Questions: Thermodynamic Systems and System Boundaries

Question 1

An engineer is analyzing a steam turbine at steady state. She writes the energy balance as Q̇ − Ẇ_shaft = ṁ(h_out − h_in). Why does enthalpy appear instead of internal energy?

• A. Enthalpy is always larger than internal energy, giving a conservative estimate
• B. Flowing steam does work (Pv) pushing itself through the inlet and outlet; h = u + Pv bundles this flow work into a single term
• C. Internal energy is conserved only in closed systems; open systems require a different conservation law
• D. The engineer chose enthalpy for convenience; either h or u gives the same answer

Answer: (B)

When mass crosses a system boundary, it must push against the pressure at the inlet to enter, doing work Pv per unit mass (flow work). This term is not zero and must appear in the energy balance. Rather than writing u + Pv separately for every stream, engineers use h = u + Pv, which incorporates flow work automatically. Using internal energy alone would undercount the energy carried by flowing mass — option D is wrong because u and h give different numerical answers.

Question 2

A student draws the system boundary tightly around a turbine blade passage; her classmate draws it around the entire turbine casing. Which statement best describes their analyses?

• A. Only the boundary aligned with the physical turbine walls is thermodynamically valid
• B. Both boundaries are valid analytical choices; each gives different information about the same physical system
• C. The larger boundary is always preferred because it captures all irreversibilities
• D. The smaller boundary violates the First Law because it omits shaft work

Answer: (B)

The system boundary is an analytical choice, not a physical fact. A tight boundary around a blade passage gives detailed local information (pressure and velocity distributions); a boundary around the entire casing gives the overall Q, W, and enthalpy change in one equation. Both satisfy the First Law — they just track different crossing terms. The art of engineering thermodynamics is choosing the boundary that most efficiently reveals what you need to know.

Question 3

A closed system can seldom exchange work with its surroundings — mainly heat can cross its boundary.

• T. True
• F. False

Answer: False

A closed system cannot exchange *mass* with its surroundings, but it can exchange both heat and work. Boundary work (W = ∫P dV) occurs whenever the boundary moves, as in a piston-cylinder device. Shaft work (e.g., a stirrer driven by a motor through a sealed shaft) and electrical work can also cross a closed system boundary. The defining constraint of a closed system is fixed mass, not fixed energy.

Question 4

Enthalpy is the natural thermodynamic potential for open systems because it already incorporates the work done by flowing mass at the system boundary.

• T. True
• F. False

Answer: True

Every mass stream crossing an open system boundary does flow work Pv on entry and receives flow work Pv on exit. Including these terms alongside internal energy u gives h = u + Pv. The steady-state First Law for open systems therefore involves enthalpy changes (Δh) per unit mass rather than internal energy changes — not because the physics is different, but because h is the appropriate accounting unit when mass is flowing. This is the mechanistic reason enthalpy appears throughout power generation and refrigeration analysis.

Question 5

Why is the choice of system boundary an engineering decision rather than a physical fact, and what criteria guide a good choice?

Model answer: A system boundary is an imaginary surface drawn by the analyst to define what is 'inside' (the system) versus 'outside' (the surroundings). Nature does not prescribe a unique boundary; multiple valid choices exist for the same device. A good boundary minimizes complexity by maximizing what is already known. Practical criteria: (1) Does the desired unknown appear cleanly in the resulting energy balance? (2) Can all crossing terms — heat, shaft work, mass streams and their enthalpies — be identified from available data? (3) Can the system be treated as steady-state or must transient effects be tracked? Drawing the boundary around the whole turbine casing is simpler if only net power output is needed; drawing it inside reveals blade-level inefficiencies.

This insight distinguishes engineering problem setup from mere formula application. Students who treat the system boundary as fixed often get stuck when standard configurations are not present. The freedom to choose the boundary is a problem-solving tool: a well-drawn boundary converts a complex system into a tractable one-equation energy balance.