MAE 201– Spring 2025
PREPARING FOR FINAL EXAM
Some Additional Practice Problems
Section 1: Circle the correct answer for each.
1. Does hfg change with pressure? How?
2. If an energy transfer between a closed system and its surroundings is not heat, it must be
a. Work
b. Energy transfer by mass
c. Work or energy transfer by mass
d. Mechanical energy
e. Thermal energy
3. Work is done on a 0.5-kg closed system by a rotating shaft in the amount of 10 kJ during a
period of 20 s. The work per unit mass and the power are
a.
, 5 kW
b.
, 2 kW
c.
, 0.5 kW
d.
, 50 W
e.
, 5 kW
4. Is there any difference between the intensive properties of saturated vapor at a given
temperature and the vapor of a saturated mixture at the same temperature?
5.
Is the boundary work associated with constant-volume systems always zero?
6. Steam in a stationary adiabatic piston-cylinder device is being stirred by mixer driven by a
shaft. The piston is free to move. The energy balance relation for this process in its simplest
form is
a. 𝑊sh,in = 𝛥𝑈
b. 𝑊sh,in − 𝑄out = 𝛥𝑈
c. 𝑊sh,in − 𝑊𝑏, out = 𝛥𝑈
d. 𝑊sh,in − 𝑊𝑏, out = 𝛥𝐻
e. 𝑊sh,in + 𝑄in = 𝛥𝐻
7. What is the total mechanical energy of a fluid flowing in a horizontal pipe at a velocity is
? The pressure of the fluid is 200 kPa and its specific volume is
.
a.
b.
c.
d.
e.
8. 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?
9. In the absence of any friction and other irreversibilities, can a heat engine have an efficiency
of 100 percent?
10. Are the efficiencies of all the work-producing devices, including the hydroelectric power
plants, limited by the Kelvin–Planck statement of the second law?
11. Consider the process of baking potatoes in a conventional oven. Can the hot air in the oven
be treated as a thermal energy reservoir?
12. A mechanic claims to have developed a car engine that runs on water instead of gasoline.
What is your response to this claim?
13. A piston–cylinder device contains helium gas. During a reversible, isothermal process, the
entropy of the helium will (never, sometimes, always) increase.
14. A piston–cylinder device contains nitrogen gas. During a reversible, adiabatic process, the
entropy of the nitrogen will (never, sometimes, always) increase.
15. Consider the air-conditioning process during which heat is absorbed from the indoors at 230C
and is discarded to the outdoors at 400C. Identify the heat source and the heat sink for this
process.
(a)
Indoors: Sink, Outdoors: Source
(b)
Indoors: Source, Outdoors: Sink
(c)
Indoors: Sink, Outdoors: Sink
(d)
Indoors: Source, Outdoors: Source
16. Steam is accelerated as it flows through an actual adiabatic nozzle. The entropy of the steam
at the nozzle exit will be (greater than, equal to, less than) the entropy at the nozzle inlet.
17. Work is entropy free, and sometimes the claim is made that work will not change the entropy
of a fluid passing through an adiabatic steady-flow system with a single inlet and outlet. Is
this a valid claim?
Section 2: Answer the following questions. No need to follow the formal solution procedure
but must provide appropriate justification using basic equations and/or property relations
to receive credit.
1. A heat engine absorbs heat from a source at 1000 K at a rate of 250 kW and rejects 100 kW
of it to a sink at 300 K. The thermal efficiency of this heat engine is
i.
0.40
ii.
0.50
iii. 0.60
iv.
0.75
v.
1
2. A heat pump absorbs heat from the cold outdoors at a rate of 3 kW and transfers 8 kW of
heat to the warm indoors. What is the COP of this heat pump and the power input,
respectively?
a.
b.
c.
d.
e.
3. A steam power plant receives heat from a furnace at a rate of 280 GJ/h. Heat losses to the
surrounding air from the steam as it passes through the pipes and other components are
estimated to be about 8 GJ/h. If the waste heat is transferred to the cooling water at a rate of
165 GJ/h, determine (a) net power output and (b) the thermal efficiency of this power plant.
4. Liquid water flows through a
cross-section pipe at a velocity of
. If the
density of water is
, the mass and volume flow rates of water, respectively, are
a)
b)
c)
d)
e)
5. Consider a fluid flowing in a pipe at 150 kPa and 30C. Various specific energy terms of this
fluid are given as
What is the
flow energy of this fluid?
(a)
(b)
(c)
(d)
(e)
6. An initially evacuated adiabatic tank is charged by air flowing in a line at (in kPa) and
(in K). The valve at the inlet of the tank is closed when the air pressure inside the tank
becomes . The temperature of the air in the tank at this point is
(a)
(b)
(c)
(d)
(e)
7. A tank initially contains air at
and
with a mass of . Now, a valve is opened and air is
allowed to leave the tank until the temperature and pressure in the tank drop to
and .
Heat is also lost from the tank during this process. Taking the tank as the system, the energy
balance for this process can be expressed as
(a)
(b)
(c)
(d)
(e)
8. Air enters a 16-cm-diameter pipe steadily at 200 kPa and 200C with a velocity of 5 m/s. Air
is heated as it flows, and it leaves the pipe at 180 kPa and 400C. Determine (a) the volume
flow rate of air at the inlet, (b) the mass flow rate of air, and (c) the velocity and volume flow
rate at the exit.
9. A well-insulated piston-cylinder device contains m kg of saturated liquid water at T and P.
Now, an electrical resistance heater submerged in water is turned on. The piston is free to rise
at constant pressure. What is the amount of electrical energy consumed when the last drop of
liquid in the cylinder is vaporized?
(a) 𝑊𝑒,in = 𝑚ℎ𝑓 @𝑃
(b) 𝑊𝑒,in = 𝑚𝑢𝑔 @𝑇
(c) 𝑊𝑒,in = 𝑚ℎ𝑔 @𝑃
(d) 𝑊𝑒,in = 𝑚ℎ𝑓𝑔 @𝑇
(e) 𝑊𝑒,in = 𝑚𝑢𝑓𝑔 @𝑇
10. An adiabatic piston-cylinder device contains 1 kg air at 10C. Now, an electric heater in the
cylinder is turned on and kept on until the air temperature rises by 1C while the pressure
remains constant. What is the amount of boundary work done during this process? For air,
take 𝑐𝑝 = 1 kJ/kg ⋅∘ 𝐶 and 𝑐𝑣 = 0.7 kJ/kg ⋅∘ 𝐶.
(a) 0.3 kJ
(b) 0.7 kJ
(c) 1 kJ
(d) 1.3 kJ
(e) 0 kJ
11. A tank contains 1 kg water at 50C. Now, 20,000 J of heat is lost from the water to its
surroundings. What is the final temperature of the water in the tank? For water, take 𝑐𝑝 =
4 kJ/kg ⋅° 𝐶.
(a) 45C
(b) 40C
(c) 30C
(d) 20C
(e) 0C
12. What is the pressure of water at T = 170C and
(a) Less than 792.18 kPa
(b) 792.18 kPa
(c) Greater than 792.18 kPa
(d) 100 kPa
(e) 1000 kPa
? See water tables.
13. A piston-cylinder device contains a fixed mass of air at a given pressure and temperature. If
both the temperature and pressure are doubled, the volume of the air will
(a) Double
(b) Decrease by half
(c) Remain constant
(d) Insufficient information
Section 3:
Clearly state all assumptions, fundamental equations, and steps in your solution.
1. A piston–cylinder device initially contains 0.6 m³ of saturated water vapor at 250 kPa. At
this state, the piston is resting on a set of mechanical stops, preventing any movement. The
mass of the piston is such that it begins to rise only when the pressure inside the cylinder
exceeds 300 kPa.
Heat is slowly added to the system, causing the steam to expand. Once the pressure reaches
300 kPa, the piston starts to move, and the heating continues until the total volume of the
steam doubles.
1. Sketch the process on a P–v diagram relative to the saturation lines.
2. Determine:
a. The final temperature of the steam.
b. The work done during the process.
c. The total heat transfer to the steam.
2. A room measuring 4 m × 5 m × 6 m is to be heated using an electric resistance heater
installed in a short, adiabatic duct located inside the room. Initially, the room air is at 15°C,
and the local atmospheric pressure is 98 kPa. The room loses heat to the outside at a constant
rate of 150 kJ/min. A 200-W fan circulates air steadily through the duct and the electric
heater at an average mass flow rate of 40 kg/min. Assume that no air leaks in or out of the
room. If it takes 25 minutes for the room temperature to rise to 25°C, determine:
(a) The power rating of the electric heater (in kW), and
(b) The temperature rise of the air each time it passes through the heater.
3. A 10-gram sample of computer chips (specific heat c=0.3 kJ/kg.K) is initially at 20°C. These
chips are placed in thermal contact with 5 grams of saturated liquid R-134a at –40°C inside
an insulated container (no heat exchange with the surroundings). Assume the pressure
remains constant during the process.
Determine:
(a) The entropy change of the computer chips,
(b) The entropy change of the R-134a,
(c) The total entropy change of the system.
Also, comment on whether the process is possible, based on the second law of
thermodynamics.
4. Refrigerant-134a enters a compressor as a saturated vapor at 160 kPa with a volumetric flow
rate of 0.03 m³/s, and it exits at a pressure of 800 kPa. The compressor receives a power input
of 10 kW. The surroundings (at 20°C) experience an entropy increase at a rate of 0.008
kW/K due to heat loss from the compressor.
Determine:
(a) The rate of heat loss from the compressor (in kW),
(b) The exit temperature of the refrigerant,
(c) The rate of entropy generation (in kW/K).