TP: Thermodynamic States

Thermodynamic States

Definition:

The thermodynamic state of a system is the condition in which it is currently in. Knowing the state fixes all of the system's intensive properties.

Definition:

A process is the act of bringing a system from one state to another.

A process may be adiabatic, isothermal, isobaric, isochoric (also isenthalpic, isentropic, etc. but we will discuss those later).

What is interesting about a process is that values of thermodynamic intensive properties do not depend on the path that the process takes from one state to the next. They simply depend on the states achieved.

fictitious paths

Definition:

A state function or property of state is independent of path (only depends on state). Examples: T, P, v, u

Because of this path independence, changes in the state functions are often calculated via paths that are simply convenient/hypothetical rather than real.

Nevertheless, there are other important quantities, such as heat and work, that are path dependent. These are often called path functions.

Heat and work are path functions (path dependent).

Explain the difference between state and path variables

Test Yourself:

Is the specific volume of the following materials the same or different at the end of the process?

  • 100.0 kg of methane gas initially at 100C is cooled to room temperature and atmospheric pressure
  • 1.0 g of liquid methane at -10C is vaporized rapidly to form a gas at room temperature and pressure

While the answer to this question may not be immediately obvious, hopefully the answer to an analogous question is: what is the corresponding final height (above sea level) of each of the following students?

  • Student A takes the bus from Shadyside, arrives at Benedum Hall and takes the elevator to the 12th floor
  • Student B gets on a helicopter on Mt. Washington, lands on the room of Benedum Hall and climbs down the stairs to the 12th floor