Equilibrium

Thermodynamic Equilibrium

Requires a system to be in mechanical, thermal, and chemical equilibrium.

Mechanical Equilibrium

The sum of the forces/pressures/torques is 0. Thus no work being done.

\sum F = 0, \sum W = 0, P_{i n} = P_{o u t}

Thermal Equilibrium

Temperature of system the same as the surroundings. Thus no heat.

\sum Q = 0, T_{i n} = T_{o u t}

Chemical Equilibrium

Same chemical potential. Thus no change in chemical composition.

Quasi-Equilibrium

"Slow" transformation with each steps in equilibrium.
Could stop transformation at any (intermediate) time