Ch 11 - Thermodynamics11th Physics

The Distinction

• Kinetic Theory (Microscopic): Involves tracking the detailed coordinates, velocities, and individual collisions of the huge number of discrete molecules constituting the gas.
• Thermodynamics (Macroscopic): Completely avoids molecular descriptions. It deals exclusively with bulk systems and is specified by macroscopic state variables that can be directly felt or measured, such as Pressure ($P$), Volume ($V$), Temperature ($T$), Mass ($M$), and Composition.

The Distinction

Imagine an equilibrium system divided perfectly in half:

• Extensive Variables: Variables whose values get halved in each part. They indicate the overall size or amount of the system.
  Examples: Volume ($V$), Mass ($M$), Internal Energy ($U$).
• Intensive Variables: Variables that remain identically unchanged for each part. They are strictly independent of the system's size.
  Examples: Pressure ($P$), Temperature ($T$), Density ($\rho$).

The Distinction

• State Variable ($U$): Its value depends only on the current equilibrium state of the system (determined by $P$, $V$, $T$), regardless of the history of how that state was achieved.
• Path Variables ($Q$ and $W$): Heat and Work are not inherent properties possessed by a system; they are modes of energy transfer. The amount of heat absorbed or work done depends entirely on the specific path or process taken to go from the initial state to the final state.