# Quasi-static and reversible processes in thermodynamics

• misko
In summary, the conversation discusses the relationship between reversible and quasi-static processes, as well as the applicability of the formula for calculating entropy change. It is clarified that reversible processes are always quasi-static, but the reverse is not necessarily true. The formula is only applicable for reversible processes, and a special method must be used to calculate entropy change for irreversible processes.

#### misko

Is every reversible process quasi static? If not, what are some examples?

If process is irreversible then it doesn't need to be non quasi-static, I understand that. (eg. free expansion of the gas)
Can irreversible process be quasi static? If so what are some examples?

When is the formula for calculating entropy change applicable (dS=dQ/T)? Is it for reversible or it can also be for irreversible quasi-static (if they exists)?

misko said:

Is every reversible process quasi static? If not, what are some examples?
Yes.
If process is irreversible then it doesn't need to be non quasi-static, I understand that. (eg. free expansion of the gas)
Can irreversible process be quasi static? If so what are some examples?
Quasistatic refers only to the mechanical features of a process. Even if a system is quasi-static mechanically, you can still have irreversibilities if there are significant temperature gradients (heat conduction) in the system or significant concentration gradients (diffusion).

A reversible process is one for which, at each increment of the process causing the transition of the system from the initial thermodynamic equilibrium state to the final thermodynamic equilibrium state, the system is only slightly removed from being at thermodynamic equilibrium throughout. Therefore, a reversible process can be viewed as a continuous sequence of thermodynamic equilibrium states.

When is the formula for calculating entropy change applicable (dS=dQ/T)? Is it for reversible or it can also be for irreversible quasi-static (if they exists)?
The formula is only applicable for reversible changes. For an irreversible process in which Q is the heat flow and TB is the temperature at the boundary of the system where the heat flow is occurring, ##\Delta S>Q/T_B##. To get the change in entropy for an irreversible process, you need to follow the following 3-step recipe:

1. Forget about the irreversible process path entirely, and focus only on the initial and final thermodynamic equilibrium end states.
2. Dream up a reversible path to take the system from the initial state to the final state. This reversible path does not have to bear any resemblance whatsoever to the actual irreversible process path. For example, even if the actual irreversible path is adiabatic, the reversible path can involve exchange of heat with the surroundings. There are an infinite number of reversible paths between the two end states. They all give the same result for the change in entropy, as calculated from Step 3. So choose one that is simple for you.
3. Calculate the integral of dQ/T for the reversible path that you have chosen. This will be the change in entropy for both the reversible path and the irreversible path.

If you would like to gain additional understanding of these concepts, see my Physics Forums Insights article: https://www.physicsforums.com/insights/understanding-entropy-2nd-law-thermodynamics/

## 1. What is the difference between quasi-static and reversible processes?

Quasi-static processes are those that occur slowly enough for the system to remain in thermal equilibrium throughout the process. Reversible processes, on the other hand, are those that can be reversed without any change in the system or its surroundings. Essentially, quasi-static processes focus on the speed of the process, while reversible processes focus on the ability to reverse the process.

## 2. How do quasi-static and reversible processes relate to thermodynamic equilibrium?

Quasi-static and reversible processes are both considered to be in thermodynamic equilibrium. In quasi-static processes, the system is always in thermal equilibrium, while in reversible processes, the system is in both thermal and mechanical equilibrium. This means that the pressure and temperature are the same throughout the system and there is no net transfer of energy.

## 3. Can all thermodynamic processes be considered quasi-static and reversible?

No, not all thermodynamic processes can be considered quasi-static and reversible. Quasi-static processes require the system to remain in thermal equilibrium throughout the process, which may not be possible for certain rapid or spontaneous processes. Reversible processes also require very specific conditions and may not be possible in all systems.

## 4. How are quasi-static and reversible processes represented in a thermodynamic cycle?

Quasi-static processes are typically represented as straight lines on a thermodynamic cycle, as they occur slowly and continuously. Reversible processes are represented as curved lines, as they can be reversed and do not have a definite beginning or end. Both types of processes are important in understanding the overall behavior of a thermodynamic system.

## 5. What are some real-life examples of quasi-static and reversible processes?

Examples of quasi-static processes include slowly heating a pot of water on a stove or compressing a gas in a piston slowly enough for the temperature to remain constant. Reversible processes can be seen in systems with minimal energy loss, such as an ideal gas expanding and contracting within a perfectly insulated container. Biological processes, such as muscle contraction and expansion, can also be considered reversible processes.