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Physics 41 Exam 3 Practice Fall 2016
Name: _________________________ T W TH
Short Conceptual Questions (2 points each) Circle the best answer.
1. When bringing water to a boil in the mountains, the time needed to reach the boiling point is
a. less than at sea level
b. more than at sea level
2.. When snow forms in clouds, the surrounding air
A) warms.
B) cools.
c. the same as at sea level
C) neither warms nor cools.
3. In a thermodynamic process, the internal energy of a system in a container with adiabatic walls increases by 800 J.
Which statement is correct?
a.
b.
c.
d.
e.
The system lost 800 J by heat transfer to its surroundings.
The system gained 800 J by heat transfer from its surroundings.
The system performed 800 J of work on its surroundings.
The surroundings performed 800 J of work on the system.
The 800 J of work done by the system was equal to the 800 J of heat transferred to the system from its
surroundings.
4. Hydrogen and oxygen molecules in a gas sample have the same temperature. This means the hydrogen molecules, on
the average, have the same
A) speed and the same kinetic energy. B) speed, but more kinetic energy.
C) speed, but less kinetic energy.
D) kinetic energy, but more speed.
E) kinetic energy, but less speed.
5. The food in a refrigerator is cooled by
A) vaporization of the refrigerating fluid.
C) the ice in your nearby freezer.
B) condensation of the refrigerating fluid.
6. What, if anything, is wrong with this refrigerator? Which Law of Thermo does it violate?
A. It violates the first law. B. It violates the second law.
B. It violates the third law.
C. Nothing is wrong.
7. Systems that are left alone, tend to move toward a state of
A) less entropy.
B) more entropy.
C) no entropy.
Pour a liter of water at 40 degrees C into a liter of water at 20 degrees C and the final temperature of the
two becomes
A) less than 30 degrees C.
B) at or about 30 degrees C.
C) more than 30 degrees C.
8.
9. The second law of thermodynamics says that
A. the entropy of an isolated system never decreases. B. heat never flows spontaneously from cold to hot.
C. the total thermal energy of an isolated system is constant. D. both a and b.
E. both a and c.
10. The specific heat of an ideal gas at constant pressure is greater than the specific heat of an ideal gas at constant
volume because
a.
c.
d.
e.
work is done by a gas at constant pressure.
b.
work is done by a gas at constant volume.
no work is done by a gas at constant pressure.
the temperature remains constant for a gas at constant pressure.
the temperature remains constant for a gas at constant volume.
Short Problems (10 points each). Show your work in the space provided for credit. BOX FINAL ANSWERS!
1. A child has a temperature of 101°F. If her total cross-sectional area is 2 m 2, find the energy lost each second
(in W) due to radiation, assuming the emissivity is 1. (A ssume the room temperature is 70°F.)
a.
b.
c.
d.
e.
217
180
90
68
850
2. A super-insulated house is at a temperature of 20°C. The temperature outside is 0°C. The surface area of the house is
200 m2, and the emissivity is 1. Approximately how much energy is radiated (in W) per second. Show explicitly how
the units work out.
a.
20 000
b.
2000
c.
200
d.
2
e.
0.2
3. A heat pump has a coefficient of performance of 4. If the heat pump absorbs 20 cal of heat from the cold
outdoors in each cycle, the heat expelled (in cal) to the w arm indoors is
a.
b.
c.
d.
e.
34
27
36
40
80
4. A refrigerator has a coefficient of performance of 4. If the refrigerator absorbs 30 cal of heat from the cold
reservoir in each cycle, the heat expelled (in cal) into the heat reservoir is
a.
b.
c.
d.
e.
40.5
37.5
36.5
34.5
22.5
A 0.0500 kg ice cube at -30.0 degrees C is placed in 0.400 kg of 35.0 degrees C water in a very well insulated container.
What is the final temperature?
a.
b.
c.
d.
25C
24C
21C
19C
The Long Problem: (20 points) This must be NEAT and SHOW ALL THE WORK! BOX ANSWERS!
A heat engine using a monatomic gas follows the cycle shown.
(a) Find the temperature, volume and pressure at each point and the change in internal energy, work done ON the gas,
and Q for each process and the net for the cycle. Clearly label and show ALL your work and attach on additional paper.
, naming each process. Put your results in the tables.
(b) Find the total work done by the engine during one cycle and the
thermal efficiency of the engine.
(c) Draw the heat transfer diagram, labeling and showing what TH, TC,
QH, QC and Work.
SHOW WORK ON ADDITIONAL PAPER. LABEL CLEARLY!
n
T (K)
P(kPa)
V (cm3)
1
2
3
W (J)
12
23
31
Net
e
Q (J)
Eint (J)
Eint (J)