Science

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EGME 304 Thermodynamics Fall 2014

Homework 1, Due on 9/17 Problem 1. (15’) The specific volume of water vapor at 0.3 MPa, 160 °C is 0.651 m3/kg. Molecular weight of water vapor is 18.02 kg/kmol. If the water vapor occupies a volume of 2 m3, determine: (a) The amount present, in kg and kmol. (5’+5’) (b) The number of molecules. (5’) Problem 2. (20’) A gas in a piston-cylinder assembly undergoes a process for which the relationship between pressure and volume is = . The initial pressure is 1 bar, the initial volume is 0.1 m3, and the final pressure is 9 bar. Determine: (a) The final volume, in m3. (10’) (b) The work for the process, in kJ. (10’) Problem 3. (35’) A gas contained within a piston-cylinder assembly, initially at a volume of 0.1 m3, undergoes a constant-pressure expansion at 2 bar to a final volume of 0.12 m3, while being slowly heated through the base. The change in internal energy of the gas is 0.25 kJ. The piston and cylinder walls are fabricated from heat-resistant material and the piston moves smoothly in the cylinder. The local atmosphere pressure is 1 bar. (a) For the gas alone as the system, find work and heat transfer, each in kJ. (5’+10’) (b) For the piston alone as the system, find work and change in potential energy, each in kJ. (10’+10’)

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Problem 4. (20’) The outer surface of a transistor is cooled convectively by a fan-induced flow of air at a temperature of 25 °C and a pressure of 1 atm. The transistor’s outer surface area is 5 × 10 . At steady state, the electrical power to the transistor is 3 W. The heat transfer coefficient is 100 W/m2·K. Determine: (a) The rate of heat transfer in W. (10’) (b) The temperature at the transistor’s outer surface, in °C. Given = ( ) (10’) Problem 5. (10’) Helium gas is contained in a closed rigid tank. An electric resistor in the tank transfers energy to the gas at a constant rate of 1 kW. Heat transfer from the gas to its surroundings occurs at a rate of 5 watts, where is time, in minutes. Find total energy E as a function of .

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