Physics · Ch 8 — Heat and Thermodynamics
Thermodynamic State Variables and Equation of State
Thermodynamic State Variables and Equation of State
A system's state at equilibrium is described by thermodynamic (state) variables -- pressure, temperature, volume, internal energy, entropy -- whose values completely fix the equilibrium state, unlike heat and work, which are process variables, not state variables. State variables split into extensive ones, which depend on the system's size or mass (volume, total mass, entropy, internal energy, heat capacity), and intensive ones, which do not (temperature, pressure, specific heat capacity, density). The equation of state is the relation connecting these variables at equilibrium -- for an ideal gas, ; for a real gas, the more accurate van der Waals equation. Full thermodynamic equilibrium requires simultaneous thermal equilibrium (same temperature throughout, Figure 8.18), mechanical equilibrium (no unbalanced force on or by the system -- Figure 8.17 …
What this figure shows. Two side-by-side cylinder-and-piston diagrams. The left diagram shows a gas-filled cylinder with a movable piston and no extra masses resting on it, at rest in its equilibrium position. The right diagram shows the same cylinder but now with masses placed on top of the piston; after the piston settles down to a new lower resting position, it stops moving once the upward force exerted by the compressed gas exactly balances the downward gravitational force of the piston plus the added masses -- this balanced, unmoving final state is …
What this figure shows. Two-part diagram, (a) and (b). In part (a), two separate systems A and B are each shown in direct thermal contact with a third system C, but A and B are not in contact with each other at all. In part (b), the same systems A and B are now shown placed directly in thermal contact with each other (with C no longer needed). The figure sets up the exact scenario the zeroth law addresses: after A and C reach equilibrium, and B and C separately reach equilibrium, part (b) demonstrates that A and B, when finally brought into contact with each other, show no further heat flow between them -- proving they were al …