An engineer must analyze a centrifugal pump. She knows the inlet and outlet flow rates, pressures, and temperatures. Which system type and energy term should she use?
AClosed system, using internal energy — the pump casing contains a fixed mass of fluid at any instant
BIsolated system, since pumps are insulated and exchange no heat with the environment
COpen system (control volume), using enthalpy — mass flows across the boundary and carries both internal energy and flow work
DClosed system, using enthalpy — enthalpy is always the correct energy form for mechanical devices
When mass flows in and out, the correct system type is an open system (control volume). For flowing mass, the energy carried across the boundary includes not just internal energy u but also flow work Pv — the work done by upstream fluid pushing the mass through the boundary. The combination u + Pv = h (specific enthalpy) is the correct energy term for flowing streams. The closed system analysis using internal energy is appropriate when following a fixed parcel of fluid, but knowing inlet/outlet conditions makes the control volume approach far more natural for devices like pumps, turbines, and compressors.
Question 2 Multiple Choice
Why must energy balances for open systems use enthalpy (h = u + Pv) for the energy carried by flowing mass, rather than just internal energy (u)?
AEnthalpy is a more accurate measure of thermal energy because it accounts for pressure effects
BFlowing mass must push itself across the system boundary against the local pressure, doing work equal to Pv; this flow work plus internal energy gives enthalpy h = u + Pv
CInternal energy is only defined for static systems and loses meaning when mass is in motion
DEnthalpy is more convenient to measure with thermocouples than internal energy
When a parcel of mass crosses a system boundary, it is pushed through by the fluid behind it — this takes work. The upstream fluid exerts pressure P on the parcel of specific volume v, doing flow work Pv per unit mass. This work is in addition to whatever internal energy u the parcel carries. The total energy transported across the boundary per unit mass is therefore u + Pv = h, specific enthalpy. This is not a convention or approximation — it is a precise accounting of all energy transfers when mass crosses a boundary. Omitting the Pv term would give the wrong energy balance for turbines, compressors, and all other flow devices.
Question 3 True / False
The same physical apparatus (such as a pump) can be correctly analyzed using either a closed-system or an open-system framework, and both approaches yield the same physical predictions.
TTrue
FFalse
Answer: True
The system boundary is a choice the engineer makes, not a property of the apparatus. A pump can be analyzed as a control volume (fixed region in space, with fluid flowing through it) using the steady-flow energy equation, or by following a fixed parcel of fluid through the pump as a closed system. Both frameworks are thermodynamically correct and yield consistent predictions for work input, pressure rise, and efficiency. The choice is purely a matter of convenience — the control volume is usually simpler when inlet/outlet conditions are known; the closed system is simpler when following the fate of a specific mass parcel.
Question 4 True / False
An isolated thermodynamic system can exchange heat with its surroundings, but not work.
TTrue
FFalse
Answer: False
An isolated system exchanges neither heat nor work (nor mass) with its surroundings — it has no interaction with the surroundings whatsoever. This is the definition of isolation in thermodynamics. A system that exchanges heat but not work (or work but not heat) is a closed system with specific boundary properties (adiabatic for no heat, rigid for no work). Isolated systems are mainly theoretical tools used to apply the entropy principle: for an isolated system, entropy can only increase or stay constant, never decrease. Most real engineering devices are open systems (exchanging mass, heat, and work).
Question 5 Short Answer
Why is defining the system boundary the essential first step in any thermodynamic analysis, rather than just a bookkeeping formality?
Think about your answer, then reveal below.
Model answer: The system boundary determines which physical interactions appear as terms in the energy and mass balance equations, and which belong to the surroundings and are excluded. Choosing a rigid boundary means no boundary work appears (W_boundary = ∫P dV = 0). Choosing an open boundary means mass flow terms enter the energy balance, and those terms must use enthalpy rather than internal energy. Choosing an adiabatic boundary eliminates heat transfer terms. Getting these choices wrong means writing incorrect balance equations — the subsequent algebra will be solving the wrong problem, regardless of how carefully it's executed. The system definition is not a preliminary step before the real work begins; it is the step that determines what the real work will be.
The deeper point from the Explainer is that the same apparatus admits multiple valid system definitions. A competent thermodynamicist chooses the boundary that matches what is known (flow rates and inlet/outlet states → control volume; known mass and process path → closed system). The boundary is a tool for matching the mathematical framework to the available information, not a fact about the world to be discovered.