# MECH 371 Thermodynamics - Portal

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27 Οκτ 2013 (πριν από 4 χρόνια και 8 μήνες)

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Multiple
-
Choice Problems

Tutorial
:

Energy Analysis of Closed Systems

1
.
A 1.2
-
m
3

rigid tank contains nitrogen gas at 200 kPa and 350 K. Now heat is transferred to the nitrogen in
the tank and the pressure of nitrogen rises to 900 kPa. The work done du
ring this process is

(a) 840 kJ

(b) 0 kJ

(c) 156 kJ

(d) 240 kJ

(e) 1080 kJ

Answer

(b) 0 kJ

2
.
An 8
-
L cylinder contains air at 300 kPa and 300 K. Now air is compressed isothermally to a volume of 2
L. The work done on air during this compressio
n process is

(a) 119 kJ

(b) 1.8 kJ

(c) 3.3 kJ

(d) 4.5 kJ

(e) 0 kJ

Answer

(c) 3.3 kJ

3
.
Air in a piston
-
cylinder device is compressed from 25

C and 100 kPa to 500 kPa during a polytropic
process for which
Pv
1.3

= constant. The air temperature a
fter compression is

(a) 33

C

(b) 156

C

(c) 1207

C

(d) 115

C

(e) 182

C

Answer

(b) 156

C

4
.
A well
-
sealed room contains 90 kg of air at 1 atm and 20

C. Now a 1
-
kW electric heater and a 200
-
W fan
are turned on. If heat transfer through the walls is negligible, the air tempe
rature in the room in 15 min will be

(a) 36.7

C

(b) 31.9

C

(c) 33.9

C

(d) 20.3

C

(e) 49.5

C

Answer

(a) 36.7

C

5
.
An air
-
tight room contains 80 kg of air, and a 2
-
kW baseboard electric resistance heater in the room is
turned on and kept on

for 15 min. The temperature rise of air at the end of 15 min is

(a) 22.4

C

(b) 31.3

C

(c) 0.52

C

(d) 13.4

C

(e) 42.8

C

Answer

(b) 31.3

C

6
.
A room contains 25 kg of air at 100 kPa and 10

C. The room has a 250
-
W refrigerator (the refrigerator
consu
mes 250 W of electricity when running) and 1
-
kW electric resistance heater. During a cold winter day,
it is observed that both the refrigerator and the resistance heater are running continuously but the air
temperature in the room remains constant. The rat
e of heat loss from the room during that day is

(a) 3600 kJ/h

(b) 2700 kJ/h

(c) 4500 kJ/h

(d) 1000 kJ/h

(e) 1250 kJ/h

Answer

(c) 4500 kJ/h

7
.
A piston
-
cylinder device contains 3 kg of air at 300 kPa and 25

C. During a quasi
-
equilibrium isotherm
al
expansion process, 20 kJ of boundary work is done by the system, and 8 kJ of paddle
-
wheel work is done
on the system. The heat transfer during this process is

(a) 28 kJ

(b) 20 kJ

(c) 36 kJ

(d) 12 kJ

(e) 4 kJ

Answer

(d) 12 kJ

8
.
A 1
-
kW electric

resistance heater submerged in 10
-
kg water is turned on and kept on for 15 min. During
the process, 400 kJ of heat is lost from the water. The temperature rise of water is

(a) 21.5

C

(b) 12.0

C

(c) 31.1

C

(d) 50.0

C

(e) 8.8

C "

Answer

(b) 12.0

C

9
.
Eggs with a mass of 0.12 kg per egg and a specific heat of 3.32 kJ/kg.

C are cooled from 30

C to 10

C at
a rate of 800 eggs per minute. The rate of heat removal from the eggs is

(a
) 8.0 kW

(b) 53 kW

(c) 478 kW

(d) 106 kW

(e) 159 kW

Answer

(d) 106 kW

10
.
An orange with an average mass of 0.20 kg and average specific heat of 3.70 kJ/kg.

C is cooled from

C to 5

C. The amount of heat transferred from the o
range is

(a) 7.4 kJ

(b) 24.8 kJ

(c) 18.5 kJ

(d) 55.5 kJ

(e) 11.1 kJ

Answer

(e) 11.1 kJ