Homework 4 Due Wednesday, March
5, 2008
1. A frictionless
piston-cylinder device initially contains 200 L of saturated liquid
refrigerant-134a. The piston is free to
move, and its mass is such that it maintains a pressure of 800 kPa on the refrigerant.
The refrigerant is now heated until its temperature rises to 50 ºC. Calculate the work done during this
process. (C&B) W = 5226 kJ
2. A mass of 2.4 kg of air
at 150 kPa and 12 ºC is contained in a gas-tight,
frictionless piston-cylinder device. The
air is now compressed to a final pressure of 600 kPa. During the process, heat is transferred from
the air such that the temperature inside the cylinder remains constant. Calculate the work input during the process. (C&B)
W = -272 kJ
3. A university campus has
200 classrooms and 400 faculty offices.
The classrooms are equipped with 12 fluorescent tubes, each consuming
110 W, including the electricity used by the ballasts. The faculty offices, on the average, have
half as many tubes. The campus is open
240 days a year. The classrooms and
faculty offices are not occupied an average of four hours a day, but the lights
are kept on. If the unit cost of
electricity is $0.082/kWh, determine how much the campus will save a year if
the lights in the classrooms and faculty offices are turned off during
unoccupied periods. (C&B) $41564
4. A 3-m3 rigid
tank contains hydrogen at 250 kPa and 500 K. The gas is now cooled until its temperature
drops to 300 K. Determine (a) the final
pressure in the tank and (b) the amount of heat transfer. (C&B)
Qout = 753.1
kJ
5. An insulated rigid tank
is divided into two equal parts by a partition.
Initially, one part contains 6 kg of an ideal gas at 800 kPa and 50 ºC, and the other part is evacuated. The partition is now removed, and the gas
expands into the entire tank. Determine
the final temperature and pressure in the tank.
(C&B) P2 = 400 kPa
6. An insulated
piston-cylinder device initially contains 0.3 m3 of carbon dioxide
at 200 kPa and 27 ºC.
An electric switch is turned on, and a 110-V source supplies current to
a resistance heater inside the cylinder for a period of 10 min. The pressure is held constant during the
process, while the volume is doubled.
Determine the current that passes through the resistance heater. (C&B)
T2 = 600 K and I = 4.71 amp
7. One mole of an ideal gas
at T = 25 °C is held in a cylinder
by a piston at a pressure of 1 x 107 N m-2. The piston pressure is released in three
stages,, first to 5 x 106 N m-2, then to 1 x 106
N m-2, and then to 1 x 105 N m-2. Calculate the work done by the gas during
these irreversible, isothermal expansions.
Compare this result to work done in a quasi-static (reversible),
isothermal expansion from 1 x 107 N m-2 to 1 x 105
N m-2 at T = 25 °C.
W = 5452 J and Wrev = 11409.6 J.
"(C&B)"
indicates a problem taken from Y. A. çengel and M. A. Boles, "Thermodynamics – An
Engineering Approach," fifth edition.