Thermodynamics [FINAL EXAM]

Work is not a property

True

The total energy of a closed system can change as a result of energy transfer across the system boundary by heat and work and energy transfer accompanying mass flow across the boundary.

False. A closed system does not allow mass transfer. Thus, there is no energy transfer by mass flow in a closed system.

If a closed system consisting of a simple compressible substance is at equilibrium, only one phase can be present.

False. A mixture of phases can be presented in a closed system, e.g., a mixture of saturated liquid and saturated vapor mixture.

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

False. A process that violates the second law does not necessarily violate the first law of thermodynamics and vice versa.

A process that is adiabatic cannot involve work.

False. Adiabatic process is a process that has no heat transfer. Work can transfer during an adiabatic process.

For control volume at a steady state, mass can accumulate within the control volume.

False. All properties, including mass, must be constant within the control volume at a steady-state.

A common assumption, neglecting the change in kinetic energy and potential energy between the inlet and the outlet, can be applied when analyzing the first law of thermodynamics for all steady flow devices, including nozzle and diffuser.

False. Although the change in KE and PE is typically small compared to the change of enthalpy in the first law, the KE term cannot be neglected when analyzing nozzle and diffuser because the main purpose of these devices is to decrease/increase the fluid velocity (e.g., changing the kinetic energy of the fluid).

A significant increase in pressure can be achieved by introducing a restriction into a line through which a gas or liquid flows.

False. As the fluid flows through a restriction (e.g., undergoes a throttling process), the pressure decreases.

A velocity decreases in a diffuser, pressure decreases.

False. Diffuser is a flow passage of varying cross-section areas in which the fluid velocity decreases and the pressure increases in the direction of flow. Fluid kinetic energy is converted into fluid pressure in a diffuser.

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

False. Entropy can transfer to/from control volumes by 2 modes, i.e., heat and mass flow.

Flow work is the work done on a flowing stream by a paddle wheel or piston.

False. Flow work is work associated with fluid movement at it entering and leaving the control volume.

It is correct to say that a system contains heat.

False. Heat is a mode of energy transfer and can only be recognized at a system boundary. It is not a property of a system.

A two-phase liquid-vapor mixture with equal volumes of saturated liquid and saturated vapor has a quality of 0.5.

False. Quality is defined as the ratio of mass (not volume) of saturated vapor to the total mass. Thus, the quality of the mixture with equal volumes of saturated liquid and saturated vapor does not need to be 0.5 (or 50%).

For gases modeled as ideal gases, the ratio Cv/Cp must be greater than one.

False. Since Cp = Cv + R where R is a gas constant and a positive value. Cp of an ideal gas is always greater than Cv, making the ratio Cv/Cp always less than one.

The energy of an isolated system can only increase.

False. Since an isolated system does not allow neither mass nor energy flow, the energy of an isolated system can only stay constant.

The Carnot efficiency also limits the efficiency of wind turbines in generating electricity.

False. The Carnot efficiency only applies to a power cycle operating between two thermal reservoirs. The wind turbine does not operate in a power cycle thus, the Carnot efficiency does not apply.

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

False. The energy of an isolated system must remain constant, but the entropy can only increase (according to the increase of entropy principle).

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. The increase of entropy principle, one corollary of the second law of thermodynamics, states that the total entropy change of an isolated system during a process always increases or, in the limited case of a reversible process, remains constant.

If a system's temperature increases, it must have experienced heat transfer.

False. The system temperature could increase due to other mode of energy transfer such as work.

A gas can be modeled as an ideal gas with constant specific heats when Z = 1.

False. When Z is close to 1, real gas can be modeled as an ideal gas. To assume the specific heat as a variable or a constant, it depends on the temperature change during the process. If the change is small, the specific heat can be assumed constant. Otherwise, it is more accurate to assume that the specific heat is variable.

Kinetic Energy and Potential Energy are extensive properties of a system.

True

Factors that may allow one to model a control volume as having negligible heat transfer include: (1) the outer surface of the control volume is well-insulated, (2) the outer surface area of the control volume is too small to permit effective heat transfer, (3) the temp. difference between the control volume and its surroundings is so small that the heat transfer can be ignored, and (4) the working fluid passes through the control volume so quickly that there is not enough time for significant heat transfer to occur.

True.

For a system at steady state, no property values at the same location change with time.

True.

The pressure listed in thermodynamics tables are absolute pressure, not gage pressures.

True. All pressures listed in thermodynamic tables and equations are absolute pressure.

At a steady state, conservation of energy asserts the total rate at which energy is transferred into the control volume equals the total rate at which energy is transferred out.

True. At steady state, there is no rate of change of energy within the control volume. Thus, the total rate of energy transfer into the control volume must be equal to the total rate of energy transfer from the control volume.

There is no boundary work associated with a rigid system.

True. Boundary work is work that occurs at the boundary. Since the boundaries of a rigid system are fixed, there is no boundary work in the system.

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

True. Carnot cycle is a reversible cycle. No entropy is generated in a reversible cycle.

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. Entropy change in a closed system is only caused by heat transfer and irreversibility. Since entropy is always generated in an irreversible process, i.e., making the entropy change increase, the only way entropy change can be decreased is when heat transfers from the system to its surroundings. Entropy is transferred along with the heat.

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

True. Entropy is a property of a system. Since the property is a point function and path independent, the change in entropy of a closed system depends only on the two specified states.

Friction associated with flow of fluids through pipes and around objects is one type of irreversibility.

True. Examples of irreversibility include (1) Friction, (2) Heat transfer across a finite temperature difference, (3) Unrestrained expansion of a gas to a lower pressure, etc.

As pressure increases toward the critical pressure, the values of Vf and Vg approach each other.

True. On the p-v plot, the saturated liquid and the saturated vapor curve meet at the critical points to form the saturation dome. Thus, as the pressure increases toward the critical pressure, Vf and Vg approach each other.

Pump is a device in which work is done on a liquid to increase the pressure.

True. Pump raises the pressure/the elevation of a liquid through work transfer.

For liquid water, the approximation v (T, p) = Vf (T) is reasonable for many engineering applications.

True. The availability of compressed liquid properties is often limited. However, since the compressed liquid is an incompressible substance, its properties depend primarily on temperature, not pressure. Thus, the compressed liquid properties can be approximated to the properties of a saturated liquid at the given temperature.

The change in total energy of a closed system other than changes in kinetic and gravitational potential energy are accounted for by the change in internal energy.

True. The change in total energy of a closed system consists of the changes in KE, PE, and U.

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

True. The entropy change of an incompressible substance depends on mass, specific heat, and the temperature difference. When T2 is greater than T1 (or the temperature increases), the entropy change will be a positive value (i.e., the entropy of the substance increases).

For a one-inlet, one-exit control volume at steady state, the mass flow rates at the inlet and exit are equal but the inlet and exit volumetric flow rates may not be equal.

True. The mass flow rates must be equal for one-inlet and one-exit control volume at a steady state, but the inlet and exit volumetric flow rates may not be equal. The volumetric flow rates of liquid flow must be equal, but the volumetric flow rates of gas flow are not equal.

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

True. The specific entropy depends on two independent intensive properties (e.g., T and p)

When a substance undergoes a throttling process through a valve, the specific enthalpies of the substance at the valve inlet and the valve exit are equal.

True. Valve is an isenthalpic device.

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

True. When a closed system undergoes an adiabatic and reversible process (i.e., isentropic process), the entropy change is zero (i.e., the entropy remains constant).