Thermodynamic State Variables and PV Diagrams
Pressure, volume and temperature describe the state of a system regardless of how it got there, and a path on a PV diagram shows a process. The area under the path is the work done.
What a learner can do afterwards
- Distinguishes state variables from path quantities such as work and heat
- Reads the work done from the area under a path on a PV diagram
- Explains why a closed cycle returns every state variable to its starting value
1 · Read
Pressure, volume, temperature, and internal energy describe the current state of a gas, however it got there. Work and heat instead describe one particular path, so they are not state variables.
On a PV diagram each path is a process, and the area under the path gives the work done. Hotter isotherms sit higher and steeper as hyperbolas that never cross. The equation of state links pressure, volume, and temperature, so any two fix the third.
Trace a closed loop and every state variable returns to its start. The work of each leg comes from its own area. A clockwise loop expands at high pressure and squeezes at low pressure, so net work flows out, and the enclosed area equals both the net work out and the net heat in.
Never read work off the endpoints alone. Two paths between the same endpoints can carry different work and heat, because work cares about the route.
State variables fix the now, paths fix the work, and loops return every state variable home.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.