# I Is a quasi-static but irreversible process possible?

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1. Apr 1, 2017

### goodphy

Hello.

I read the textbook of the thermodynamic and it said the definition of the reversible process as "thermodynamic process which is slow enough so the system state is always infinitesimally close to the thermodynamic equilibrium (quasi-static) during the process. Such a process can always be reversed without changing the thermodynamic state of the universe". I accepted this definition in a way that "quasi-static" and "reversible" is equivalent.

Is this true? Is there any process which is quasi-static but not reversible?

2. Apr 2, 2017

### Staff: Mentor

Heat transfer from a hot slab to a cold slab through an intervening wall of very low thermal conductivity.

Last edited: Apr 2, 2017
3. Apr 2, 2017

### Mapes

A reversible process doesn't increase entropy and thus cannot exist in the real world (although we can come arbitrarily close). Every real process is irreversible. You can generally slow down a process as much as you wish to satisfy your criterion for "quasi-static". Another simple example to complement Chester's is a container of water with a pinhole opening, left to evaporate. Others are a battery attached to a very high resistance, a load applied to a solid at a low temperature (i.e., a solid with little propensity to creep) or a high-viscosity liquid, a nonspherical asteroid, or an amorphous solid whose equilibrium state is crystalline.

4. Apr 5, 2017

### goodphy

Thanks you! you and Chestermiller suggest examples of a quasi-static and irreversible process. I've searched the internet and found that the reversible process is quasi-static but its converse is not guaranteed. So, I think the definition of the reversible process given in my book has some hole. So, the true definition of the reversible process is that the reversible process is the process which doesn't change a total entropy of the system and its surrounding or simply the entropy of the universe, right?

Could you tell me why the reversible process has to be quasi-static? Why the process which is done pretty much fast so never be quasi-static has to be an irreversible process?

5. Apr 5, 2017

### Mapes

Whenever energy transfer is driven by a gradient (e.g., a pressure difference causing a change in volume, a voltage difference causing electric charge transfer, or a temperature difference that heats something up), entropy is produced and reversibility is violated. In contrast, steep gradients are associated with rapid process because the driving force is large. So reversibility can only be approached through very slight gradients, resulting in low rates (i.e., quasi-static processes).

6. Apr 6, 2017

### Staff: Mentor

To add to what Mapes said, in a system featuring finite velocity gradients, entropy generation is caused by viscous dissipation of mechanical energy to internal energy, and the local rate of entropy generation per unit volume is proportional to the square of the velocity gradient and to the viscosity. In a system featuring finite temperature gradients, the local rate of entropy generation per unit volume is proportional to the square of the temperature gradient and to the thermal conductivity.