Chapter 6: Entropy Concepts

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Questions and Answers

What does the subscript 'int rev' signify in the equation for entropy change?

It indicates the process is internally reversible. (correct)

It refers to the integral for any process.

It specifies a constant property of the system.

It denotes the total change in entropy.

What units are used for specific entropy?

kJ/°C

kJ/kg∙K (correct)

kJ/K

Btu/O

How can the change in entropy between two states be determined?

Only through an internally irreversible process.

By relying on the temperature alone.

Using the integral of the defining equation for any process. (correct)

By using values from internal energy tables.

What happens to the entropy of a closed system when energy is removed by heat transfer?

The entropy decreases. Signup and view all the answers

Which of the following statements about entropy is incorrect?

The change in entropy can only be positive. Signup and view all the answers

What type of diagrams are often used to show states with specific entropy as a coordinate?

Mollier diagrams. Signup and view all the answers

Which equation indicates the differential basis for entropy change?

Eq. 6.2b. Signup and view all the answers

In what scenario will a closed system experience an increase in entropy?

When it receives energy by heat transfer. Signup and view all the answers

What is the implication of having a value of zero for entropy during a closed system process?

No irreversibilities are present within the system. Signup and view all the answers

Which equation represents the entropy balance for closed systems as derived from the Clausius inequality?

ΔS = ∫ (Q/T) + s_gen Signup and view all the answers

What does a positive value of entropy in a closed system indicate?

Irreversibilities are present within the system. Signup and view all the answers

During an internally reversible process, how is energy transfer by heat represented in thermodynamics?

By an area on a temperature-entropy diagram. Signup and view all the answers

What is indicated by the Clausius inequality within the context of a closed thermodynamic cycle?

The net change in entropy can be zero if no irreversibilities occur. Signup and view all the answers

In a closed system undergoing an adiabatic expansion to lower pressure, what is expected in terms of entropy production?

Entropy production may be positive due to irreversibilities. Signup and view all the answers

What are the components of the entropy balance in a closed system?

Amount of entropy contained, heat transfer across boundaries, and entropy produced. Signup and view all the answers

If the entropy balance for a closed system shows a negative value for produced entropy, what does this indicate?

The process is impossible. Signup and view all the answers

What is an important extensive property that accounts for the measure of disorder in a system?

Entropy Signup and view all the answers

According to the Clausius inequality, which of the following statements about entropy cycles is TRUE?

For cycles without irreversibilities, $s_{cycle} = 0$ Signup and view all the answers

Which process represents a scenario where entropy change is dependent only on the end states?

Internally reversible processes Signup and view all the answers

What can be concluded if the integral of the entropy change between two states is independent of the process used?

The integral represents a property of the system Signup and view all the answers

How is entropy accounted for in a system?

Through an entropy balance Signup and view all the answers

Which equation would indicate the presence of irreversibilities in a system?

s_{cycle} > 0 Signup and view all the answers

In the context of closed systems, what does it mean if the entropy change is found to be negative?

The process is impossible Signup and view all the answers

What aspect of an internally reversible process is represented as an area on a temperature-entropy diagram?

Heat transfer Signup and view all the answers

Study Notes

Chapter 6: Using Entropy

This chapter introduces the concept of entropy and its applications in thermodynamics.
Learning outcomes include explaining entropy concepts, evaluating entropy changes, analyzing isentropic processes, and representing heat transfer in reversible processes.
Entropy is an extensive property, similar to mass and energy, which can be transferred across system boundaries.
Entropy balances account for entropy changes in a system, similar to mass and energy balances.
Concepts are developed using the Clausius inequality.
The inequality is used to determine if a process is irreversible, reversible, or impossible.
Entropy change is a property of the system, independent of the path taken.
The change in entropy between two states can be determined considering an internally reversible process.
Units for entropy are kJ/K and Btu/°R, and for specific entropy, kJ/kg·K and Btu/lb·°R.

Entropy Facts

Entropy is an extensive property, and its change can be positive, negative, or zero.
Specific entropy values are provided in tables (A-2 through A-18).
Specific entropy values are calculated using similar procedures as those used for specific volume, internal energy, and enthalpy.
Entropy calculations include two-phase liquid-vapor mixtures and liquid water.

Defining Entropy Change

Entropy change can be calculated using the equation S₂-S₁ =∫ (δQ/T)_{int rev}
The subscript "int rev" indicates an internally reversible process.
The integral's value depends on the end states only, not on the process path.

Entropy and Heat Transfer

Entropy transfer accompanies heat transfer.
Heat transfer from a closed system decreases its entropy.
In an internally reversible and adiabatic process, entropy remains constant.
For an internally reversible process, heat transfer can be calculated using TdS.
On a T-s diagram, heat transfer is represented by the area under the curve.

Entropy Balance for Closed Systems

The entropy balance for a closed system uses the Clausius inequality and the entropy change equation.
The result is an equation that sums entropy change, transfer, and production.
The value of σ, in the equation, represents entropy generated within the system due to irreversibilities.
σ = 0 means no irreversibilities. σ > 0 means irreversibilities are present. σ < 0 is impossible.

Entropy Rate Balance

The entropy rate balance for a closed system is expressed as ds/dt = ∑ (Q̇_j/T_j) + σ̇.
Q̇_j represents the rate of heat transfer across the boundary at temperature T_j.
σ̇ is the rate of entropy production due to irreversibilities.

Entropy Rate Balance for Control Volumes

The entropy rate balance for control volumes is modified to account for entropy transfer by mass flow.
With steady-state conditions, the entropy rate balance simplifies.
Applications include one-inlet, one-exit control volumes.

Isentropic Turbine Efficiency

Isentropic efficiency for turbines is the ratio of actual turbine work to theoretical work.
It is related to entropy changes during an expansion process.
Calculations involve comparing the actual exit state with the theoretical isentropic exit state.

Isentropic Compressor and Pump Efficiencies

The same principle of comparing actual with isentropic conditions applies for compressors and pumps.
These efficiencies are calculated by considering work input per unit mass.

Heat Transfer and Work in Internally Reversible Steady-State Flow Processes

Calculations for heat transfer and work in internally reversible, steady-state flow processes involve integrating specific expressions along an internally reversible path.
TdS equations for flow systems link the different relevant thermodynamic properties.
Heat transfer is represented on a T-s diagram by the area beneath the curve.
Calculating work involves considering changes in enthalpy and pressure along the process path.

Calculating Entropy Change of an Ideal Gas

Calculating entropy change involves utilizing tabulated properties and integrating TdS equations.
Applying relationships involving entropy change to systems that are mostly ideal gases.
Using equations that relate entropy changes to temperatures, pressures, and volumes.
Practical applications of these equations.

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Description

Explore the fascinating world of entropy in thermodynamics with this quiz. Understand entropy's extensive properties, evaluate entropy changes, and analyze isentropic processes. This quiz is designed to reinforce your knowledge of how entropy plays a crucial role in determining the reversibility of processes.