Test 3

Question 1

5–1C When is the flow through a control volume steady?

Answer 1

5-1C Flow through a control volume is steady when it involves no changes with time at any specified position.

Question 2

5–2C Define mass and volume flow rates. How are they

related to each other?

Answer 2

5-2C Mass flow rate is the amount of mass flowing through a cross-section per unit time whereas the volume flow rate is the amount of volume flowing through a cross-section per unit time.

Question 3

5–3C Does the amount of mass entering a control volume
have to be equal to the amount of mass leaving during an
unsteady-flow process?

Answer 3

5-3C The amount of mass or energy entering a control volume does not have to be equal to the amount of mass or energy leaving during an unsteady-flow process.

Question 4

5–4C Consider a device with one inlet and one outlet. If the
volume flow rates at the inlet and at the outlet are the same,
is the flow through this device necessarily steady? Why?

Answer 4

5-4C No, a flow with the same volume flow rate at the inlet and the exit is not necessarily steady (unless the density is
constant). To be steady, the mass flow rate through the device must remain constant.

Question 5

5–17C What is flow energy? Do fluids at rest possess any

flow energy?

Answer 5

5-17C Flow energy or flow work is the energy needed to push a fluid into or out of a control volume. Fluids at rest do not possess any flow energy.

Question 6

5–18C How do the energies of a flowing fluid and a fluid
at rest compare? Name the specific forms of energy associated
with each case.

Answer 6

5-18C Flowing fluids possess flow energy in addition to the forms of energy a fluid at rest possesses. The total energy of a fluid at rest consists of internal, kinetic, and potential energies. The total energy of a flowing fluid consists of internal, kinetic, potential, and flow energies.

Question 7

5–23C A diffuser is an adiabatic device that decreases the
kinetic energy of the fluid by slowing it down. What happens
to this lost kinetic energy?

Answer 7

5-23C It is mostly converted to internal energy as shown by a rise in the fluid temperature.

Question 8

5–24C The kinetic energy of a fluid increases as it is accelerated
in an adiabatic nozzle. Where does this energy come from?

Answer 8

5-24C The kinetic energy of a fluid increases at the expense of the internal energy as evidenced by a decrease in the fluid temperature.

Question 9

5–40C Consider an air compressor operating steadily. How
would you compare the volume flow rates of the air at the
compressor inlet and exit?

Answer 9

5-40C The volume flow rate at the compressor inlet will be greater than that at the compressor exit.

Question 10

5–41C Will the temperature of air rise as it is compressed

by an adiabatic compressor? Why?

Answer 10

5-41C Yes. Because energy (in the form of shaft work) is being added to the air.

Question 11

5–42C Somebody proposes the following system to cool a
house in the summer: Compress the regular outdoor air, let it cool back to the outdoor temperature, pass it through a turbine, and discharge the cold air leaving the turbine into the house. From a thermodynamic point of view, is the proposed system sound?

Answer 11

No

Question 12

5–58C Why are throttling devices commonly used in refrigeration and air-conditioning applications?

Answer 12

5-58C Because usually there is a large temperature drop associated with the throttling process.

Question 13

5–59C Would you expect the temperature of air to drop as it undergoes a steady-flow throttling process? Explain.

Answer 13

5-59C No. Because air is an ideal gas and h = h(T) for ideal gases. Thus if h remains constant, so does the temperature.

Question 14

5–60C Would you expect the temperature of a liquid to

change as it is throttled? Explain

Answer 14

5-60C If it remains in the liquid phase, no. But if some of the liquid vaporizes during throttling, then yes.

Question 15

5–61C During a throttling process, the temperature of a

fluid drops from 30 to 2208C. Can this process occur adiabatically?

Answer 15

Yes

Question 16

5–68C Consider a steady-flow mixing process. Under what conditions will the energy transported into the control volume
by the incoming streams be equal to the energy transported out of it by the outgoing stream?

Answer 16

5-68C Under the conditions of no heat and work interactions between the mixing chamber and the surrounding medium.

Question 17

5–69C Consider a steady-flow heat exchanger involving
two different fluid streams. Under what conditions will the
amount of heat lost by one fluid be equal to the amount of
heat gained by the other?

Answer 17

5-69C Under the conditions of no heat and work interactions between the heat exchanger and the surrounding medium.

Question 18

5–70C When two fluid streams are mixed in a mixing

chamber, can the mixture temperature be lower than the temperature of both streams? Explain.

Answer 18

5-70C Yes, if the mixing chamber is losing heat to the surrounding medium.

Question 19

6–1C Describe an imaginary process that violates both the

first and the second laws of thermodynamics.

Answer 19

6-1C Transferring 5 kWh of heat to an electric resistance wire and producing 6 kWh of electricity.

Question 20

6–2C Describe an imaginary process that satisfies the first

law but violates the second law of thermodynamics.

Answer 20

6-2C Transferring 5 kWh of heat to an electric resistance wire and producing 5 kWh of electricity.

Question 21

6–3C Describe an imaginary process that satisfies the second law but violates the first law of thermodynamics.

Answer 21

6-3C An electric resistance heater which consumes 5 kWh of electricity and supplies 6 kWh of heat to a room.

Question 22

6–4C An experimentalist claims to have raised the temperature of a small amount of water to 1508C by transferring heat from high-pressure steam at 1208C. Is this a reasonable claim? Why? Assume no refrigerator or heat pump is used in the process.

Answer 22

6-4C No. Heat cannot flow from a low-temperature medium to a higher temperature medium.

Question 23

6–5C What is a thermal energy reservoir? Give some examples

Answer 23

6-5C A thermal-energy reservoir is a body that can supply or absorb finite quantities of heat isothermally. Some examples are the oceans, the lakes, and the atmosphere.

Question 24

6–6C Consider the process of baking potatoes in a conventional oven. Can the hot air in the oven be treated as a thermal energy reservoir? Explain.

Answer 24

6-6C Yes. Because the temperature of the oven remains constant no matter how much heat is transferred to the potatoes.

Question 25

6–7C What are the characteristics of all heat engines?

Answer 25

6-7C Heat engines are cyclic devices that receive heat from a source, convert some of it to work, and reject the rest to a sink.

Question 26

6–8C What is the Kelvin–Planck expression of the second

law of thermodynamics?

Answer 26

6-8C It is expressed as “No heat engine can exchange heat with a single reservoir, and produce an equivalent amount of work”.

Question 27

6–9C Is it possible for a heat engine to operate without
rejecting any waste heat to a low-temperature reservoir?
Explain.

Answer 27

6-9C No. Such an engine violates the Kelvin-Planck statement of the second law of thermodynamics.

Question 28

6–10C Baseboard heaters are basically electric resistance
heaters and are frequently used in space heating. A home
owner claims that her 5-year-old baseboard heaters have a conversion efficiency of 100 percent. Is this claim in violation of any thermodynamic laws? Explain.

Answer 28

6-10C No. Because 100% of the work can be converted to heat.

Question 29

6–11C Does a heat engine that has a thermal efficiency of
100 percent necessarily violate (a) the first law and (b) the
second law of thermodynamics? Explain.

Answer 29

6-11C (a) No, (b) Yes. According to the second law, no heat engine can have and efficiency of 100%.

Question 30

6–12C In the absence of any friction and other irreversibilities, can a heat engine have an efficiency of 100 percent? Explain.

Answer 30

6-12C No. Such an engine violates the Kelvin-Planck statement of the second law of thermodynamics.

Question 31

6–13C Are the efficiencies of all the work-producing

devices, including the hydroelectric power plants, limited by the Kelvin–Planck statement of the second law? Explain.

Answer 31

6-13C No. The Kelvin-Plank limitation applies only to heat engines; engines that receive heat and convert some of it to work.

Question 32

6–14C Consider a pan of water being heated (a) by placing it on an electric range and (b) by placing a heating element in the water. Which method is a more efficient way of heating water? Explain.

Answer 32

6-14C Method (b). With the heating element in the water, heat losses to the surrounding air are minimized, and thus the desired heating can be achieved with less electrical energy input

Question 33

6–30C In a refrigerator, heat is transferred from a lower temperature medium (the refrigerated space) to a higher temperature one (the kitchen air). Is this a violation of the
second law of thermodynamics? Explain.

Answer 33

6-30C No. Because the refrigerator consumes work to accomplish this task.

Question 34

6–31C A heat pump is a device that absorbs energy from the cold outdoor air and transfers it to the warmer indoors. Is this a violation of the second law of thermodynamics? Explain.

Answer 34

6-31C No. Because the heat pump consumes work to accomplish this task.

Question 35

6–32C Define the coefficient of performance of a refrigerator in words. Can it be greater than unity?

Answer 35

6-32C The coefficient of performance of a refrigerator represents the amount of heat removed from the refrigerated space for each unit of work supplied. It can be greater than unity.

Question 36

6–33C Define the coefficient of performance of a heat

pump in words. Can it be greater than unity?

Answer 36

6-33C The coefficient of performance of a heat pump represents the amount of heat supplied to the heated space for each
unit of work supplied. It can be greater than unity.

Question 37

6–34C A heat pump that is used to heat a house has a COP of 2.5. That is, the heat pump delivers 2.5 kWh of energy to the house for each 1 kWh of electricity it consumes. Is this a violation of the first law of thermodynamics? Explain.

Answer 37

6-34C No. The heat pump captures energy from a cold medium and carries it to a warm medium. It does not create it.

Question 38

6–35C A refrigerator has a COP of 1.5. That is, the refrigerator removes 1.5 kWh of energy from the refrigerated space for each 1 kWh of electricity it consumes. Is this a violation of the first law of thermodynamics? Explain.

Answer 38

6-35C No. The refrigerator captures energy from a cold medium and carries it to a warm medium. It does not create it.

Question 39

6–36C What is the Clausius expression of the second law

of thermodynamics?

Answer 39

6-36C No device can transfer heat from a cold medium to a warm medium without requiring a heat or work input from the surroundings.

Question 40

6–37C Show that the Kelvin–Planck and the Clausius

expressions of the second law are equivalent.

Answer 40

6-37C The violation of one statement leads to the violation of the other one, as shown in Sec. 6-4, and thus we conclude that the two statements are equivalent.