What is Quasi-Equilibrium ?

One example of a quasi-equilibrium process is a piston and cylinder device in which the contents are compressed very slowly.

Quasi-Equilibrium Process

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A process during which the system only deviates from equilibrium by an infinitessimal amount.
Cylinder
Piston
Piston
Piston
Piston
Piston
Piston
Piston
Piston
Piston
Piston
Piston
Piston
Piston
Piston
As the piston compresses the gas inside the cylinder, the pressure inside the gas remains very nearly uniform at all times during the process. So, the system never deviates significantly from mechanical equilibrium.

No system can be in complete equilibrium as it undergoes a real process.

However, some processes are almost in perfect equilibrium throughout the process.

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Ch 1, Lesson D, Page 6 - What is Quasi-Equilibrium ?