Thermodynamics

4–62 A piston–cylinder device whose piston is resting ontop of a set of stops initially contains 0.5 kg of helium gas at100 kPa and 25°C. The mass of the piston is such that 500kPa of pressure is required to raise it. How much heat mustbe transferred to the helium before the piston starts rising?Answer: 1857 kJ4–63 An insulated piston–cylinder device contains 100 L ofair at 400 kPa and 25°C. A paddle wheel within the cylinderis rotated until 15 kJ of work is done on the air while thepressure is held constant. Determine the final temperature ofthe air. Neglect the energy stored in the paddle wheel.4–64E A piston–cylinder device contains 25 ft 3 of nitrogenat 40 psia and 700°F. Nitrogen is now allowed to cool at constantpressure until the temperature drops to 200°F. Usingspecific heats at the average temperature, determine theamount of heat loss.4–65 A mass of 15 kg of air in a piston–cylinder device isheated from 25 to 77°C by passing current through a resistanceheater inside the cylinder. The pressure inside the cylinder isheld constant at 300 kPa during the process, and a heat loss of60 kJ occurs. Determine the electric energy supplied, in kWh.Answer: 0.235 kWhW eAIRP = constantFIGURE P4–654–66 An insulated piston–cylinder device initially contains0.3 m 3 of carbon dioxide at 200 kPa and 27°C. An electricswitch is turned on, and a 110-V source supplies current to aresistance heater inside the cylinder for a period of 10 min.The pressure is held constant during the process, while thevolume is doubled. Determine the current that passes throughthe resistance heater.4–67 A piston–cylinder device contains 0.8 kg of nitrogeninitially at 100 kPa and 27°C. The nitrogen is now compressedslowly in a polytropic process during which PV 1.3 constant until the volume is reduced by one-half. Determinethe work done and the heat transfer for this process.4–68 Reconsider Prob. 4–67. Using EES (orother) software, plot the process described in theproblem on a P-V diagram, and investigate the effect ofthe polytropic exponent n on the boundary work and heatQChapter 4 | 207transfer. Let the polytropic exponent vary from 1.1 to1.6. Plot the boundary work and the heat transfer versus thepolytropic exponent, and discuss the results.4–69 A room is heated by a baseboard resistance heater.When the heat losses from the room on a winter day amountto 6500 kJ/h, the air temperature in the room remains constanteven though the heater operates continuously. Determine thepower rating of the heater, in kW.ROOMT air = constantW eFIGURE P4–694–70E A piston–cylinder device contains 3 ft 3 of air at 60psia and 150°F. Heat is transferred to the air in the amount of40 Btu as the air expands isothermally. Determine the amountof boundary work done during this process.4–71 A piston–cylinder device contains 4 kg of argon at250 kPa and 35°C. During a quasi-equilibrium, isothermalexpansion process, 15 kJ of boundary work is done by thesystem, and 3 kJ of paddle-wheel work is done on the system.Determine the heat transfer for this process.4–72 A piston–cylinder device, whose piston is resting on aset of stops, initially contains 3 kg of air at 200 kPa and27°C. The mass of the piston is such that a pressure of 400kPa is required to move it. Heat is now transferred to the airuntil its volume doubles. Determine the work done by the airand the total heat transferred to the air during this process.Also show the process on a P-v diagram. Answers: 516 kJ,2674 kJ4–73 A piston–cylinder device, with a set of stops onthe top, initially contains 3 kg of air at 200 kPaand 27°C. Heat is now transferred to the air, and the pistonrises until it hits the stops, at which point the volume is twicethe initial volume. More heat is transferred until the pressureinside the cylinder also doubles. Determine the work doneand the amount of heat transfer for this process. Also, showthe process on a P-v diagram.Closed-System Energy Analysis: Solids and Liquids4–74 In a manufacturing facility, 5-cm-diameter brass balls(r 8522 kg/m 3 and c p 0.385 kJ/kg · °C) initially at 120°Care quenched in a water bath at 50°C for a period of 2 min atQ