# Irreversibility Definition and 21 Threads

In science, a process that is not reversible is called irreversible. This concept arises frequently in thermodynamics.
In thermodynamics, a change in the thermodynamic state of a system and all of its surroundings cannot be precisely restored to its initial state by infinitesimal changes in some property of the system without expenditure of energy. A system that undergoes an irreversible process may still be capable of returning to its initial state. However, the impossibility occurs in restoring the environment to its own initial conditions. An irreversible process increases the entropy of the universe. Because entropy is a state function, the change in entropy of the system is the same, whether the process is reversible or irreversible. The second law of thermodynamics can be used to determine whether a process is reversible or not.
Intuitively, a process is reversible if there is no dissipation. For example, Joule expansion is irreversible because initially the system is not uniform. Initially, there is part of the system with gas in it, and part of the system with no gas. For dissipation to occur, there needs to be such a non uniformity. This is just the same as if in a system one section of the gas was hot, and the other cold. Then dissipation would occur; the temperature distribution would become uniform with no work being done, and this would be irreversible because you couldn't add or remove heat or change the volume to return the system to its initial state. Thus, if the system is always uniform, then the process is reversible, meaning that you can return the system to its original state by either adding or removing heat, doing work on the system, or letting the system do work. As another example, to approximate the expansion in an internal combustion engine as reversible, we would be assuming that the temperature and pressure uniformly change throughout the volume after the spark. Obviously, this is not true and there is a flame front and sometimes even engine knocking. One of the reasons that Diesel engines are able to attain higher efficiency is that the combustion is much more uniform, so less energy is lost to dissipation and the process is closer to reversible.All complex natural processes are irreversible. The phenomenon of irreversibility results from the fact that if a thermodynamic system, which is any system of sufficient complexity, of interacting molecules is brought from one thermodynamic state to another, the configuration or arrangement of the atoms and molecules in the system will change in a way that is not easily predictable. Some "transformation energy" will be used as the molecules of the "working body" do work on each other when they change from one state to another. During this transformation, there will be some heat energy loss or dissipation due to intermolecular friction and collisions. This energy will not be recoverable if the process is reversed.
Many biological processes that were once thought to be reversible have been found to actually be a pairing of two irreversible processes. Whereas a single enzyme was once believed to catalyze both the forward and reverse chemical changes, research has found that two separate enzymes of similar structure are typically needed to perform what results in a pair of thermodynamically irreversible processes.

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1. ### I Feynman's Brownian Ratchet analysis

Hi, as in a previous thread I would like to better understand the Feynman's analysis of brownian ratchet as described here: https://www.feynmanlectures.caltech.edu/I_46.html https://en.wikipedia.org/wiki/Brownian_ratchet Consider the case in which the two boxes (i.e. heat baths) are at the same...
2. ### B Irreversibility of Aristotle's law

"Reversibility means that we never lose information, that at a fundamental level we can always retrodict the past as well as predict the future in the laws of physics". Susskind in his Theoretical minimum Lecture 2 describes about the laws which are true and which are not. He tells the law of...
3. ### A Statistical physics : Irreversibility

I was reading mehran kardar (books and lectures) it says the concept of irreversibility comes from an assumption (in which we increase the length scale by interaction disctance between two particles). So My question is the concept of irreversibility is still valid in the case of 1 particle...
4. ### How many reversible thermodynamic cycles are there between two heat reservoirs?

Hi, I was revisiting my (high school level) understanding of thermodynamic cycles and I think I still have some doubts. Last year and more recently I posted a few questions which surely helped me, but I think I need more clarifications. In a nutshell, what I'd like to know is the following...
5. ### What is the entropy for an irreversible adiabatic process?

Homework Statement The change in entropy is zero for: A. reversible adiabatic processes B. reversible isothermal processes C. reversible processes during which no work is done D. reversible isobaric processes E. all adiabatic processes Homework Equations ## dS = \frac{dQ}{T} ## The Attempt...
6. ### Logarithm in entropy shows irreversibility of the Universe?

Dear community, I stumbled upon this ecology article (https://www.witpress.com/elibrary/dne/4/2/402, page 4) and have some confusion about a statement in there: "Before further unpacking the formal defnition of entropy, one would be justifed in asking why not simply choose (1 – p) instead of...
7. ### B Irreversibility vs reversibility

Decoherence is when the system is entangled with the environment irreversibile... But when it is reversible.. is it still called Decoherence? However, the universe is said to be a closed system. Does the irreversibility in decoherence means it is just our ignorance that we can't track it. For...
8. ### I The typical and the exceptional in physics

For properly normalized extensive macroscopic properties (and this includes the center of mass operator), there is such a proof in many treatises of statistical mechanics. It is the quantum analogue of the system size expansion for classical stochastic processes. For example, see Theorem 9.3.3...
9. ### Prove irreversibility in this quasi-static process.

Consider a piston-cylinder arrangement with an ideal gas inside the cylinder. The region outside this arrangement or thermodynamic system is absolute vacuum. The piston has some mass and initially everything is in equilibrium. The inner surface of the cylinder is rough hence friction force comes...
10. ### Reversible Work and Irreversibility

I am trying to grasp the concepts of reversible work, irreversible work and irreversibility.(Last one is the difference between them if i am not mistaken.) Let us consider a rigid and evacuated container at volume V. Then, a valve opens and athmospheric air (P0, T0 is filling the tank. The wall...
11. ### Less irreversibility = less change in entropy

I have a conceptual question that I am trying to clear up. A reversible process means that the net change in entropy is zero. If we have two scenarios, where case 1 has a ΔS= 0.1 J/K and case 2 has a ΔS= 0.5 J/K. Is it a correct statement to say that case 1 is "less irreversible (i.e...
12. ### "Lost work" and irreversibility

I'd like some help to understand how to calculate "lost work" for irreversible heat transfer across a finite temperature difference. I'd also be grateful for any links to the derivation of a clear, general expression for lost work (in terms of entropy). Thanks
13. ### Spontaneous Process and Irreversibility

Hi,all My problem is following this picture! http://mykomica.org/boards/shieiuping/physics/src/1325318780058.jpg If the system is isolated, dQ is always zero. Sb-Sa ≥0 And it satisfies the requirement of spontaneousness in isolated systems. So ,I do wonder if irreversible process is always...
14. ### Irreversibility of a Process: Finding ΔQ and ΔS of a Mass Dropped into the Sea

Homework Statement A mass m with heat capacity c at temperature T_1 is dropped into the sea at T_2. What is the irreversibility of the process? Homework Equations I = T_0 (\Delta S + \Delta S_E ) I = W_{\text{rev}} - W The Attempt at a Solution Heat in is Q=mc\Delta...
15. ### Time Irreversibility and the Weak Nuclear Force

I've been writing a lot recently about whether the reason for the increase of entropy over time (i.e. the second law of thermodynamics) is the time irreversibility of the weak nuclear force. I would like to get some people's thoughts on this topic (perhaps links to important papers on this...
16. ### Irreversibility temperature in superconductors

Hello, I would be grateful is someone could explain what the irreversibly temperature in type II superconductors is. I have just done some NMR experiments on MgB2 and get a lineshape change at the the irreversibility temperature as opposed to the field corrected Tc, so I believe it has...
17. ### Boltzmann collision term irreversibility

Hello, Here is the Boltzmann collision term as it is expressed on http://en.wikipedia.org/wiki/Boltzmann_equation" : Deriving the H theorem from this equation is the way to usually prove of the irreversibility of this term. Of course, this collision term should not be symmetric by...
18. ### Reflection and the measurement, and time irreversibility

Reflection is not considered to be a measurement. Reflection, like using lenses is considered to be a time-reversible process. It is not a measurement and it does not erase the coherence (interference pattern) Here is what I don't understand What happens to the momentum of the photon? The...
19. ### Quantum computations, irreversibility, unitarity

A stupid couple of questions... In quantum computations, one typically starts with some initial quantum state on which an operator is applied. This operator must be unitary , right? (I guess that otherwise, it would not corerspond to an actual physical quantum setup). And this implies...
20. ### Understanding of irreversibility

The thermodynamic explanation of irreversibility - does it include the microscopic view of a process, i.e. the expansion of a ideal gas (the random movement of the molecules) or the melting of an ice cube? Or is it only defined within the macroscopic world?
21. B

### Mechanical Reversibility vs Thermodynamical Irreversibility

I'm a bit confused with this topic we're supposed to be writing a paper on: "Thermodynamical Irreversibility VS Mechanical Reversibility (Microscopic Nature of the 2nd Law of Thermodynamics)" I think I know the concept of the irreversible nature of thermodynamics...such as the flow of...