Question #1 - Pressure in a piston-cylinder
As shown in the figure, air is contained in a vertical piston-cyclinder assembly fitted with an electrical resistor. The atmosphere exerts a pressure of 14.7 lbf/in2 on the top of the piston, which has a mass of 100 lbm and a face area of 1 ft2. As electric current passes through the resistor, the volume of the air increases while the piston moves smoothly in the cylinder.
The local acceleration of gravity is 32.0 ft/s2. Determine
A) The initial absolute pressure in lbf/in2.
B) The final absolute pressure in lbf/in2
Question #2 - Energy Balance A gas is compressed in a piston-cylinder assembly from p1 = 1.5bar to p2 = 7bar, V2 = 0.02 m 3 in a polytropic process for which n = 1.3 . The mass of the gas is 0.2 kg. The specific internal energy of the gas increases by 60 kJ/kg during the process, Kinetic and potential energy changes are negligible. A) Determine the heat transfer in kJ, use correct sign.
Question #3 - Piston Energy
The figure to the right shows a gas contained in a vertical piston-cylinder assembly. A vertical shaft with a crosssectional area of 1.0 cm2 is attached to the top of the piston. The total mass of the piston and shaft is 20 kg. While the gas is slowly heated, the internal energy of the gas increases by 0.15 kJ, the potential energy of the piston-shaft combination increases by 0.2 kJ, and a force of 1300 N is exerted on the shaft as shown in the figure. The piston and cylinder are both good insulators and friction between them is negligible. The local atmospheric pressure is 1 bar and g = 9.81 m/s2.
Determine:
A) The work done by the shaft (kJ)
B) The work done in displacing the atmosphere (kJ)
C) The heat transfer to the gas (kJ)
Question #4 - Energy Balance, Cycles
A piston-cylinder assembly undergoes a cycle consisting of the following three processes:
Process 1-2: Compression with constant internal energy
Process 2-3: Constant-volume cooling to p3 = 150 kPa, V3 = 0.025 m3
Process 3-1: Constant-pressure expansion with W31 =12.5 kJ
Sketch the cycle on a p-V diagram, labeling each of the states and indicating the process directions with arrows. If the work for the entire cycle is Wcycle = -18.3 kJ and changes in kinetic and potential energy are negligible, determine:
A) The volume at state-1 in m3
B) The work for process 1-2 (in kJ)
C) The heat transfer for process 1-2 (in kJ)
D) Is this cycle a power cycle or a refrigeration/heat pump cycle?