- A refrigeration engineer analyzes a compressor where refrigerant enters as vapor and exits at higher pressure and temperature. The compressor is driven by an electric motor. To determine the electrical power consumption, she defines the system boundary to include both the compressor and the motor. What is the primary advantage of this system definition compared to defining the system as the compressor only?
Answer: It eliminates the need to determine the mechanical work transfer between motor and compressor components
When analyzing thermodynamic systems, defining your system boundary strategically can simplify calculations by eliminating internal energy transfers that are difficult to measure. This question tests your understanding of how system boundaries affect the complexity of energy balance equations.
Choice A is correct because when you include both the motor and compressor within the system boundary, the mechanical work transfer between these components becomes an internal energy transfer rather than a system input. Since internal transfers cancel out in energy balance equations, you can directly relate electrical power input to the useful compression work output without needing to determine the efficiency of the motor-compressor coupling or measure the intermediate mechanical work.
Choice B is wrong because refrigerant cycle analysis depends on following the refrigerant through all cycle components (evaporator, condenser, expansion valve, compressor), not on whether you include the motor in your compressor analysis. Both system definitions allow cycle analysis.
Choice C incorrectly suggests that including the motor affects steady-state conditions. Steady state depends on whether system properties change with time, not on where you draw the boundary. Both system definitions can operate in steady state.
Choice D misunderstands system classification. Both definitions remain open systems because refrigerant mass flows into and out of the compressor regardless of whether you include the motor. Mass flow considerations exist in both cases.
Study tip: When facing system boundary problems, ask yourself "What energy transfers can I eliminate by drawing the boundary differently?" Including more components often simplifies analysis by making external transfers internal.
- A laboratory setup consists of an electric heating element submerged in water within an insulated beaker. The heating element is connected to an external power supply. A thermodynamics student wants to analyze the temperature rise of the water. She defines the system as 'the water only.' What is the most significant limitation of this system definition for her analysis?
Answer: Energy transfer to the system will appear to come from a source at the same temperature as the system boundary
When analyzing thermodynamic systems, your choice of system boundary fundamentally determines how energy transfers appear in your analysis. This question tests whether you understand how system definitions affect the apparent source of energy inputs.
The correct answer is C because when you define only the water as your system, the heating element becomes part of the surroundings. Since the heating element is in direct contact with the water, the system boundary passes right at the water-element interface. From the system's perspective, heat appears to be flowing from the boundary itself—which is at the same temperature as the adjacent water. This creates a thermodynamic impossibility: heat cannot spontaneously flow between objects at the same temperature, violating the Second Law of Thermodynamics. The analysis loses track of the true driving force (the electrical energy being converted to thermal energy).
Option A is wrong because system boundaries can have well-defined temperatures even when passing through objects. Option B misses the point—thermal equilibrium isn't the issue here; the problem is that the energy source appears to be at the wrong temperature. Option D reflects a common misconception; system boundaries don't have to follow material interfaces and regularly pass through solids in thermodynamic analysis.
Remember this key principle: when electrical heating is involved, include the heating element in your system definition. This ensures that electrical work appears as the energy input, rather than creating the impossible scenario of heat transfer with no temperature difference.
- A student observes an insulated rigid tank containing nitrogen gas. The tank has a pressure gauge and thermometer. Initially, the readings are 300 kPa and 25°C. Later, the readings are 350 kPa and 45°C. The student concludes that the system surroundings must have done work on the system. What is the primary flaw in this reasoning?
Answer: The system boundary characteristics prevent work transfer, making the conclusion about work interaction impossible
When analyzing work interactions in thermodynamics, you must carefully examine the system boundary and its characteristics. Work transfer requires either a moving boundary (like a piston) or some form of deformation that allows the system to exchange energy mechanically with its surroundings.
The correct answer is B because an insulated rigid tank fundamentally cannot allow work transfer. The key word here is "rigid" – this means the tank walls cannot move or deform. Since work in thermodynamics is defined as energy transfer due to a force acting through a displacement, and the tank boundary cannot move, no work interaction is possible regardless of what happens to the gas inside. The pressure and temperature changes observed are entirely due to the constraint that the gas volume remains constant in the rigid container.
Looking at the incorrect options: A suggests the conclusion about work is correct but reasoning incomplete – this is wrong because no work can occur at all with rigid boundaries. C claims the state change is impossible, but the described pressure and temperature increase is perfectly consistent with heating gas at constant volume. D incorrectly states that gas alone cannot be a thermodynamic system, when in fact any defined mass of substance can serve as a system.
Study tip: Always identify boundary characteristics first when analyzing energy interactions. Rigid boundaries eliminate work transfer possibilities, while adiabatic (insulated) boundaries eliminate heat transfer. This systematic approach will help you quickly eliminate impossible energy interactions in thermodynamics problems.
- A research team studies a gas-filled balloon that can expand or contract freely. They define two different systems: System A includes only the gas inside the balloon, and System B includes both the gas and the balloon material. Both systems are initially in equilibrium. When the external atmospheric pressure decreases, which statement best compares the system boundaries?
Answer: System A boundary moves outward and remains impermeable to mass, while System B boundary moves outward and remains impermeable to mass
When analyzing thermodynamic systems with movable boundaries, you need to carefully track how system definitions affect boundary behavior and permeability properties. This question tests your understanding of how different system boundaries respond to external changes.
Let's trace what happens when atmospheric pressure decreases. For System A (gas only), the gas will expand due to the pressure drop, so the boundary between the gas and balloon material moves outward. Since System A contains only gas, mass cannot cross this boundary - it remains impermeable to mass transfer. For System B (gas + balloon), the entire system expands outward as the balloon stretches, moving the boundary between the balloon material and atmosphere outward. This boundary also remains impermeable to mass since we're studying a sealed balloon system.
Choice A incorrectly suggests System B's boundary remains fixed, but the balloon material itself expands outward with decreasing pressure. It also wrongly introduces energy permeability changes that aren't relevant here. Choice C falsely claims System A's boundary stays fixed - the gas-balloon interface clearly moves as the gas expands. Choice D incorrectly states that System A becomes permeable to mass, which would mean gas escaping through the balloon wall - not described in this scenario.
Choice B correctly identifies that both boundaries move outward (System A's gas boundary and System B's outer balloon boundary) while both maintain impermeability to mass transfer throughout the expansion process.
Remember: always visualize exactly where each system boundary lies and track how physical changes affect boundary position and properties separately.
- An inventor claims that the thermodynamic state of a pure substance can be completely specified by measuring only its temperature and volume. Under what conditions would this claim be valid, and what does this reveal about the relationship between properties and state?
Answer: Valid for any pure substance in single-phase regions since two independent intensive properties define state, but volume must be converted to specific volume
For a pure substance in single-phase regions, any two independent intensive properties define the state. Temperature and specific volume (volume per unit mass) are independent intensive properties in single-phase regions. Choice A is too restrictive - this applies to real substances too. Choice C misses the key point about two independent properties and focuses incorrectly on equilibrium. Choice D is wrong because this doesn't work in two-phase regions where temperature and pressure are dependent.
- A power plant operator monitors a steam generator where liquid water enters and steam exits. The unit operates continuously. For safety analysis, regulations require tracking the total thermal energy content within the steam generator at any time. Which system definition and boundary characteristics would be most appropriate for this regulatory requirement?
Answer: Open system with fixed boundary around the steam generator internal volume, tracking energy content of whatever fluid is present
When analyzing continuous flow equipment like steam generators, you need to choose a system definition that matches your analysis goal. Since the regulatory requirement focuses on tracking thermal energy content within the generator at any given time, you're performing an instantaneous energy inventory analysis.
The most appropriate approach is an open system with a fixed boundary around the steam generator's internal volume (answer B). This definition captures exactly what regulations need: the total energy content of whatever fluid happens to be inside the generator at any moment. As water flows in and steam flows out, the boundary stays fixed while the mass and energy content within that boundary changes. This allows direct tracking of the thermal energy inventory without worrying about the complexities of mass flow rates.
Answer A is problematic because moving boundaries create unnecessary complications and don't align with how equipment is physically defined. The steam generator has fixed internal dimensions regardless of inventory levels. Answer C incorrectly includes the generator walls and structure, which adds irrelevant thermal mass that isn't part of the regulatory concern about fluid energy content. Answer D fundamentally misses the point by only looking at the inlet stream rather than the total energy inventory within the generator.
For thermodynamics problems involving continuous equipment, remember that your system boundary choice should directly support your analysis objective. When the goal is tracking energy inventory within equipment, use fixed boundaries around the equipment's internal volume with an open system approach to account for mass flow.
- A chemical reactor vessel contains a gas mixture undergoing an exothermic reaction. The vessel walls are thick steel that can store significant thermal energy. An engineer needs to predict the gas temperature during the reaction. She considers two system definitions: (1) gas mixture only, and (2) gas mixture plus vessel walls. What is the most important difference in how these system definitions will affect the temperature prediction?
Answer: System (1) will predict higher gas temperatures because the thermal mass of the system is smaller
When analyzing thermodynamic systems, the choice of system boundaries dramatically affects how energy flows and temperature changes. This question tests your understanding of thermal capacitance and heat capacity effects on temperature predictions.
The key insight is that thermal mass (heat capacity) acts like a "thermal buffer." In an exothermic reaction, chemical energy converts to thermal energy, raising the system temperature. However, the temperature rise depends on how much mass must be heated. System (1) includes only the gas mixture, which has relatively low thermal mass. System (2) includes the thick steel walls, which have much higher thermal mass due to steel's density and heat capacity.
When the same amount of heat energy is released, it gets distributed among all components in your defined system. With more thermal mass to heat up (System 2), the temperature rise will be smaller. Conversely, System (1) concentrates that same heat energy into a smaller thermal mass, resulting in higher predicted temperatures.
Answer A correctly identifies this relationship. Answer B incorrectly suggests the vessel walls generate additional energy—they don't; they only absorb and store energy. Answer C wrongly implies that excluding the vessel walls improves accuracy, when in reality, those walls significantly affect the actual gas temperature through heat transfer. Answer D describes temperature stability correctly but misses that System (2) predicts lower temperatures, not just more stable ones.
Remember: larger thermal mass = smaller temperature changes for the same heat input. Always consider what's inside your system boundaries when predicting temperature responses.
- A research facility has a vacuum chamber with multiple ports connected to different pumping systems. During operation, some ports actively remove gas while others are sealed. For thermodynamic analysis of the gas removal process, how should the system boundary be defined, and what challenges does this create for state analysis?
Answer: Boundary at the chamber walls creating an open system; challenges include multiple mass outflows with potentially different compositions and properties at each port
The boundary should be at the chamber walls since we're analyzing what happens to the gas inside. With active pumping ports, mass flows out, making it an open system. The challenge is that different ports may remove gas at different rates, compositions, or conditions. Choice A incorrectly classifies it as closed when mass flows out. Choice C unnecessarily expands the boundary beyond the process of interest. Choice D incorrectly places the boundary through the ports rather than around the volume being analyzed.
- An automotive engineer studies an internal combustion engine cylinder during the compression stroke. She defines the system as the gas mixture within the cylinder. During compression, the gas temperature increases from 350 K to 650 K while pressure increases from 101 kPa to 1800 kPa. She measures that the cylinder volume decreases from 500 cm³ to 50 cm³. Based on these measurements, what can be concluded about the system boundary behavior?
Answer: The boundary moved inward following the piston motion, and boundary work was performed on the system during compression
When analyzing thermodynamic systems, you must carefully define what constitutes the system and understand how the system boundary behaves. In this engine cylinder problem, the system is defined as the gas mixture itself, not the physical cylinder walls.
The key insight is that when you define a gas as your system, the system boundary moves with the gas molecules. As the piston compresses the gas from 500 cm³ to 50 cm³, the system boundary contracts inward following the piston motion. The volume decrease directly indicates boundary movement, and since compression work is being done on the gas (evident from the pressure increase from 101 kPa to 1800 kPa), boundary work is indeed being performed. This makes option B correct.
Option A incorrectly treats the cylinder walls as the system boundary, but the engineer explicitly defined the gas mixture as the system, so the boundary moves with the gas, not the walls. Option C contradicts the measured data—the volume clearly decreased during compression, and thermal expansion effects don't override the mechanical compression work being done by the piston. Option D incorrectly suggests that gas system boundaries are undefined, when in fact they're precisely defined by the outer surface of the gas molecules.
Remember that in thermodynamics, the system definition determines boundary behavior. When the system is a gas, the boundary moves with the gas surface, making volume changes direct indicators of boundary movement. Always match your boundary analysis to the given system definition, not the physical container.
- An engineer is analyzing a steam turbine where superheated steam enters at one condition and exits as wet steam. The turbine casing is the physical boundary, but for thermodynamic analysis, the engineer must define the system boundary. Which choice represents the most appropriate system boundary definition and reasoning?
Answer: The system boundary should be drawn through the inlet and outlet pipes to create an open system, excluding the turbine material to focus on the working fluid
For thermodynamic analysis of a turbine, the system boundary should be drawn as a control volume through the inlet and outlet, creating an open system that focuses on the working fluid. This allows proper analysis of mass flow and energy transfers. Choice A incorrectly suggests a closed system when mass flows through. Choice C unnecessarily complicates the analysis by including piping. Choice D misunderstands the purpose of system boundaries, which define what is being analyzed, not pressure regions.
- An engineer is designing a control volume analysis for a turbine. She initially considers the turbine casing and all internal components as the system, then later redefines the system to include only the flowing working fluid within the turbine at any instant. What is the most significant change in the system boundary characteristics between these two definitions?
Answer: The boundary changes from impermeable to mass flow to permeable to mass flow while remaining permeable to energy transfer
Control volume analysis is fundamental in thermodynamics, and understanding how system boundary definitions affect mass and energy transfer is crucial for proper analysis.
In the first case, when the system includes the turbine casing and all internal components, you're dealing with a closed system. The physical boundary is the outer casing, which prevents mass from crossing (impermeable to mass flow). However, energy can still transfer through this boundary via heat transfer and work output, making it permeable to energy.
When the engineer redefines the system to include only the flowing working fluid, she creates an open system (control volume). Now the boundary cuts through the inlet and outlet pipes, allowing working fluid to continuously enter and exit - making it permeable to mass flow. Energy transfer remains possible through heat transfer and work interactions, so it stays permeable to energy transfer.
Looking at the wrong answers: Choice A incorrectly suggests the boundary becomes impermeable to energy, which is impossible since turbines must transfer energy to do work. Choice C describes the boundary changing from movable to fixed, but neither definition involves movable boundaries - both are spatially fixed. Choice D claims the final system remains impermeable to mass, which contradicts the fundamental nature of control volumes where mass must flow through.
Study tip: Remember that closed systems have fixed mass (impermeable to mass flow) while open systems/control volumes allow mass flow. Both can transfer energy, but the key distinction is always about mass crossing the boundary.
- A rigid tank contains water vapor at 150°C and 0.5 MPa. Heat is removed from the tank until the pressure drops to 0.1 MPa. During this process, which statement best describes the changes to the system boundary and thermodynamic state?
Answer: The boundary remains fixed while both intensive and extensive properties change, representing a change in thermodynamic state
In a rigid tank (fixed boundary), when heat is removed and pressure drops, both intensive properties (temperature, pressure, specific volume) and extensive properties (internal energy, entropy) change, representing a definite change in thermodynamic state. Choice A is incorrect because extensive properties like internal energy change. Choice C is wrong because a rigid boundary cannot deform and the state clearly changes. Choice D is incorrect because intensive properties also change and temperature/pressure are state variables, not just process variables.
- A heat exchanger has hot fluid flowing through tubes and cold fluid flowing around the tubes. An analyst wants to study the temperature change of the hot fluid and defines the system as 'the hot fluid currently inside the tubes.' A colleague suggests this system definition will lead to analysis difficulties. What is the most likely reason for this concern?
Answer: The system mass is continuously changing as fluid flows, requiring complex accounting for mass transfer across boundaries
When analyzing thermodynamic systems, one of the most fundamental decisions is defining your system boundary. This choice determines whether you'll use an open system (control volume) or closed system (control mass) approach, each with distinct governing equations and complexity levels.
The suggested system definition creates a control mass that moves and deforms as hot fluid flows through the tubes. While this might seem intuitive since you want to track "the hot fluid," it creates significant analytical challenges. As fluid flows, the system mass continuously changes because new hot fluid enters the tubes while other fluid exits. This requires accounting for mass transfer across boundaries at every instant, making the energy balance much more complex. You'd need to track which specific fluid particles are "in" versus "out" of your system at each moment.
Looking at the wrong choices: (A) is incorrect because temperature differences across system boundaries are completely normal and don't violate any thermodynamic principles. (C) misrepresents the system definition—the analyst specifically defined the system as only the hot fluid, not the tubes themselves. (D) is wrong because the system boundary can be clearly defined (it's wherever the hot fluid currently is), even though this boundary moves and changes shape.
The better approach would be defining the tube interior as a control volume with fixed boundaries, then applying steady-flow energy equations that naturally handle the mass flow terms.
Study tip: When you see fluid flow problems, immediately ask whether the proposed system boundaries move with the fluid (complex) or remain stationary (simpler analysis).
- A materials scientist studies a metal rod that is heated at one end while the other end is cooled. The rod reaches steady state with a linear temperature profile from hot to cold end. She defines the system as a 1-cm segment in the middle of the rod. After steady state is established, what is the most accurate description of the thermodynamic state of this system?
Answer: The system cannot be assigned a thermodynamic state because temperature varies spatially within the system boundaries
When you encounter questions about thermodynamic states, remember that a system can only be assigned a well-defined thermodynamic state when its properties are uniform throughout and unchanging with time. This is a fundamental requirement for applying thermodynamic principles.
In this scenario, the 1-cm segment has a linear temperature gradient from the hot end to the cold end of the rod. Even though the system has reached steady state (meaning properties don't change with time), the temperature varies spatially within the system boundaries. Since temperature is a state property, this spatial variation means you cannot assign a single, well-defined thermodynamic state to the entire system. Different points within the segment exist at different temperatures, making option B correct.
Option A is wrong because while properties remain constant with time, they vary with position - uniformity in both space and time is required for a well-defined state. Option C incorrectly conflates steady state with thermodynamic equilibrium. In steady state, there's continuous heat flow through the system, meaning energy transfer rates are constant but non-zero across boundaries. True thermodynamic equilibrium requires no net energy transfer and uniform properties. Option D misunderstands the situation - the system isn't alternating between states; rather, it maintains a steady but non-uniform condition.
For thermodynamics problems, always check whether the system has uniform properties throughout its boundaries. If temperature, pressure, or other intensive properties vary spatially within the system, you cannot define a single thermodynamic state, even if the system appears stable over time.
- A thermodynamics student observes that a rigid, sealed container of gas has the following measured properties: pressure = 150 kPa, temperature = 300 K, volume = 0.5 m³. After heating, the measurements are: pressure = 200 kPa, temperature = 400 K, volume = 0.5 m³. The student claims these two sets of measurements represent two different thermodynamic states. What is the most accurate evaluation of this claim?
Answer: Correct, because at least one independent intensive property changed between the measurements, defining two distinct equilibrium states
When you encounter questions about thermodynamic states, remember that a thermodynamic state is defined by the values of all intensive properties at equilibrium. A system is in a different state if any independent intensive property changes.
Looking at the data, you have two sets of measurements with different pressures (150 kPa vs 200 kPa) and temperatures (300 K vs 400 K). Since pressure and temperature are both intensive properties (independent of system size) and they've both changed, these measurements represent two distinct thermodynamic states. The student's claim is correct.
Option A is wrong because it confuses the type of process with the definition of state. A constant volume process (isochoric) can absolutely connect different thermodynamic states - volume staying constant doesn't mean the state is unchanged if other properties vary.
Option B misunderstands how states are defined. While intensive properties are indeed the key defining characteristics, extensive properties like volume are still valid state properties. More importantly, this option ignores that the intensive properties (pressure and temperature) did change.
Option D reaches the right conclusion but for the wrong reason. Internal energy is a state function, but stating that increased internal energy "automatically" defines a new state is imprecise reasoning. The correct logic is that changed intensive properties define the new state.
Remember this pattern: thermodynamic states are characterized by intensive properties like pressure, temperature, and density. If any independent intensive property changes between two equilibrium conditions, you're looking at different states, regardless of what happens to extensive properties.
- A student measures the pressure, temperature, and volume of steam in a rigid vessel and declares these three measurements completely define the thermodynamic state. The professor points out an error in this reasoning. What is the most likely issue with the student's approach?
Answer: Three properties provide redundant information since only two independent properties are needed to define the state of a pure substance
For a pure substance, only two independent properties are needed to define the thermodynamic state. Having pressure, temperature, and volume provides redundant information since these three properties are related by an equation of state. Choice B is incorrect because extensive properties can be used if converted to intensive form or if mass is known. Choice C assumes non-equilibrium without evidence. Choice D addresses two-phase regions but doesn't identify the main issue of redundancy.
- A piston-cylinder assembly contains refrigerant R-134a. During a process, the piston moves while maintaining constant pressure. If the refrigerant transitions from saturated liquid to saturated vapor, how do the system boundary characteristics and state properties change?
Answer: The boundary is movable and impermeable; intensive properties remain constant while extensive properties change due to phase transition
In a piston-cylinder assembly, the boundary is movable (piston can move) and impermeable (no mass crosses). During constant pressure phase transition, intensive properties (P, T) remain constant while extensive properties (V, U, H, S) change significantly. Choice A incorrectly states the boundary is permeable. Choice C is wrong because the volume changes during phase transition (not constant volume). Choice D incorrectly states that intensive properties change significantly during constant pressure phase transition.
- A thermodynamic system consists of air in a sealed container placed inside a larger insulated chamber. The container has a small leak that allows air to escape slowly. Which statement correctly identifies the system boundary location and classification for analyzing the air mass?
Answer: Boundary at container walls creating an open, adiabatic system since mass crosses the boundary through the leak
When analyzing the air mass, the boundary must be at the container walls where the air is initially located. Since air leaks out through the hole, mass crosses this boundary, making it an open system. The outer insulation makes it adiabatic. Choice A incorrectly classifies it as closed despite mass flow. Choice C places the boundary incorrectly at the chamber walls. Choice D incorrectly suggests isothermal conditions and wrong boundary placement.
- A thermodynamics instructor presents a piston-cylinder assembly where the piston is held fixed by external stops. The cylinder contains steam at 200°C and 1.5 MPa. The instructor states that heat is removed until the steam temperature drops to 150°C while the piston remains against the stops. A student claims that the steam undergoes a change of state during this process. What additional information is minimally required to verify this claim?
Answer: The final pressure of the steam after cooling, since pressure and temperature together determine the state of steam
When analyzing phase change problems involving steam, you need to determine whether the substance crosses phase boundaries during the process. This requires understanding how pressure and temperature relate to the steam's state.
In this fixed-volume process, the steam starts at 200°C and 1.5 MPa, then cools to 150°C. To determine if a phase change occurs, you must know whether the steam crosses the saturation curve - the boundary between liquid and vapor phases. Since the volume is constant (piston against stops), as temperature decreases, pressure will also decrease following a constant-volume path. The critical question is: does this path cross into the two-phase region?
Answer A is correct because knowing the final pressure allows you to plot both initial and final states on a temperature-pressure diagram. If the final state (150°C at the final pressure) lies in the two-phase region while the initial state is superheated steam, then a phase change definitely occurred. Pressure and temperature together uniquely determine the intensive state of steam.
Answer B is wrong because heat removal alone doesn't indicate phase boundaries - heat transfer occurs in both single-phase and two-phase processes. Answer C is incorrect since mass is an extensive property and doesn't affect whether phase boundaries are crossed; intensive properties (pressure, temperature) determine the phase. Answer D is flawed because while volume affects the process path, you need the final pressure (not initial volume) to determine if the final state is in a different phase region.
Remember: For steam problems, always consider whether the process path crosses phase boundaries by examining pressure-temperature relationships.
- Using the information provided above, which system definition and analysis approach would be most appropriate for determining the power output of the turbine?
Answer: Control volume with boundary around the internal fluid volume, because power output depends on mass flow rate through the turbine
When analyzing thermodynamic systems, you must first decide whether to treat the system as a closed system (fixed mass, no mass flow) or an open system with a control volume (mass flows in and out). The key is identifying what physical quantity you're trying to calculate and whether mass flow is essential to that calculation.
For turbine power output, the fundamental relationship is that power depends directly on the mass flow rate of steam passing through the turbine and the energy change per unit mass. Since steam continuously flows into and out of the turbine during operation, you need a control volume analysis that accounts for this mass flow. Answer C is correct because it recognizes that power output inherently depends on mass flow rate, requiring a control volume approach with boundaries around the internal fluid volume where the thermodynamic processes occur.
Answer A incorrectly suggests a closed system approach. While the turbine casing does remain constant, this completely ignores the flowing steam, which is what actually generates the power. Answer B also wrongly chooses a closed system by focusing on the working fluid's thermodynamic process, but this misses that the fluid is continuously flowing—not a fixed mass undergoing a process. Answer D selects the right analysis type (control volume) but draws the boundary incorrectly around the entire turbine including the casing, when the relevant thermodynamic processes occur within the fluid volume inside the turbine.
Remember: when power output or efficiency involves flowing fluids, always think control volume analysis. The mass flow rate is typically a critical parameter in power calculations for turbomachinery.
