4: Processes and cycles

Section 1: Fundamental Concepts 4: Processes and Cycles

A process is a transformation of a system from one equilibrium state to another. It is the path of successive states through which the system passes. Since a change in state occurs, at least one property of the system must change. Processes are often described by the property that remains constant, such as:

• Isobaric: Constant pressure
• Isochoric (or Isometric): Constant volume
• Isothermal: Constant temperature
• Adiabatic: No heat transfer ($Q = 0$)

A quasi-equilibrium or quasi-static process is an idealized, infinitely slow process where the system remains infinitesimally close to an equilibrium state at every instant. This is a crucial concept because it allows us to represent the path of a process on property diagrams (like a P-V or T-s diagram) with a continuous line and to use the system's properties (like pressure $P$ and temperature $T$) to calculate work and heat transfer. Real processes are not truly quasi-static, but the assumption is often made to simplify analysis.

The work done during a process is path-dependent. It is calculated by the integral of $PdV$ for a quasi-equilibrium compression or expansion process. Because it depends on the path, work is not a property of the system.

When a system undergoes a series of processes and returns to its initial state, it has executed a cycle. At the conclusion of a cycle, all properties ($P$, $T$, $V$, $U$, etc.) return to their initial values. The net change in any property for a cycle is therefore zero.

However, the net work and net heat transfer for a cycle are not zero. This is the fundamental principle behind all power generation and refrigeration systems. The First Law of Thermodynamics for a cycle simplifies to: $Q_{net} = W_{net}$, meaning the net heat transfer into the system is equal to the net work output from the system. Cycles are graphically represented as closed loops on property diagrams.