Thermodynamic Systems

Classification of open, closed, and isolated systems, thermodynamic boundaries and states.

General idea of the topic

Before applying any thermodynamic law, you must decide which boundary defines the system and what can or cannot cross it. This topic classifies systems according to whether they exchange matter, energy, both, or neither with their surroundings.

What you will truly understand

• Why an open system exchanges both matter and energy, and in which problems it is mandatory to use it.
• Why a closed system exchanges only energy, never matter, even though its volume may change.
• Why an isolated system exchanges nothing with its surroundings and is the idealization used to state conservation laws.
• How the choice of boundary completely changes which terms appear in the energy balance.

Recommended learning route

1. Open System: boundary permeable to both matter and energy.
2. Closed System: boundary permeable only to energy.
3. Isolated System: boundary impermeable to any exchange.

Physical criterion you must acquire

Before writing any balance, explicitly draw the system boundary and decide what crosses it and what does not. A misclassified system invalidates the entire subsequent setup, even if the algebra is correct.

Connection with real problems

Turbines and compressors are modeled as open systems; a gas in a closed cylinder with a piston, as a closed system; the entire thermodynamic universe, as an isolated-system approximation for applying the second law.

Final goal of the topic

Unambiguously identify which type of system describes each real situation and choose accordingly which matter and energy balance applies.