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Questions and Answers
What is the change in internal energy for 1 kg of air moving from state 1 to state 2?
What is the change in internal energy for 1 kg of air moving from state 1 to state 2?
In a closed system where no work is done, what does the energy balance equation simplify to?
In a closed system where no work is done, what does the energy balance equation simplify to?
What is a characteristic of a pure substance?
What is a characteristic of a pure substance?
What does the exponent 'n' represent in the equation pV^n = constant for a polytropic process?
What does the exponent 'n' represent in the equation pV^n = constant for a polytropic process?
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Which properties are considered intensive properties for a simple compressible system?
Which properties are considered intensive properties for a simple compressible system?
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What defines a phase in thermodynamics?
What defines a phase in thermodynamics?
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Which of the following describes a process when n equals 0?
Which of the following describes a process when n equals 0?
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For an ideal gas, what type of process occurs when n equals 1?
For an ideal gas, what type of process occurs when n equals 1?
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Which scenario would be classified as a two-phase liquid-vapor mixture?
Which scenario would be classified as a two-phase liquid-vapor mixture?
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Why are velocity and elevation excluded from intensive properties in simple compressible systems?
Why are velocity and elevation excluded from intensive properties in simple compressible systems?
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What does the state principle indicate for a simple compressible system?
What does the state principle indicate for a simple compressible system?
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What does quality refer to in a two-phase liquid-vapor mixture?
What does quality refer to in a two-phase liquid-vapor mixture?
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When is the ideal gas model generally applicable?
When is the ideal gas model generally applicable?
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What state is referred to when the temperature is lower than the saturation temperature at a given pressure?
What state is referred to when the temperature is lower than the saturation temperature at a given pressure?
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What happens to the specific volume when a two-phase liquid-vapor mixture is heated at constant pressure?
What happens to the specific volume when a two-phase liquid-vapor mixture is heated at constant pressure?
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What is the quality, x, of a saturated liquid state?
What is the quality, x, of a saturated liquid state?
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At which state is the entire liquid converted into vapor?
At which state is the entire liquid converted into vapor?
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What type of diagram is created by projecting the p-v-T surface onto the pressure-specific volume plane?
What type of diagram is created by projecting the p-v-T surface onto the pressure-specific volume plane?
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In a two-phase liquid-vapor mixture, what does the ratio of the mass of vapor to the total mass represent?
In a two-phase liquid-vapor mixture, what does the ratio of the mass of vapor to the total mass represent?
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Which state corresponds to the maximum specific volume within a two-phase mixture during heating?
Which state corresponds to the maximum specific volume within a two-phase mixture during heating?
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What is a characteristic of the saturated liquid state in terms of temperature and pressure?
What is a characteristic of the saturated liquid state in terms of temperature and pressure?
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What is the formula for specific enthalpy (h) in terms of internal energy (u) and pressure-volume work (pv)?
What is the formula for specific enthalpy (h) in terms of internal energy (u) and pressure-volume work (pv)?
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Which property is developed as an intensive property in Chapter 6?
Which property is developed as an intensive property in Chapter 6?
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What is the specific volume (v) of superheated water vapor at 10 MPa and 400°C?
What is the specific volume (v) of superheated water vapor at 10 MPa and 400°C?
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How is specific volume determined for a state that does not exactly match property table values?
How is specific volume determined for a state that does not exactly match property table values?
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At 8°C, how is the specific volume of saturated liquid calculated from the property table?
At 8°C, how is the specific volume of saturated liquid calculated from the property table?
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What does the quality, x, represent in a two-phase liquid-vapor mixture?
What does the quality, x, represent in a two-phase liquid-vapor mixture?
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At states where the pressure is small relative to the critical pressure, what value can the compressibility factor Z be approximated to?
At states where the pressure is small relative to the critical pressure, what value can the compressibility factor Z be approximated to?
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Which of the following properties is not included in the tabulated properties of Tables A-4 and A-5?
Which of the following properties is not included in the tabulated properties of Tables A-4 and A-5?
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What does the equation $pV = mRT$ (Eq. 3.33) represent in the context of the ideal gas model?
What does the equation $pV = mRT$ (Eq. 3.33) represent in the context of the ideal gas model?
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In the context of thermodynamics, what is the significance of specific enthalpy (h)?
In the context of thermodynamics, what is the significance of specific enthalpy (h)?
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Which equation indicates that the specific internal energy for a gas modeled as an ideal gas depends primarily on temperature?
Which equation indicates that the specific internal energy for a gas modeled as an ideal gas depends primarily on temperature?
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Which of the following equations correctly relates specific enthalpy, internal energy, and the ideal gas equation?
Which of the following equations correctly relates specific enthalpy, internal energy, and the ideal gas equation?
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What is the main criterion for using the ideal gas model effectively in engineering thermodynamics?
What is the main criterion for using the ideal gas model effectively in engineering thermodynamics?
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What does the universal gas constant R equal when expressed in kJ/kmol∙K?
What does the universal gas constant R equal when expressed in kJ/kmol∙K?
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In an ideal gas, how does the specific heat $c_v$ vary?
In an ideal gas, how does the specific heat $c_v$ vary?
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Under what conditions is the ideal gas model justified for use?
Under what conditions is the ideal gas model justified for use?
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What is the relationship between specific enthalpy and temperature for an ideal gas?
What is the relationship between specific enthalpy and temperature for an ideal gas?
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Which equation represents the change in specific internal energy for an ideal gas under constant specific heat?
Which equation represents the change in specific internal energy for an ideal gas under constant specific heat?
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What is the suggested method for evaluating changes in specific internal energy and enthalpy for ideal gases?
What is the suggested method for evaluating changes in specific internal energy and enthalpy for ideal gases?
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When is it inappropriate to use the equation h2 - h1 = cp[T2 - T1]?
When is it inappropriate to use the equation h2 - h1 = cp[T2 - T1]?
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In a rigid tank with air heated from 300 K to 1500 K, what should be neglected to determine the heat transfer?
In a rigid tank with air heated from 300 K to 1500 K, what should be neglected to determine the heat transfer?
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Which parameters are necessary to determine the change in specific enthalpy using Table A-22?
Which parameters are necessary to determine the change in specific enthalpy using Table A-22?
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What does the specific heat capacity cp for an ideal gas depend on?
What does the specific heat capacity cp for an ideal gas depend on?
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What is the value of h2 - h1 if h1 = 300.19 kJ/kg and h2 = 1635.97 kJ/kg?
What is the value of h2 - h1 if h1 = 300.19 kJ/kg and h2 = 1635.97 kJ/kg?
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Study Notes
Chapter 3: Evaluating Properties
- This chapter covers evaluating properties within a thermodynamic context.
- Learning outcomes include explaining phases, pure substances, and the state principle for simple compressible systems.
- Knowledge of p-v-T surfaces, saturation temperature, pressure, two-phase liquid-vapor mixtures, quality, enthalpy, and specific heats is essential.
- Analyzing closed systems, including applying energy balances with property data, is another key element.
- Students should be able to sketch and interpret T-v, p-v, and phase diagrams and retrieve data from Tables A-1 through A-23.
- Applying the ideal gas model is pertinent, including when using it is warranted.
Phase
- A phase is a quantity of matter that is homogeneous throughout in chemical composition and physical structure.
- A homogeneous phase is either all solid, all liquid, or all vapor (gas).
- Examples include:
- Air is a gas phase.
- Water with ice is liquid and solid water.
- Salad dressings are different liquid phases.
Pure Substance
- A pure substance displays a uniform and invariable chemical composition across all its phases.
- A pure substance can have multiple phases but must maintain the same chemical composition within each phase.
- Examples include:
- Drinking water (with ice cubes) is a pure substance because each phase (ice and liquid) has the same composition.
- A fuel/air mixture in an engine can be considered a pure substance pre-ignition.
State Principle for Simple Compressible Systems
- Simple compressible systems comprise commonly encountered pure substances listed in Tables A-2 through A-18, A-22, and A-23.
- At equilibrium, a simple compressible system's intensive state is defined by its intensive properties, including temperature, pressure, specific volume, density, specific internal energy, and specific enthalpy.
- Properties like velocity and elevation are not relevant to the state principle as they are based on arbitrary datums.
- Not all intensive properties are independent. Some are related by definition (e.g., density = 1/v). Others are defined through experimental data.
- For a simple compressible system, values of any two independent intensive properties define all other intensive properties (state principle/state postulate).
- Commonly used alternative sets include (T, v) and (p, v). (p, T) is not always an independent set.
p-v-T Surface
- The p-v-T surface shows how pressure varies with temperature and specific volume for pure substances.
- Single-phase regions inside the surface include solid, liquid, and vapor states.
- Two-phase regions are between the single-phase regions, where two phases (e.g., liquid-vapor) coexist in equilibrium.
- The dome-shaped region formed by the two-phase liquid-vapor states is known as the vapor dome.
- Lines bordering the vapor dome are the saturated liquid and saturated vapor lines.
- The critical point is where the saturated liquid and vapor lines meet.
- Critical temperature (Tc) is the highest temperature at which liquid and vapor can coexist.
- Critical pressure (Pc) is the pressure at the critical point.
Projections of the p-v-T Surface
- Projections provide graphical representations of thermodynamic properties.
- A phase diagram (projection onto the pressure-temperature plane) is a graphical tool.
- Saturation temperature is the temperature at which a phase change occurs at a given pressure.
- Saturation pressure is the pressure at which a phase change occurs at a given temperature.
- Pressure and temperature are not independent within two-phase regions.
Phase Change
- A closed system containing liquid water at a given temperature and pressure may undergo a phase change with increasing temperature and/or pressure.
- Compressed liquid states have temperatures below the saturation temperature of the liquid, for given pressure.
- Saturated liquid states represent the start of phase changes, where temperature and pressure are now fixed.
- Saturated vapor states represent a temperature at which the last of the liquid is vaporized under given pressure.
- Superheated vapor states represent temperatures above the saturation temperature under given pressure.
Property Approximations for Liquids/Compressed Liquid Approximation
- Approximate values for v, u, and h in the compressed liquid region can be obtained using saturated liquid data.
- The values of v and u vary little with pressure at a fixed temperature in compressed liquids, so approximations (Eqs. 3.11 and 3.12) can be used
- An approximate value of h can also be found (Eqs. 3.11, 3.12 and 3.13)
- When the term (p − psat(T)) is small, h(T,p) is approximately equal to h(T)
Steam Tables
- Steam tables present the properties of water in a specific format.
- Table A-4 encompasses superheated vapor.
- Table A-5 is related to compressed liquid water.
- Tables A-2 and A-3 describe two-phase liquid-vapor mixtures.
Single-Phase Regions
- Independent temperature and pressure typically define single phases.
- Tables A-4 and A-5 contain properties for superheated water vapor and compressed liquid water, expressed as functions of pressure and temperature.
- Properties such as temperature (T), pressure (P), specific volume (v), specific internal energy (u), and specific enthalpy (h) are tabulated.
- Enthalpy is defined as u + pv and specific entropy, s, is an intensive property described in Chapter 6.
Two-Phase Liquid-Vapor Region
- Pressure and temperature are dependent within a two-phase region; the quality or x is needed to fix the state.
- Specific volume, internal energy (u), and enthalpy (h) can be calculated with the quality parameter in this region
- V = Vf + x(Vg − Vf)
- u = uf + x(ug − uf)
- h = hf + x(hg − hf)
Ideal Gas Model
- The ideal gas model is applicable when pressure is lower than critical pressure.
- The behavior of ideal gases at different states can be studied under a generalized compressibility chart. - In the ideal gas model, the relationship between PV and T is mathematically derived via R.
- Internal energy and enthalpy are only dependent on temperature in an ideal gas.
- Ideal gas properties like enthalpy (h) and internal energy (u) can be found using tables A-21, A-22, & A-23
Specific Heats
- Specific heats (cp and cv) are closely related to internal energy (u) and enthalpy (h), offering valuable insights into thermodynamic behavior.
- The specific heats can vary based on the substance and conditions.
- For an incompressible substance, cp=cv = c.
- In the ideal gas model, specific heats are also temperature-dependent.
Polytropic Process
- A polytropic process describes a quasiequilibrium process obeying pvn = constant, where the exponent n changes per condition.
- Constant-pressure, constant-volume, and constant-temperature processes are subsets of the polytropic process based on various values of n.
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Description
Test your knowledge on key concepts in thermodynamics, including internal energy changes, properties of pure substances, and phase characteristics. This quiz covers various processes and fundamental principles relevant to simple compressible systems and ideal gases.
