaching a lower undergraduate level class and would like to derive it with simpler terms of only using ...
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0answers
32 views

Did the early universe violate the 2nd law of thermodynamics? [duplicate]

If the universe started out isotropic and homogenous and of all fundamental particles then how could there now be any concentration of energy anywhere? If you say that nothing is really homogenous ...
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1answer
57 views

Is entropy violated inside black holes and worm holes?

Do the laws of thermodynamics hold true everywhere in universe ? What about black holes and worm holes ?
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4answers
14k views

Entropy Change During Reversible Processes

I'm confused about the Second Law of Thermodynamics. The Second Law of Thermodynamics prohibits a decrease in the entropy of a closed system and states that the entropy is unchanged during a ...
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2answers
104 views

The statistical interpretation of Entropy

I recently got introduced to the Statistical Mechanics, particularly, the Statistical Interpretation of Entropy and am utterly confused regarding the following problem: Imagine a box with two ...
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1answer
189 views

If entropy is a state function, then why is all the talk about reversible vs. irreversible processes?

So I'm preparing for my Thermodynamics undergrad exam, and I just can't wrap my head around the significance of reversibility vs. irreversibility of a process in relation to entropy. I mean if entropy ...
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6answers
2k views

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 ...
4
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1answer
217 views

H-theorem and Boltzmann equation applied to Boltzmann distribution

Using the Boltzmann equation: $$ \frac{dH}{dt} = \int_0^{\infty} dr \int_0^{\infty} ds W(r,s)[p_r - p_s][\ln{p_r} - \ln{p_s}],$$ and assuming $p_r = e^{-\beta r}$, the equation looks like $$ ...
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0answers
55 views

difference between reversible and irreversible entropy

Before I proceed, let me first say I have done research and understand the general idea between the two: summed up crudely reversible happens in a slower continuous manner while irreversible happens ...
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2answers
256 views

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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4answers
1k views

Why don't photons split up into multiple lower energy versions of themselves?

A photon could spontaneously split up into two or more versions of itself and all the conservation laws I'm aware of would not be violated by this process. (I think.) I've given this some thought, and ...
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1answer
408 views

If Hawking radiation is proved to be true won't black holes lose entropy which is against laws of thermodynamics?

Hawking radiation states that black holes continuously emit radiation. But if its the case won't entropy of black holes decrease which violates the laws of heat, entropy and thermodynamics?
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1answer
325 views

Name of experiment

I'm seeking the name of or reference for an experiment I once saw in a college physics class. At the beginning of one class the instructor repeatedly wound a wiper that spread a blot of some type of ...
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0answers
34 views

Facing a difficulty in calculating thermal entropy of a massless scalar field in a d-dimensional box at temperature T

I try to calculate the thermal entropy of a massless scalar field in a box at finite temperature T. The calculation is almost mindless but I meet a math problem that I don't know how to solve. My ...
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6answers
3k views

Perpetual motion machine of the second kind possible in nano technology?

First of all sorry for my English - it is not my native language. During my engineering studies at the university the thermodynamics professor told us that the "second law of thermodynamics is not ...
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3answers
845 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 ...
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7answers
3k views

Why does heat added to a system at a lower temperature cause higher entropy increase?

Entropy is defined in my book as $\Delta\ S = \frac{Q}{T}$. To derive the formula it says that entropy should be directly proportional to the heat energy as with more energy the particles would be ...
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0answers
38 views

Do the entropy of the universe and third law of thermodynamics conflict? [duplicate]

We know that our universe was in low entropy state at the earlier time after the Big bang. But we also know that the temperature at that times was enormously high. But what the third law of ...
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2answers
338 views

Does a universe experiencing “heat death” have a temperature?

As defined by Wikipedia: The heat death of the universe is a suggested ultimate fate of the universe in which the universe has diminished to a state of no thermodynamic free energy and ...
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0answers
52 views

Entropy definitions

So I have learned that entropy is the measure of disorder of a system. For the IPhO this was of course not enough as we need to be able to calculate entropy changes of ideal gases. Those equations ...
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4answers
2k views

How does the entropy of an isolated system increase?

The change of entropy is defined $$\Delta S = \int \frac{dQ_\mathrm{rev}}{T}.$$ If a system is isolated the heat transfer between the system and the surroundings is zero ($dQ = 0$), thus $\Delta S = ...
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3answers
116 views

If heat can't be transformed into other forms of entropy, why do hot things radiate electromagnetic waves?

The laws of entropy says entropy can only increase. On the other hand, if I take a hot object, it will naturally convert its heat into EM radiation. How is this possible? Does EM radiation count as ...
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1answer
376 views

The definition of entropy

As history of thermodynamics say, it was a mystery that what is the required condition for a given energy conversion to take place? Like there are two possible events each conserving energy but only ...
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1answer
2k views

Thermodynamics for Dummies: Entropy and temperature

I do not study physics and I have never had a course in thermodynamics. I have no idea what it is about, but I am currently taking a course where we had something about entropy. Would be great if ...
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1answer
85 views

Entropy and the $2^{nd}$ law of thermodynamics

I have just been introduced to the word "entropy" and as it is my understanding that it is a measure of the randomness and chaos of particles in as system. My textbook list the 2nd law of ...