DoAssignment study guide

1.1 · Define the system, working fluid, state, and process

Learn to define the system, working fluid, state, and process through clear examples and targeted practice.

University of Alberta MEC E 340: Applied Thermodynamics

Cycle Analysis Foundations

MEC E 340 Applied Thermodynamics — Study topic 1.1

Applied thermodynamics problems begin by deciding what is being studied. That choice fixes the system boundary, identifies the matter whose behaviour matters, and determines how to describe changes between conditions. A closed system contains a fixed amount of matter; a control volume is a chosen region through which matter may flow. In either case, state numbers identify conditions, and a process describes a change from one state to another. These definitions prepare you to apply mass and energy balances later. This lesson focuses on defining the system, working fluid, state, and process rather than solving a full cycle.

What you will learn

Distinguish a closed system from a control volume by identifying what can cross its boundary.
Identify the working fluid and describe the chosen system boundary.
Explain what a thermodynamic state and process mean in an applied-cycle or device description.
Use numbered states and consistent assumptions to prepare a problem for later balance calculations.

1. Choose the system and boundary

A thermodynamic system is the matter or region selected for study. Its boundary separates it from everything outside, called the surroundings. The boundary may be a real surface, such as a vessel wall, or an imaginary surface drawn around a device. Before using an equation, say what lies inside that boundary.

A closed system contains a fixed amount of matter. Energy may cross its boundary as heat or work, but matter does not. For example, the gas inside a sealed piston–cylinder can be treated as a closed system if the cylinder remains sealed. The piston may move, so the boundary can change shape while still enclosing the same matter.

An open system is commonly described by selecting a control volume: a defined region in space. Matter can enter or leave through its boundary, and energy can also cross. A turbine casing is often represented as a control volume, with fluid entering and leaving. The turbine is not the working fluid; the fluid passing through it is.

The choice is a modelling decision, so it should match the question. A sealed container and its contents may be one closed system. A component with inlet and outlet streams is often more clearly described as a control volume. State the boundary and whether mass crosses it rather than relying on the device name alone.

For a closed system, the mass inside the chosen boundary remains constant. m=constant

A system is the selected matter or region; its boundary separates it from the surroundings.
A closed system has no mass crossing its boundary.
A control volume is a region that may have mass entering or leaving.

2. Name the working fluid and describe a state

The working fluid is the substance, or mixture, whose thermodynamic behaviour is being studied. It might be air in a gas-cycle device, water or steam in a vapour system, or a refrigerant in a refrigeration device. For a mixture, identify the mixture as the working fluid and give its composition only when the problem provides it.

A state is the condition of the working fluid at a specified instant or location. It is described using properties such as pressure, temperature, specific volume, and, where relevant, phase or composition. A complete description must contain enough information to identify the condition for the model being used. Do not assume that a state is known merely because it has a number.

Number states in a consistent order that follows the process or flow path. For example, state 1 can mark a control-volume inlet and state 2 its outlet. In a cycle, state numbers identify successive conditions around the loop. Numbers are labels, not property values: state 2 is not inherently hotter, higher-pressure, or later in time unless the stated process establishes that.

Use only supplied information or an identified property source to assign numerical properties. If pressure and temperature are given, the appropriate property model or table may be needed to determine other values. This lesson does not assume particular table entries or phase boundaries.

(P_i,T_i,v_i,\ldots)

Identify the fluid separately from the device containing or processing it.
A state is a condition described by properties, not just a numbered point.
Number states in the order relevant to the process, and define what each number denotes.

3. A process connects states

A process is the change of a system from one state to another. Its endpoints can be labelled state 1 and state 2. The process description may include what is held constant or what physical interaction occurs, but do not add conditions that the problem has not supplied. For example, calling a process constant-pressure requires that assumption to be given or justified.

A process is not the same thing as a state. State 1 and state 2 describe endpoint conditions; the process describes the change between them. A cycle is a sequence of processes that returns the working fluid to its starting state. A device may instead be described by an inlet state and an outlet state, without claiming that the fluid completes a cycle inside that component.

The first-law balance is a useful reminder of why the boundary matters: energy transfers and, for a control volume, mass flows are counted at that boundary. Choose a sign convention and keep it consistent if a later calculation requires one. You do not need to calculate heat or work merely to define a system or process.

A pressure–specific-volume or temperature–entropy sketch can help show state order and process connections, but it is schematic unless enough property information is given to place states quantitatively. Do not infer numerical coordinates, phase boundaries, or an unprovided process model from the shape of a sketch.

1\rightarrow2\rightarrow\cdots\rightarrow1

A process connects two or more states; its name must match the stated assumptions.
A cycle returns to its initial state after a sequence of processes.
A sketch communicates sequence and intended process, not unprovided property data.

4. Set up before calculating

Before writing balances, make a short setup: name the system or control volume; identify the working fluid; label the states; list the assumptions supplied by the problem; and note whether mass crosses the boundary. For a device, mark the inlet and outlet. For a closed system, make clear which fixed amount of matter is included.

Then select the balance appropriate to that boundary. For a closed system, the enclosed mass does not change. For a control volume over a time interval, the net mass entering equals the increase in mass held within the region. Here, the quantities are total masses crossing or accumulating during that interval, not mass flow rates. Energy balances likewise depend on the selected system and the transfers that cross its boundary. These are orientation rules, not a request to calculate an unspecified device.

A consistent setup prevents common errors: using a device as though it were the fluid, treating an open component as a closed system without justification, or assigning physical meaning to state numbers that are only labels. When information is missing, state what is unknown instead of inventing a property or process.

Writing the mass balance explicitly helps keep the sign convention clear. Define the incoming and outgoing amounts over the same interval, then compare their net amount with the change in mass stored inside the control volume.

m_{in}-m_{

\mathrm{out}

}=\Delta m_{

\mathrm{CV}

}

Define the boundary before choosing a balance.
Record the fluid, state labels, flow crossings, and supplied assumptions.
Do not fill missing property or process information with guesses.

Worked example

A sealed gas vessel

A rigid, sealed vessel contains air. Describe a suitable system, working fluid, and state notation when the air is examined before and after heating.

Choose the boundary
Take the air inside the sealed vessel as the system. Since the vessel is sealed, no air crosses this boundary. The vessel is rigid, but rigidity is not what makes the system closed; the absence of mass flow does.
Name the fluid and states
The working fluid is air. Use state 1 for its condition before heating and state 2 for its condition afterward. The labels alone do not provide pressure or temperature values.
$1\rightarrow2$
State the limitation
Heating describes an energy interaction, but no amount of heat or final property data is supplied. Therefore, the setup can identify the system and process without calculating the final state.

Answer: The system is the fixed amount of air in the sealed vessel; air is the working fluid; states 1 and 2 denote its conditions before and after heating. No numerical state properties can be determined from the information given.

Check: Mass stays within the chosen boundary, as required for a closed system.

Worked example

A turbine control volume

Steam enters a turbine at a location called state 1 and exits at state 2. The question concerns the turbine as a component. Define the system and the state-to-state process without assigning property values.

Turbine control volume

Schematic control volume for steam flowing from state 1 to state 2; no property values or energy-transfer magnitudes are specified.

Select the control volume
Draw an imaginary boundary around the turbine component. Steam crosses that boundary at the inlet and outlet, so this is an open-system description, represented by a control volume.
Identify the working fluid
The working fluid is steam. State 1 is the inlet condition and state 2 is the outlet condition. These labels locate conditions along the flow path; they do not specify pressure, temperature, or phase details beyond the statement that the inlet fluid is steam.
$1\rightarrow2$
Limit the conclusion
The state-to-state process is the change in the fluid as it passes through the turbine. No energy balance or performance value can be calculated without the needed flow, property, and energy-transfer information.

Answer: Use the turbine as the control volume and steam as the working fluid. Label the inlet state 1 and outlet state 2. Mass crosses the boundary, so it is not a closed-system model.

Check: The boundary includes the component while allowing inlet and outlet flow, matching the description.

Worked example

States around a simple cycle

A diagram for a gas-cycle study labels four successive conditions 1, 2, 3, and 4, then returns to 1. Explain what the labels and connections mean, without assuming a particular cycle model or numerical properties.

Four-state cycle: schematic

Schematic pressure–specific-volume representation of a closed sequence for the chosen working fluid; not to scale and not based on supplied property coordinates.

Name the fluid and system
For this description, identify the gas specified by the study problem as the working fluid. The system must still be stated: it could be a closed amount of gas followed around a cycle, or the relevant plant could be divided into control volumes. The state labels alone do not settle that choice.
Read the state order
States 1 through 4 mark successive conditions. The directed connections represent processes between those conditions, and the return to state 1 indicates a cycle for the chosen working fluid.
$1\rightarrow2\rightarrow3\rightarrow4\rightarrow1$
Avoid unsupported claims
The numbering does not tell you which process has constant pressure or temperature, nor does it provide coordinates on a property plot. Those details require the problem statement or supplied property data.

Answer: The four labels identify successive states, and the directed connections identify processes. Returning to state 1 describes a cycle. The system boundary and any specific process conditions must be supplied separately.

Check: A cycle ends at the starting state, but its state labels do not by themselves define the boundary or process models.

Common mistakes and how to avoid them

Calling a turbine the working fluid.

Correction: The turbine is a device or control volume; the gas or vapour flowing through it is the working fluid.

Treating any device as a closed system.

Correction: Check whether mass crosses the chosen boundary. Inlet or outlet flow means a control-volume description is appropriate.

Assuming state 2 must have a particular pressure or temperature because it follows state 1.

Correction: State numbers establish labels and order only. Use stated process conditions and supplied data to determine property changes.

Reading exact properties from a schematic cycle sketch.

Correction: Unless coordinates or property data are supplied, use the sketch to understand state order and connections, not to infer numerical values.

Lesson summary

Choose and describe the system boundary before applying a balance.
A closed system contains fixed mass; a control volume can have mass crossing its boundary.
The working fluid is the substance or mixture being studied.
States identify conditions; processes describe changes between states.
State numbering, diagrams, and assumptions must be consistent, and missing data must not be invented.

Check your understanding

Question 1

A sealed vessel contains a fixed amount of refrigerant. Which description fits the contents when studying their heating?

A closed system; the refrigerant is the working fluid.
A control volume with an inlet and outlet, because it is a vessel.
The vessel is the working fluid; the refrigerant is the system boundary.
A cycle, because the refrigerant has two numbered states.

Show answer and explanation

A closed system; the refrigerant is the working fluid.

The sealed boundary prevents mass from crossing, and the refrigerant is the substance being studied. Heating alone does not make the process a cycle.

Question 2

A gas enters and exits a component through separate openings. What is the most direct system description?

A closed system containing a fixed amount of gas.
A control volume around the component, with the gas as working fluid.
The component is the working fluid and the gas is the surroundings.
A cycle, regardless of the outlet condition.

Show answer and explanation

A control volume around the component, with the gas as working fluid.

The selected region can have mass entering and leaving, so a control-volume description fits. The gas is the working fluid.

Question 3

What does the sequence

1\rightarrow2

establish by itself?

That pressure is constant from state 1 to state 2.
That temperature rises from state 1 to state 2.
That state 2 follows state 1 in the described process.
That the process returns the fluid to its starting state.

Show answer and explanation

That state 2 follows state 1 in the described process.

The arrow and labels establish process direction and state order only. They do not specify property changes or a return to the starting state.

Key terms

System: The selected matter or region being studied.
Boundary: The real or imaginary surface separating a system from its surroundings.
Closed system: A system across whose boundary mass does not pass.
Control volume: A selected region used to study a system in which mass may cross the boundary.
Working fluid: The substance or mixture whose thermodynamic behaviour is being studied.
State: The condition of a working fluid, described by relevant properties.
Process: A change of a system from one state to another.

Continue through MEC E 340

View the complete MEC E 340 University of Alberta MEC E 340: Applied Thermodynamics curriculum and lessons

About this lesson and its review

Published by DoAssignment. This AI-assisted lesson follows University of Alberta MEC E 340: Applied Thermodynamics, study topic 1.1. It is a study resource, not an official curriculum publication.

Before publication, the draft is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability, and then requires administrator approval. Errors can still occur, so corrections are welcomed.

Official curriculum reference

Report a correction or ask a question