22 votes
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Is there a quasistatic process that is not reversible?

Most quasi-static processes are irreversible. The issue comes down to the following: the term quasi-static applies to the description of a single system undergoing a process, whereas the term ...
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  • 4,062
21 votes
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Reversibility = non-causality. Can this be right?

You're right as far as it goes -- if you can come up with a Newtonian system that reaches a stationary state from a non-stationary one, then the system must be non-deterministic. The point (to the ...
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20 votes

Where does the irreversiblity came from if all the fundamental interaction are reversible?

There's a distinction between microscopic reversibility and macroscopic reversibility. Or if you will, a difference between something being irreversible in theory versus irreversible in practice. (Or ...
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  • 300
16 votes

Quantum and classical physics are reversible, yet quantum gates have to be reversible, whereas classical gates need not. Why?

Though your language is vague and handwavy, I would say you pretty much got it right. Classical physics is reversible at a microscopic level when you look at ALL degrees of freedom. However, we don’t ...
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12 votes
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Thermodynamic reversible process

This is a very interesting, important and at the same time subtle matter which is useful not only in thermodynamics, but other areas as well, and not everyone quite understands it. First off, two ...
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12 votes

How is entropy a state function?

Your question goes right in the kernel of the meaning of the term state function. A state function is a function defined over all possible states of the system such that its value for every state ...
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12 votes

Where does the irreversiblity came from if all the fundamental interaction are reversible?

Irreversibility comes from the thermodynamics: the probability that we return to the same state in any reasonable amount of time is extremely small. In more technical terms: the entropy is increasing. ...
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12 votes

In the Langevin dynamics: neglecting inertia. A mathematician trying to understand physics terminology

A physicist would often write these equations as $$ m\ddot{x}=-\gamma\dot{x}+\xi(t)\Leftrightarrow \begin{cases}\dot{x}=v,\\ m\dot{v}=-\gamma v+\xi(t)\end{cases} $$ Admittedly, it lacks mathematical ...
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11 votes
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Change in entropy when mixing water at different temperature

The bottom line is that hot water loses heat at high temperature, giving a small negative entropy change while the cold water gain heat at low temperature resulting in a high entropy change. The net ...
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10 votes

Change in entropy when mixing water at different temperature

To get the entropy change for a system experiencing an irreversible process, the first step is to forget entirely about the actual irreversible process and, instead, devise a reversible process that ...
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10 votes
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When doing entropy calculations, why do you use the reversible heat for the system but the actual heat for the surroundings?

It is a standard practice in textbook thermodynamics analyses to tacitly equip the surroundings with equipment that does not generate entropy during a process. One such piece of equipment is the so-...
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9 votes
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Why is heat transfer reversible when temperature difference is infinitesimal?

To do it reversibly, you can heat the body from $T_1$ to $T_2$ (i.e., over a finite temperature change) using an infinite sequence of constant temperature reservoirs, in which each reservoir in turn ...
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9 votes

Is there a quasistatic process that is not reversible?

In your question you mentioned two examples -- (1) slowly moving something that has friction, and (2) gradually mixing two chemicals that react spontaneously ($\Delta G\gg0$). Then you said neither ...
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9 votes

What are entropy and reversible processes really?

The practical definition of a reversible process is one for which the system (no mass entering or leaving) passes through a continuous sequence of thermodynamic equilibrium states. No, the entropy ...
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9 votes
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Why is the work done in reversible process greater than work done in irreversible process?

Its almost true. Of course, Jamie would beg to differ. However, the general logic is simple. If you have something that's irreversible, energy is lost. That energy was work that could have been ...
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8 votes
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What is the difference between reversible and irreversible adiabatic expansion?

The difference is that one expansion is quasi-static (the reversible one) while the other is spontaneous because of a dramatic change of the external constraints (the irreversible one). In the quasi-...
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8 votes

Why does slow compression of a gas doesn't change its entropy to increase?

Squeezing the wavefunction means confining it to a smaller space. It takes more energy to confine something within a small space than within a big one. 2, 3: These are consequences of the quantum ...
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8 votes
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Which temperature does $T$ in Clausius inequality ($\oint \frac{\delta Q}T\le 0$) refer to?

The temperature appearing the the Clausius inequality is definitely the temperature of the "boundary interface (with the surroundings)", or simply the temperature of the sources. One of the best ...
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8 votes
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Why is quantum mechanics reversible?

So before college even, we students learn that there is this wonderful alternate way to look at a lot of physical problems in terms of energies. But, they seem to lose some information about the ...
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8 votes
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$dU=dQ$ and $dU=TdS$, but $dQ$ not always equal to $TdS$? Why?

In $dS = \frac{dQ}{T}$, the $dQ$ is the heat exchange on a reversible path from the initial state to the final state, irrespective of how the process is actually carried out.
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8 votes

The actual definition of entropy

THE RECIPE Apply the First Law of Thermodynamics to the irreversible process to determine the final thermodynamic equilibrium state of the system Totally forget about the actual irreversible ...
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8 votes
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Why the entropy change is not zero in the irreversible adiabatic process?

Although entropy change is defined in terms of a reversible differential transfer of heat divided by the temperature at which the heat is transferred, you can have entropy change without heat transfer....
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8 votes

Is $dS=\frac{\delta Q_{irev}}{T}$ true for non-reversible processes?

Why do we need reversibility? I do not see why this shouldn't be true for quasi-static irreversible processes. Although the definition is in terms of a reversible transfer of heat, you are correct ...
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7 votes
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Principle of Caratheodory and The Second Law of Thermodynamics

(1) Adiabatic accessibility means that by some purely mechanical, electrical, magnetic, etc. (but not thermal) method, an equilibrium state can be reached from another one. At the heart of ...
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7 votes
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Significance of Reversible and Irreversible Process

Reversible processes are important because they are related to the efficiency of a process. Take for examples a pair hotplates, one at 100C and one at 0C. In a theoretically ideal setting you could ...
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7 votes

What experiment supports the axiom that quantum operations are reversible?

One place to look for relatively direct evidence is in the cross sections of time-reversed nuclear and particle physics reactions. For instance comparing $$A + n \to B + \alpha $$ with $$B + \alpha \...
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7 votes
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How do we know that truly reversible processes don't exist?

The long story short is "we don't know." We actually don't know any laws of physics, if you get down to it. None of them. The universe is what it is, and it behaves the way it behaves. That's ...
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7 votes

Entropy change in reversible and irreversible processes

Background The form $d...$ is a differential. It means an infinitesimal change in something as it undergoes a process. The something of interest here is a thermodynamic state function of a system, its ...
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7 votes

Why free expansion of a gas is irreversible?

You went wrong in interpreting the Kelvin-Plank statement of the second law. There is no violation. The statement is (the details of the statement may differ depending on the references)(italics done ...
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7 votes

Is it possible to deduce the second law of thermodynamics from the first?

The first law is about conservation of energy. The second law is about irreversibility of processes if this involved decreasing entropy in a closed system. If there are say $N$ degrees of freedom in ...
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