• id: JTHERM-01-N01
• title: Systems, Surroundings, and State Variables
• tags: system, state-variable, equilibrium

A thermodynamic system is a definite quantity of matter or region in space chosen for study. Everything outside is the surroundings. Systems can be open (exchange matter and energy), closed (exchange energy only), or isolated (exchange nothing). State variables (P, V, T, U, S) describe the equilibrium state — they are independent of the path taken. Process variables (Q, W) depend on the path. For an ideal gas, any two of P, V, T specify the state completely (the third is determined by PV = nRT).