# I Quasi-static and reversible processes in thermodynamics

1. Apr 20, 2016

### misko

Too much confusion in my head about these concepts...

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)?

2. Apr 20, 2016

### Staff: Mentor

Yes.
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.

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/