Thermodynamics Chapter 4 Quiz

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Questions and Answers

What does the equation $ein - eout = Δesystem$ represent in the context of a constant-volume process?

The net amount of energy transferred to the system. (correct)

The change in pressure of the system.

The change in mass during the process.

The total work done by the system.

In a constant-pressure expansion or compression process, what can the PV terms be associated with?

The internal energy of the substance. (correct)

The change in temperature of the substance.

The work done on or by the system.

The heat capacity at constant volume.

What is the specific heat at constant volume denoted as?

cv (correct)

Which of the following statements about specific heats is true?

Specific heats are valid for any substance undergoing any process. Signup and view all the answers

What is the unit of measurement for specific heats?

kJ/kg·°C Signup and view all the answers

What condition must be met for the pressure at the inner surface of the piston to equal the pressure of the gas in the cylinder?

The process must be quasi-equilibrium. Signup and view all the answers

Which of the following statements is true regarding boundary work during an expansion process?

It represents energy transferred from the system. Signup and view all the answers

How is boundary work characterized in the context of a constant-volume process?

It is equal to zero. Signup and view all the answers

What must the energy transferred as work from the system equal during an expansion in a car engine?

The energy received by the crankshaft and the energy to overcome friction. Signup and view all the answers

In non-quasi-equilibrium processes, what pressure is used for calculating boundary work?

Pressure at the inner face of the piston. Signup and view all the answers

Which of the following processes allows the use of the boundary work relation for solids and liquids?

Any type of process. Signup and view all the answers

What happens if the process is isobaric?

The pressure remains constant throughout the process. Signup and view all the answers

In energy conservation, what does the boundary work performed by the system represent?

Energy that must be accounted for in other forms. Signup and view all the answers

What primarily influences the Internal Energy of an ideal gas?

Temperature Signup and view all the answers

What conclusion did Joule reach regarding the Internal Energy of Air during his experiment?

It remains constant despite changes in pressure and volume. Signup and view all the answers

For low-density gases, what is the relation between Internal Energy and pressure?

Internal Energy is independent of pressure. Signup and view all the answers

What properties can the Specific Heat at constant volume (cv) and constant pressure (cp) of an ideal gas be attributed to?

Temperature only Signup and view all the answers

In Joule's experiment, what does it indicate when there was no change in the temperature of the water bath?

No heat was transferred to or from the Air. Signup and view all the answers

Which of the following statements is true for an ideal gas under ideal conditions?

Internal Energy, Enthalpy, and Specific Heats vary only with temperature. Signup and view all the answers

As the density of a gas decreases, how does its dependence of Internal Energy on pressure change?

It diminishes. Signup and view all the answers

Which of the following factors does NOT affect the Internal Energy of an ideal gas?

Pressure Signup and view all the answers

What is the behavior of real gases at low pressures?

They behave like ideal gases. Signup and view all the answers

What is another term for the specific heats of real gases at low pressures?

Zero-pressure specific heats. Signup and view all the answers

What primarily causes the variation of specific heat with temperature in gases?

Molecular vibration. Signup and view all the answers

How do specific heats of complex molecules behave with temperature?

They increase with temperature. Signup and view all the answers

Which method is considered the easiest and most accurate for calculating changes in internal energy and enthalpy?

Using the tabulated u and h data. Signup and view all the answers

Which temperature is often chosen as the Reference Temperature when preparing ideal gas tables?

0 K. Signup and view all the answers

What is true about the variation of Cp with temperature?

It may be approximated as linear over small temperature intervals. Signup and view all the answers

What happens to internal energy and enthalpy when specific heat is taken constant over a range?

They change proportionally. Signup and view all the answers

What is the primary method of calculating specific heats when property tables are not available?

Using average specific heats Signup and view all the answers

What is the significance of the specific heat ratio for monatomic gases?

It remains essentially constant at 1.667 Signup and view all the answers

Which statement about average specific heats is true?

They are reasonably accurate for small temperature intervals Signup and view all the answers

In the context of an ideal gas, what is the relationship between internal energy change and specific heat?

du = cvdT Signup and view all the answers

What pressure is required to move the piston in the given example?

350 kPa Signup and view all the answers

How does the specific heat ratio of diatomic gases compare to that of monatomic gases at room temperature?

It is higher, around 1.4 Signup and view all the answers

What happens to the specific heat values when a gas is heated in the example?

They vary positively with temperature Signup and view all the answers

What is one reason for the convenience of computerized calculations over hand calculations for specific heats?

Computerized calculations handle complex integrations effectively Signup and view all the answers

What characterizes an incompressible substance?

Density remains constant during a process Signup and view all the answers

In the analysis of internal energy and enthalpy changes for solids, what assumption can be made about the term v ΔP?

It is insignificant and can be ignored Signup and view all the answers

For which type of process does the equation Δh = v ΔP apply?

Constant-pressure processes involving liquids Signup and view all the answers

What is the relationship between specific heats for incompressible substances?

Cv and cp values are identical Signup and view all the answers

What approximate formula can be used for enthalpy changes in solids for small temperature intervals?

Δh ≈ cavg ΔT Signup and view all the answers

What is NOT a characteristic of incompressible substances during energy analysis?

They can undergo significant volume changes Signup and view all the answers

In analyzing specific heats of incompressible substances, what is the assumption for small temperature intervals?

It can be treated as constant at the average temperature. Signup and view all the answers

What is the correct expression for enthalpy change during a constant-pressure process involving liquids?

Δh = Δu ≅ cavg ΔT Signup and view all the answers

Study Notes

Chapter 4: Energy Analysis of Closed Systems

Thermodynamics textbook by Yunus A. Cengel and Michael A. Boles (5th/8th Ed., Chapters 4 and 5) is a recommended resource.
Applied Thermodynamics by TD Eastop and A McConkey (5th Ed.) is also recommended.
Moving boundary work (PdV work) is associated with expansion and compression in piston-cylinder devices.
Exact determination of moving boundary work from thermodynamic analysis alone is not possible in real engines or compressors due to high piston speeds and variable gas equilibrium.
Process paths are not always specified or possible to draw.
Boundary work must be determined by direct measurements in real engines/compressors.
Quasi-equilibrium processes are analyzed in this context.
A quasi-equilibrium process involves nearly equilibrium conditions at all times.
Differential work (δW_b) during a quasi-equilibrium process is calculated using: δW_b= F ds = P_a ds =P dV
Total boundary work during a process is the sum of differential works from start to end, obtained by integration: W_b = ∫₁²PdV (kJ).
Positive work signifies expansion, while negative work represents compression.
Relationships between pressure (P) and volume (V) during expansion/compression are plotted on P-V diagrams.
The area under the process curve on a P-V diagram is numerically equal to the boundary work done.
Cyclic devices (e.g., car engines and power plants) will generate net work only if the work done during the cycle is not a path function.
The net work done in a cycle is determined by calculating the difference in work done by and on the system.
Pressure in the moving boundary work equation (P) is the pressure at the inner surface of the piston.
The equation W_b=∫₁² P dV (kJ) is valid for processes that are not quasi-equilibrium, provided the pressure at the inner face of piston is used instead of the instantaneous pressure inside the cylinder.
Boundary work represents energy transferred into or out of a system during expansion or compression processes.
Boundary work examples: in car engines, the work done by expanding hot gases moves the piston and crankshaft; or the work of moving boundary of the piston overcoming the frictional resistance between piston and cylinders, moving atmospheric air from the way and rotating the crankshaft
The use of boundary work relation is not limited to quasi-equilibrium processes
It is also applicable for solids and liquids, in addition to gases.

Constant-Volume Process (Isochoric)

Boundary work (W_b) for a constant-volume process is zero because the volume change is null.
W_b = ∫₁² P dV = 0

Constant-Pressure Process (Isobaric)

For this process: W_b = P(V₂ - V₁)

Isothermal Process

For an isothermal process: W_b = mRT₁ ln(V₂/V₁)

Polytropic Process

Polytropic process relationship for gases: PVⁿ = constant
The total boundary work: W_b = (P₂V₂ - P₁V₁) / (1-n)

Specific Heats

Specific heat is the energy required to raise the temperature of a unit mass of a substance by one degree.
Specific Heat at constant volume (c_v): energy required to raise the temperature of the unit mass of a substance by one degree with the volume maintained constant.
Specific Heat at constant pressure (c_p): energy required to raise the temperature of the unit mass of a substance by one degree with the pressure maintained constant.
c_p is always greater than c_v .

Internal Energy and Enthalpy

Internal energy is a function of temperature only for an ideal gas.
Enthalpy is defined by h = u + Pv and is also dependent on temperature only for an ideal gas.
Specific heats (c_p and c_v) are most often (but not always) dependent on temperature for real gas.

Ideal Gases

Internal energy (u) and enthalpy (h) are functions of temperature only for ideal gases.

Incompressible Substances (Solids and Liquids)

For incompressible substances, volume is constant.
The specific heats c_p and c_v are identical and denoted by c.
Internal energy change is du=c dT
Enthalpy change is dh=c dT + v dP

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Description

Test your understanding of the energy analysis of closed systems in thermodynamics. This quiz covers topics such as moving boundary work, quasi-equilibrium processes, and the calculation of boundary work in real engines and compressors. Recommended resources include texts by Cengel, Boles, Eastop, and McConkey.