# What is Reversible: Definition and 247 Discussions

Reversible computing is a model of computation where the computational process to some extent is time-reversible. In a model of computation that uses deterministic transitions from one state of the abstract machine to another, a necessary condition for reversibility is that the relation of the mapping from states to their successors must be one-to-one. Reversible computing is a form of unconventional computing.
Due to the unitarity of quantum mechanics, quantum circuits are usually reversible.

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1. ### I Efficiency of cycles bounded by two isotherms

I want to consider all possible reversible cycles that consist of an isothermal expansion at TH and an isothermal compression at TC. The other two processes can be isochoric, isobaric, adiabatic or anything else, but they should never leave the temperature range between the two isotherms. I also...
2. ### Reversible operation of fuel cell

Can someone explain me what does the underlined part of the following passage mean? "The reversible operation of a fuel cell implies that the external circuit exactly balances its emf, with the result that its current output is negligible. In actual operation under reasonable load, internal...
3. ### B Entropy change for reversible and irreversible processes

I came across the following statement from the book Physics for Engineering and Science (Schaum's Outline Series). I cannot seem to find a satisfactory answer to the questions. Is the statement in above screenshot talking about entropy change the statement of Second Law of Thermodynamics or is...
4. ### I Reversible / Irreversible Fundamental Equation

As far as I learned, the following statements should be correct (for closed systems, no chemical reactions), irrespective whether the process is reversible or irreversible (since S and V are state variables): dU=TdS−pdV dU=dQ+dW Does this imply, that the statement: dU≤TdS−pdV is wrong? This...
5. ### Reversible AND universal elementary cellular automata

Has any of the reversible extensions of the elementary one-dimensional cellular automata described by Wolfram in NKS (e.g. Rule 37R) been shown to be computationally universal (like Rule 110)? If so, please give me links. Otherwise, could this be the case? Or is there a proof that no nR can be...
6. ### Reversible electric motor - what am I looking at?

I have a reversible electric motor that operates an automatic skylight, vintage 1985. I'm trying to figure out how it is supposed to work, but I don't recognize the rectangular gray components that are secured to the motor case with a length of plastic zip-tie. What might they be and how would...
7. ### I Determinism in a reversible polarizer experiment

If light at a known polarization goes through a beam splitting polarizer that changes the light's polarization and then goes through the reverse orientation of that polarizer it will exit with the same polarization that it entered with. See the following picture: If the polarization state...
8. ### I About Reversible vs Irreversible Gas Compression and Expansion Work

Hi, reading the interesting Reversible vs Irreversible Gas Compression and Expansion Work insight by @Chestermiller I would like to ask for clarification on some points. In the second bullet at the beginning my understanding is as follows: consider an ideal gas contained in a cylinder...
9. ### To find total work done from multiple reversible processes

The question is given in 3 parts. For first part, process is isochoric so Work done=0. We know here that at end of the process (a), T2=T1 while V remains constant (we can take it as V1) so P2=2P1. For second part, process is isothermal so T is constant. At end of process we reach P1 again from...
10. ### I How can a process be isentropic but not reversible or adiobatic?

In the book for our thermodynamics, it states that a process that is internally reversible and adiabatic, has to be isentropic, but an isentropic process doesn't have to be reversible and adiabatic. I don't really understand this. I always thought isentropic and reversible mean the same thing...

14. ### Work done by irreversible and reversible processes

I am not able to follow the derivation of work done in a reversible and irreversible process as I don't get why the work done should be different in the two processes. a reversible process is said to be a process that occurs infinitesimally slowly and an irreversible process goes from initial to...
15. ### I The relationship between reversible and irreversible processes

Hi all. I am referencing the example given in Halliday and Resnick, Chapter 20, Section 1, Subsection "Change in Entropy". The above picture is graph of the free expansion of a gas into a volume that is double its original volume. I n a free expansion there is no heat transfer, the pressure...
16. ### Reversible Otto Cycle Efficiency: Investigating the Difference from Carnot's

So we know that every reversible engine working between the same temperatures will have the same efficiency(the same as Carnot engine). So let's consider for example reversible Otto cycle. So as you can see on the picture it is operating between ##T_1## and ##T_3##, so I was thinking that it...
17. ### Reversible processes need to be ONLY isothermal or adiabatic?

My question is: Do ALL the reversible process need to be composed of ONLY isothermal and adiabatic transformations? Carnot cycle satisfy this, but what other cycle would be also reversible? I know that for a process to be reverisble it has to be almost-static, have no dissipative force, and no...
18. ### Thermodynamics Problem: Reversible and Irreversible Processes

Attempt at A Solution Problem 1 Reversible Process - A cylinder of ideal gas at pressure P is in mechanical equilibrium with a piston of area A driven by a spring of force F = PA and thermal equilibrium with a reservoir of temperature T. The piston is moved a small distance dx toward the...
19. ### When is the propagation of an EM wave not reversible?

By reversibility, if we turn the direction of the light propagation by 180 degrees, then the new propagation path follows the old propagation path. I suspect that when there is diffraction, the light propagation is not reversible?
20. ### I Feynman's reversible and irreversible machines

I started reading "Feyman Lectures on physics" and stuck on his explanation about reversible and irreversible machines. I've tried to read other answers to this question, but I couldn't get the point. Here is the first thing: "If, when we have lifted and lowered a lot of weights and restored...
21. ### What is the change in entropy of the water in a reversible heat engine?

My attempt: I though : ΔQ_w= 1*4200 * (-100) J=-420000J Q_ice=334000*m_ice = ΔQ_w But it was totaly wrong! The solution showed : Because the heat engine is reversible the efficiency η = 1- (T_cold / T) T_cold is always 273 K while the hot temperature changes from 373 K to 273 K during this...
22. ### 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...
23. ### I Can we copy information using reversible operations?

I am trying to investigate my doubts that reversible operations can model (or at least mimic) information copy process. For simple model I take two numbers ##A \neq B##. Now I can't copy value of A into B without erasing (irreversibly) value of B. However I can use transformation that replaces...
24. ### Is the refrigeration process reversible?

The refrigeration process is used in air conditioning to dehumidify and control temperatures in homes and buildings. The heating industry preceded the air conditioning business and many evaporator coils were placed on top of furnaces to create split systems and utilize existing ductwork. The...
25. ### I Adiabatic irreversible process vs adiabatic reversible

Hi, consider an adiabatic irreversible process carrying a thermodynamic system from initial state A to final state B: this process is accompanied by a positive change in system entropy (call it ##S_g##). Then consider a reversible process between the same initial and final system state. Such...
26. ### Are all substitution reactions reversible?

Consider a reaction: H2+CuCl2= Cu+2HCl This is a substitution reaction.But is this may not be a reversible reaction since Cu is less active than .So Cu can't substitute H from HCl and make a backward reaction.Is my thinking right?
27. ### Thermal Physics: Reversible processes

Hi all, I have been having some issues trying to show that a reversible expansion of gas does not create new entropy. Assistance is greatly appreciated! So suppose that a gas expands reversibly as shown below at fixed temperature At fixed temperature, internal energy doesn't change so...
28. ### How does reversible =>no entropy change in the universe?

Homework Statement 2.Relevant equations[/B]The Attempt at a Solution How does a reversible process in the universe imply the entropy doesn't increase? I understand that the change of entropy in a closed reversible cycle is 0 in the system, but I don't get why a not closed reversible process...
29. ### Adiabatic Reversible Compression of a Solid

Homework Statement Derive an expression for the change of temperature of a solid material that is compressed adiabatically and reversible in terms of physical quantities. (The second part of this problem is: The pressure on a block of iron is increased by 1000 atm adiabatically and...
30. ### I Reversible compression of a gas - faulty reasoning?

Hello, I am trying to figure out where my reasoning falls apart in this thought experiment: To determine if a process "A" is reversible (or at the very least internally reversible), I try to picture a reversible process "B" that involves only heat transfer and links the same two endpoints that...
31. ### Work done in reversible and irreversible processes

Hello, I am encountering some confusion understanding the difference in working with reversible and irreversible processes in thermodynamics. Let's say I have a process where an ideal gas at a certain starting temperature ##T_i## expands from volume ##V_i## to ##V_f##. The temperature of the...
32. ### I Reversible Measurement: Is It Possible?

Reversible computation is a somewhat well-known topic. (Quantum computers, for instance, must use reversible gates). Apparently, though, quantum measurements can be reversible too. This also means you could recover the original state by “unmeasuring” the system. Imagine being able to “see” a...
33. ### Feynman: Reversible machines, no perpetual motion?

Greetings, I have begun reading the Feynman Lectures to repeat the most important ideas from my undergraduate studies and to improve my intuitive understanding of physics. In one of the first chapters, the one about the conservation of energy, he demonstrates that the conservation of energy is...
34. ### Is the photoelectric effect in a photocell reversible?

Is the photoelectric effect in a photocell reversible? Suppose both the cathode and the anode of a photocell are from cesium. The anode and the cathode are externally (outside the photocell) connected by a copper wire. Cesium has a threshold frequency of 470 THz. The cathode is illuminated with...
35. ### MATLAB Matlab Sim Code for ODE & Gillespie for Reversible Reaction

Here is the code you input into matlab. Aini etc, are the initial values of the population densities. A for predator, B for Prey. % example for ODE and Gillespie % one reversible reaction b1 = .033; bo = .00047; a1 = .0022; ao = .00055; Aini = 5168; Bini = 34; %% Basic ODE simulation...
36. ### How to know whether a given process is reversible?

Homework Statement How do we know that an adiabatic process is reversible , while isothermal process is not? Do we have to calculate the entropy change of the process or we have to check experimentally whether the system is in eqbm at infinitesimal time interval( but this seems impossible)...
37. ### I Reversible Process: Why is it Important?

Does this mean that in reversible process\ quasistatic process, the energy is not lost as heat and the process in which the energy is not lost as heat is known as reversible process\quasistatic process? I want to know why we want to have reversible \ quasistatic process.
38. Z

### Reversible Isothermal Expansion Steam

Homework Statement 2kgs (total mass) of steam goes through a revesible isothermal expansion at 500 degrees celcius. During the expansion the pressure drops from 300 kpa to 200 kpa. What is the heat absorbed by the steam during this process? Homework Equations U=W and W=nrt ln(v2/v1) The...
39. A

### B Understanding Reversible Machines: Exploring Feynman's Limitations

I am no able to understand the reasoning of Feynman in deducing that it is impossible to build a machine that will lift a weight higher than it will be lifted by a reversible machine. I am also not able to understand what reversible machine is. So, please help me.
40. Y

### Reversibility of Two Stroke Engines: Exploring the Impact of Crosshead Bearings

Are large two stroke engines with cross-head bearings reversible?
41. ### Reversible work of compression

Homework Statement Air enters a compressor at 100 [Kpa] and 10 [c], the air leave the compressor at 12[Mpa] the isentropic efficiency η is 87%, calculate: work done reversible work irreversibility Homework Equations [/B] T2=T1(P2/P1)(1.4-1)/1.4 W = (Cp(T2-T1))/η Wrev=Δh - T0Δs Δs =...
42. ### B Reversible Processes Explained: Entropy, Time, & More

Can someone explain to me what a reversible process means, because I am not sure I really understand. Intuitively it should be a process that should work both ways, but I am not sure I understand how it is related to entropy (the change of entropy should be 0, but I am not sure I understand...
43. ### Relation between Decomposition and Reversible reactions....

Okay...I read that the decomposition of water is a reversible reaction (because the constituents can react to form water and water can decompose to form constituents)...This lead me to another thought that almost all compounds can be decomposed (although it is true that their conditions for...
44. M

### I Understanding Reversible Fluid Flow and Its Relationship to Reynolds Number

Can someone explain what it means for a fluid flow to be reversible and how this relates to Reynolds number, both intuitively and mathematically? Thanks!
45. ### I Why is Stokes flow time reversible?

What is the intuition behind the fact that stokes flow is time reversible?
46. ### An ideal gas closed system reversible process

Homework Statement an Ideal gas at T = 70 C and 1 bar undergoes following reversible processes: a: Adiabatically compressed to 150 C b: then, cooled from 150 to 70 C at constant pressure c: finally, expanded isothermally to the original state (T=70 C and P = 1 bar) Homework Equations...
47. ### Entropy Change in Reversible vs Irreversible Gas Expansions

Hello! I have this GRE question: In process 1, a monoatomic ideal gas is heated from temperature T to temperature 2T reversibly and at constant temperature. In process 2, a monoatomic ideal gas freely expands from V to 2V. Which is the correct relationship between the change in entropy ##\Delta...
48. ### I Energy Level Change in Adiabatic Reversible Process

Hello, I am currently trying to get my head around the concept of entropy. One way to understand it is that it can be related to the amount of available energy levels in a system. From what I read, the availability of energy levels in a system: 1) increase with an increase in the system...
49. ### Definition(s) of "reversible" -- thermodynamics vs mechanical?

Is there a definition for "reversible process" that applies to mechanical systems (such as a block sliding down a frictionless inclined plane) that is distinct from the definition of "reversible process" in thermodynamics? If we apply the thermodynamic definition of "reversible process" to...
50. ### Reversible adiabatic expansion -- calculate change in E

Homework Statement Calculate the change in internal energy of an ideal gas the expands reverisbly and adiabatically from ##v_i## to ##v_f## Homework Equations ##pV^{\gamma} = constant ##*, where ##\gamma=\frac{c_p}{c_v}## for a reversible adiabatic. ##E=\alpha p V##, ##\alpha## a constant...