PHYS 313
THERMODYNAMICS
SPRING 2012
EXAM II
All work on this exam must be your own. You may use a calculator, handouts, and a 3"X5" card of notes.
Clearly indicate your final answer for each question.
Section 1: Multiple choice. No partial credit will be given for this section. Circle only one answer for each
question. No justification required. (2 pts each)
SELECT ONE OF THE MULTIPLE CHOICE QUESTIONS TO BE WORTH ZERO POINTS
1.
A pure substance undergoes an infinitesimal change in temperature, dT. The infinitesimal change in
enthalpy, dh =cpdT only if
a)
pressure is constant
b) volume is constant
c)
no phase change occurs
d) none of the above
2.
3.
As an ideal gas passes through a diffuser, its pressure and specific volume change. The work done by the
gas (per kg of gas) during this process is
a)
0
b)
 Pdv
c)
 vdP .
d)
 Pdv +  vdP
Process A: 2 kg of air at 10oC and 1 atm of pressure undergoes a reversible isothermal process ending with
a pressure of 2 atm.
Process B: 2 kg of air at 10oC and 1 atm of pressure undergoes an irreversible adiabatic process followed
by a constant volume process ending with a temperature of 10oC and pressure of 2 atm
Which of the following is true of the entropy change of the 2 kg of air during these processes?
a)
SA>SB
b) SA<SB
c)
SA=SB
d) any of the above may be true, depending on other factors
4.
A pure substance undergoes a throttling process from point A to point B. Which of the following is true?
a)
vA=vB
b) PA=PB
c)
hA=hB
d) uA=uB
5.
Two systems, called A and B, interact with each other, but not with anything else. During this interaction,
the entropy of system A increases by 4 kJ. Which of the following must be true of the entropy of system
B?
a)
b)
c)
d)
6.
it must increase
it must decrease
it might increase or decrease, but there is a limit to how much it could increase
it might increase or decrease, but there is a limit to how much it could decrease
The two gas-cycle engine processes are drawn on a T-s diagram below. The cycle for engine A is twice as
wide, and that for engine B is twice as tall.
Which of the follwing is true?
a) Wnet,out for engine A is greater in magnitude than that for engine B
b) Qnet,in for engine A is greater in magnitude than that for engine B
c) engine B is more efficient than engine A
d) none of the above
Section 2: Problems and questions. (15-20 pts each) Partial credit will be given for this section. Show all work
and justify all answers. For gas-cycle engines, you may assume air-standard assumptions.
7.
8.
9.
10.
11.
(20 pts) 0.8 kg water are placed in a cylinder with a tight-fitting but frictionless piston that keeps
the pressure at a constant 5 kPa. The liquid is 7oC below its vaporization point. The pistoncylinder system is placed in contact with a heat reservoir which is held at 500 oC. Heat is added to
the water, until 10% of it boils. Assume constant specific heats.
a)
Find the change in entropy of the water as it is heated to the vaporization temperature.
b)
Find the change in entropy of the water as it vaporizes
c)
Find the total change in entropy of the water
d)
Find the total change in entropy of the whole universe (including the water).
(15 pts) Air at 7oC and 1 atm pressure enters a turbojet engine of an airplane traveling at a velocity
of 300 m/s. The jet fuel has a heating value of 40 MJ/kg and is burned completely. It leaves the
engine at 427oC with a speed of 1100 m/s. The engine provides a thrust of 2000 N.
a)
Find the mass flow rate of the air through the engine.
b)
Find the rate at which the fuel is being burned.
(20 pts) An ideal Otto cycle consists of the following processes: 1-2 adiabatic compression, 2-3
constant volume heating, 3-4 adiabatic expansion, and 4-1 constant volume heat loss. A particular
engine running this cycle begins the adiabatic compression with air at 1 tm and 10 oC. Its
compression ratio is 10.5.
a)
Find the temperature of the air at the end of the adiabatic compression.
b)
Find the work done during the adiabatic compression (on or by the air?).
c)
Find the efficiency of the cycle.
(20 pts) An ideal Rankine cycle is drawn in fig. 1. A particular Rankine engine uses water/steam
as a working fluid flowing through it. Work is output in a turbine, during the isentropic process
from point A to point B. The steam enters the turbine at a temperature of 150 oC and a pressure of
400 kPa. It exits the turbine at a pressure of 300 kPa.
a)
Find the initial specific entropy and specific enthalpy of the steam as it enters the turbine.
b)
What is the specific entropy of the steam as it exits the turbine?
c)
What is the quality of the steam as it exits the turbine?
(15 pts) Air at 25oC and 1 atm enters a nozzle at a speed of 10 m/s through a 10-cm diameter inlet.
It exits the nozzle through an area of 8 cm2 at a speed of 70 m/s.
a)
Find the temperature of the air at the exit.
b)
Find the density of the air at the exit.
= 5.67X10 -8 W/(m2K4)
1 atm = 1.01 X105 Pa ideal gas constant Ru=8.316 J/(moleK)
T(Kelvin) = T( oC)+273
1000 L = 1 m3