1: Thermodynamic systems (open, closed, isolated)

Thermodynamic Systems: Open, Closed, Isolated

A thermodynamic system is a precisely defined region of space or quantity of matter under study. Everything external to the system is the surroundings, and the boundary separates the two. Systems are classified by how they exchange mass and energy with surroundings, critical for analyzing processes.

Open Systems

Open systems exchange both mass and energy (heat/work) with surroundings. Mass flow occurs across the boundary, making these systems control volumes. Examples include:

• A jet engine (fuel/air enter, exhaust exits).
• An open water boiler (water evaporates as steam escapes).
  Key equations (e.g., First Law) must account for incoming/outgoing mass, requiring terms for mass flow rates and associated energy.

Closed Systems

Closed systems permit energy transfer but no mass transfer across their boundary. The system contains a fixed amount of matter (control mass). Examples:

• A sealed piston-cylinder assembly (heat/work exchanged, but gas mass constant).
• A tightly capped reaction flask undergoing heating.
  Work ($W$) and heat ($Q$) interactions alter the system’s internal energy, governed by $\Delta U = Q - W$.

Isolated Systems

Isolated systems exchange neither mass nor energy with surroundings. Total internal energy ($U$) remains constant ($\Delta U = 0$). These are idealized but useful for modeling:

• A perfectly insulated thermos flask (negligible heat/mass transfer).
• The entire universe (if defined as the system).
  Real systems approximate isolation when processes are rapid (e.g., adiabatic compression in engines).

Practical Considerations

• Boundary flexibility: A piston boundary can move (allowing work via volume change), while rigid boundaries exclude $P-V$ work.
• Idealizations: Truly isolated systems are hypothetical; real applications assume near-isolation when external interactions are negligible.
• Composite systems: A refrigerator is open (refrigerant flows), but its contents may be treated as closed if the door seals.

Understanding system classification clarifies which conservation laws (mass/energy) apply and simplifies thermodynamic analysis.