A system is defined to be in thermodynamic equilibrium when the following conditions are met:
- The system is in thermal equilibrium. This means that, within the system, there is no net heat flow. For example, if your system consisted of two phases (or parts) that are kept separate across a diathermic wall, and one phase is at 350 K and the other is at 300 K, they would most likely not be within thermal equilibrium.
- The system is in mechanical equilibrium. This means that there can exist no net forces or torques on the system.
- The system must be in chemical equilibrium. Chemical equilibrium is difficult to define - the easiest way to express it is that no mixing occurs spontaneously in the system.
As well, if a system is in thermodynamic equilibrium, there cannot be gradients in any of the measurable properties of the system.
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