# B Cyclic Thermodynamic processes

1. Sep 4, 2017

### GKRM

I'm a high school student with basic knowledge about thermodynamics. I have always come across systems under going reversible cyclic processes. Are there any cases for irreversible cyclic processes? Thanks in advance.

2. Sep 9, 2017

### PF_Help_Bot

Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.

3. Sep 9, 2017

### GKRM

Basically what I'm trying to ask is if I'm given a case of cyclic processes does this always mean that it's reversible. Well I think the engines are undergoing irreversible yet cyclic processes. Although I'm not sure about this.

4. Sep 10, 2017

### Staff: Mentor

The difficulty with your question is that it's hard to figure out, what you mean by cyclic and irreversible. Let us assume that both terms refer to time. Then cyclic means a process will return to a state it already has had. This is by definition a reversion.

5. Sep 10, 2017

### GKRM

Last edited: Sep 10, 2017
6. Sep 10, 2017

### GKRM

By cyclic processes I mean the processes in which a system starting from the initial state returns to the same passing through other states( it's usually shown as a continuous closed curve in the P-V diagram of the gas undergoing the process) for eg. Suppose you reversibly compress a gas and then reversibly expand the gas so that the final temperature, pressure, volume etc. are the same as before. By reversible, I mean the usual thermodynamic term in which the system is in Thermodynamic equilibrium with the surroundings while undergoing the process. My questions regarding this remain the same as above.

7. Sep 10, 2017

### Staff: Mentor

Yes. In real life, all cyclic processes are irreversible. A reversible process is just an idealization.

8. Sep 10, 2017

### GKRM

Sir if a system undergoes an irreversible process will it reach the same state as before for example same temperature pressure and volume? And is it possible to plot an irreversible cyclic process on a graph even if it's not in equilibrium with the surroundings? (These are the probable reasons I think because of which I've not come across any irreversible cyclic processes with a possible graph)

9. Sep 10, 2017

### Lord Jestocost

If a cyclic process moves clockwise or anti-clockwise around a loop, the considered system itself always reaches the same state as before (in case the volume and temperature are the same); however, the state of the surrounding will change due to heat exchange. What is meant by "reversible cyclic processes" are idealizations in order to calculate the maximum possible efficiency of cyclic-working heat engines. As, in reality, every step within really performed cycles entails some irreversibility (friction, finite speed to carry out the step), the efficiency of real cyclic processes must always be lower compared to the analogous idealized processes. Maybe, that's the meaning when using the term "irreversible cyclic processes".

10. Sep 10, 2017

### Staff: Mentor

Yes.
Yes. In typical cyclic processes, the cycle is subdivided into individual legs, usually four. At the beginning or end of each leg, the working fluid is typically assumed to be in a thermodynamic equilibrium state. However, if the process is irreversible, one or more of the legs will not pass through a continuous sequence of thermodynamic equilibrium states (i.e., it will be irreversible). But it is still possible to plot the irreversible cyclic process on a graph of pressure vs volume if, by the pressure, we mean the force per unit area exerted by the gas on the inside face of the piston. This can be measured experimentally using, say, a flush mounted pressure transducer. The work done by the gas on the surroundings will then be the integral of this pressure over the volume change. This "external" pressure can be imposed manually using an automatic control system with feedback to control the piston movement.

In any event, for a cyclic process, at the beginning and end of each leg, the equilibrium state is supposed to be the same from cycle to cycle (irrespective of whether the individual legs are irreversible).

11. Sep 10, 2017

### GKRM

Sir, I'm not really comfortable with the idea that a system can reach it's initial state after undergoing Irreversible cyclic process. Here are my concerns regarding entropy change for an irreversible process. Although I'm not much aware about entropy but I know 2 facts regarding it. The first one that entropy is a state function. Second being that the entropy of the universe increases in an irreversible process. If I reach the same state as before in an irreversible cyclic process, then the entropy change for the system will be zero. The entropy change for the universe should also be zero because the heat liberated by the system will be zero (for if it's non zero we could have created a perpetual machine which reaches the same state as before +giving us energy in the form of heat flow). How would you then account for the fact that the system reaches the same state as before?

Last edited: Sep 10, 2017
12. Sep 10, 2017

### Staff: Mentor

The change in entropy of the surroundings per cycle is not zero. Entropy generated within the system during the cycle is transferred to the surroundings.

13. Sep 17, 2017

### PhysicsExplorer

You're mixing apples and oranges when you say

''irreversible cyclic process''

Cyclic means something comes back to its original state, irreversible does not. Anyway, aside from the semantics, yes irreversible systems exist in physics, friction is a good, easy example. In cosmology, reversible processes are known as adiabatic physics and systems or processes in the universe which do not conserve information are known as diabatic systems.

Though, we have come to accept that reversible dynamics are clearly controlling the later portion of the universe, it is actually possible the universe could have been subjected to phases which were in fact irreversible. A proper entropy equation measures both reversible and irreversible dynamics which would look like

$$ds = \frac{dQ_{rev}}{k_BT} + \frac{dQ_{irr}}{k_BT}$$

Note in this case, entropy is dimensionless. Entropy is in fact, naturally dimensionless, but it can absorb the Boltzmann constant and take on its dimensions.

14. Sep 17, 2017

### Staff: Mentor

This is not correct. The working fluid (system) returns to its original state in an irreversible cyclic process. It is the surroundings which do not return to their original state. This is, by definition, an irreversible cyclic process.

Caution: Your response is bordering on misinformation.

15. Sep 17, 2017

### PhysicsExplorer

Well I can honestly say then, I have never heard of ''irreversible cyclic process'' and hence my misunderstanding.

16. Sep 17, 2017

### PhysicsExplorer

Only an honest mistake, I have not heard the terminology.