Physics 201 MWF10:20 Fall 1994 (Ford)
Exam IV Chapts. 14--20 in SZY College Physics
Show all your work. Partial credit will be given if earned. Write your
answers in the blanks provided.
(18 pts) 1. A 0.15 kg piece of ice at an initial temperature of
-20 °C is placed in 3.00 kg of water that is in a container of
negligible mass. The water has an initial temperature T. No heat is
lost to the surroundings, and the final temperature of the system is
15 °C. What was the initial temperature T of the water?
(For water c = 4190 J/kg·C°,
LF = 333×103 J/kg, and
LV = 2256×103 J/kg.
For ice c = 2010 J/kg·C°.)
Ans. _______________
(16 pts) 2. One end of an insulated rod is in boiling water at 100 °C
and the other end is in an ice-water mixture at 0 °C. The rod has
uniform cross-sectional area of 5.00×10-4 m2.
It consists
of a 25 cm long piece of steel joined to a piece of copper. The steel end is
in the boiling water and the copper end is in the ice water. The thermal
conductivity of steel is
50.2 J/s·m·C°
and the thermal conductivity of copper is
385.0 J/s·m·C°.
The heat conducted by the
rod melts 0.0060 kg of ice in five minutes.
a) What is the heat current H in the rod?
Ans. ______________
b) What is the temperature T at the junction between the steel and copper
sections of the rod?
Ans. ______________
(15 pts) 3. Two moles of an ideal gas undergo a reversible isothermal
expansion at a temperature of 200 K. During the expansion the gas does
300 J of work and the volume of the gas increases. What is the change
in the entropy of the gas? (Be sure to indicate whether the entropy
change is positive or negative.)
Ans. ______________
(12 pts) 4. A mixture of helium and neon gas is in thermal equilibrium and
has temperature T. Both gases are monatomic and may be treated as ideal.
The average kinetic energy of a neon atom is 4.00×10-21 J. The
atomic mass of neon is 20.18 g/mol and the atomic mass of helium
is 4.00 g/mol.
a) What is the average kinetic energy of a helium atom?
Ans. _____________
b) What is the temperature T of the gas mixture?
Ans. _____________
(15 pts) 5. An ice-making machine operates in a Carnot cycle. The low
temperature reservoir is at 0 °C. To operate the device for
five minutes 200 J of mechanical work must be done on the working
substance of the machine. In five minutes the device converts 0.0120 kg of
water at 0 °C to ice at 0 °C.
a) In five minutes, what magnitude of heat is removed from the low temperature
reservoir?
Ans. ______________
b) In five minutes, what magnitude of heat is rejected by the device into the
high temperature reservoir?
Ans. ______________
c) What is the temperature (in °C) of the high temperatue reservoir?
Ans. ______________
(24 pts) 6. Three moles of an ideal gas are taken around the cycle shown in
the sketch. The gas has Cp = 29.41 J/mol·K.
The gas constant R = 8.314 J/mol·K.
Process 1 to 2 is at constant pressure, process 2 to 3
is adiabatic, and process 3 to 1 is at constant volume.
The temperature for state 1 is 300 K, for state 2 it is 455 K,
and for state 3 it is 600 K.
a) For process 1 to 2 calculate W for the gas.
(Be sure to indicate whether it is positive or negative).
Ans. __________
b) For process 2 to 3 calculate W for the gas.
(Be sure to indicate whether it is positive or negative).
Ans. ____________
c) For process 3 to 1 calculate W for the gas.
(Be sure to indicate whether it is positive or negative).
Ans. ____________
d) Calculate the net W for the entire cycle 1 to 2 to 3 to 1.
(Be sure to indicate whether it is positive or negative).
Ans. ____________________