Thermodynamics Chapter 6 True/False

44 For a specified inlet state, exit pressure, and mass flow rate, the power input to a compressor operating adiabatically and at steady state is less than what would be required if the compression occurred isentropically.

False

42 The Carnot cycle is represented on a T-s diagram as a rectangle.

True A Carnot cycle is defined as two adiabatic processes and two isothermal processes.

50 In statistical thermodynamics, entropy is associated with the notion of microscopic disorder.

True In a spontaneous process of an isolated system, the system moves toward equilibrium and the entropy increases. From the microscopic viewpoint, this is equivalent to saying that as an isolated system moves toward equilibrium our knowledge of the condition of individual particles making up the system decreases, which corresponds to an increase in microscopic disorder and a related increase in entropy.

45 For closed systems undergoing processes involving internal irreversibilities, both entropy change and entropy production are positive in value.

False As a closed system undergoes an internally reversible process, its entropy can increase, decrease, or remain constant. The second law requires that entropy production be positive or zero.

37 Entropy is produced in every internally reversible process of a closed system.

False By definition, there is no entropy production in an internally reversible production.

48 At liquid states, the following approximation is reasonable for many engineering applications: s(T,p) ≈ s(T)g

False For liquid states, entropy can be estimated by using the saturated liquid value at the given temperature s(T,p) ≈ s(T)f.

41 The energy of an isolated system must remain constant, but the entropy can only decrease.

False Since entropy is produced in all actual processes, the only processes that can occur are those for which the entropy of the isolated system increases; this is known as the increase of entropy principle.

54 The entropy change between two states of air modeled as an ideal gas can be directly read from Tables A-22 and A-22E only when the pressure at the two states is the same.

False The entropy change between two states of air modeled as an ideal gas can be directly read from Tables A-22 and A-22E when the process experiences variable specific heats.

39 The entropy of a fixed amount of an ideal gas increases in every isothermal process.

False The entropy change in an isothermal process: Δs = -R ln(p2 / p1) = R ln(v2 / v1) implies that entropy can either increase (expansion) or decrease (compression).

49 The steady-state form of the control volume entropy balance requires that the total rate at which entropy is transferred out of the control volume be less than the total rate at which entropy enters.

False The steady-state form of the control volume entropy balance requires that the rate at which entropy is transferred out must exceed the rate at which entropy enters

35 One corollary of the second law of thermodynamics states that the change in entropy of a closed system must be greater than or equal to zero.

False This is only true for isolated systems.

33 A process that violates the second law of thermodynamics violates the first law of thermodynamics.

False We would not need the second law if that were the case.

40 The specific internal energy and enthalpy of an ideal gas are each functions of temperature alone, but its specific entropy depends on two independent intensive properties.

True

47 For a specified inlet state, exit pressure, and mass flow rate, the power developed by a turbine operating at steady state is less than if expansion occurred isentropically.

True

52 The only entropy transfers to or from control volumes are those accompanying heat transfer.

True

53 Heat transfer for internally reversible processes of closed systems can be represented as areas on T-s diagrams.

True

55 When a system undergoes a Carnot cycle, no entropy is produced within the system.

True

31 The change in entropy of a closed system is the same for every process between two specified states.

True Since entropy is a property, the change in entropy of a system in going from one state to another is the same for all processes, both internally reversible and irreversible, between these two states.

51 The increase of entropy principle states that the only processes of an isolated system that are possible are those for which the entropy increases.

True Since entropy is produced in all actual processes, the only processes that can occur are those for which the entropy of the isolated system increases.

36 A closed system can experience a decrease in entropy only when there is heat transfer from the system to its surroundings during the process.

True Since entropy transfer accompanies the heat transfer, system entropy decreases.

46 The T dS equations are fundamentally important in thermodynamics because of their use in deriving important property relations for pure, simple compressible systems.

True The T dS equations allow entropy changes to be evaluated from other more readily determined property data. NOTE: In addition, they are used as a point of departure for deriving many important property relations for pure, simple com- pressible systems, including means for constructing the property tables giving u, h, and s.

32 The entropy of a fixed amount of an incompressible substance increases in every process for which temperature increases.

True The entropy change for an incompressible substance, s2 - s1 = c ln(T2 / T1), implies that Δs and ΔT are directly proportional.

34 When a net amount of work is done on a closed system undergoing an internally reversible process, a net heat transfer of energy from the system also occurs.

True The net work of any cycle is equal to the net heat transfer.

38 In an adiabatic and internally reversible process of a closed system the entropy remains constant.

True This process is called isentropic (constant entropy).

43 The entropy change of a closed system during a process can be greater than, equal to, or less than zero.

True When a closed system undergoing an internally reversible process receives energy by heat transfer, the system experiences an increase in entropy. Con- versely, when energy is removed from the system by heat transfer, the entropy of the system decreases. In an adiabatic internally reversible process, entropy remains constant.