Q.What are mechanical equilibrium and thermal equilibrium?

State variables (like P, V, T) describe the actual condition of a thermodynamic system and can change; the universal gas constant R is a fixed proportionality constant in the equation of state (PV = nRT), not itself a describable property of any particular system's state.

A thermodynamic system's macroscopic state at any instant is described by state variables such as pressure (P), volume (V), and temperature (T) (also called state variables/parameters) -- these can and do change as the system evolves, and different combinations of P, V, T represent different states.

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The gas constant R is not a thermodynamic coordinate, unlike pressure, volume, and temperature.

Thermodynamic coordinates (or state variables) are the measurable quantities — such as pressure $P$, volume $V$, and temperature $T$ — whose values together specify the state of a thermodynamic system, and which change as the system undergoes a process.

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