thermo_exam1_sol - ENGRD 221 Thermodynamics (Prof. N....

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ENGRD 221 –Thermodynamics (Prof. N. Zabaras) Prelim 1 10/11/2007 Page 1 of 9 Thursday October 11, 7:30 pm – 9:30 pm Closed books and notes. Answer all questions. Make sure your answers are legible. Circle your final answer. The TAs and instructor will not respond to any questions during the exam. If you think that something is wrong with one of the problems below, please state your concern in your exam books. Problem 1 (10 points) Multiple choice questions. Answer all questions – explanations are not needed and will not be graded, only provide your answer. (1) (2 points) An ideal gas is throttled to a lower pressure. During this process, (select the correct statement ) (a) the temperature of the gas will increase. (b) the temperature of the gas will decrease. (c) the temperature of the gas will remain constant. (d) the entropy of the gas will decrease. (e) the enthalpy of the gas will decrease. Answer: For a throttling process, h remains constant – so the temperature remains constant (correct answer (c)). Recall for ideal gases h(T). (2) (3 points) An ideal gas process for which P v k = constant, where k=c p /c v is an (select the correct answer(s)) (a) incompressible process (b) isobaric process (c) adiabatic process (d) isentropic process (e) isothermal process Answer: The correct answers are (c,d) (3) (2 points) When a closed system with internal irreversibilities undergoes a process from state 1 to state 2, the entropy change, S 2 – S 1 , and entropy production, σ , can be a) negative and positive, respectively. b) negative and zero, respectively. c) positive and negative, respectively. d) positive and zero, respectively. e) all of the above. Answer: (a) negative and positive respectively; σ is always positive for this process. (4) (3 points) A refrigeration cycle operating between two reservoirs receives energy Q C from a cold reservoir at T C = 250 K and rejects energy Q H to a hot reservoir at T H = 300
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ENGRD 221 –Thermodynamics (Prof. N. Zabaras) Prelim 1 10/11/2007 Page 2 of 9 K. If Q C = 1400 kJ and W cycle = 140 kJ, does this cycle operate reversibly, irreversibly, or is it impossible? a. Reversibly b. Irreversibly c. It is impossible Answer: 5 250 300 250 max = = = c h c T T T β and 10 140 1400 = = = cycle c W Q , since max > this is impossible, so the answer is (c). Problem 2 (5 points) 1.2 kg of liquid water initially at 15 ° C is to be heated to 95 ° C in a teapot equipped with a 1200 W electric heating element inside. The specific heat of water can be taken to be 4.18 kJ/kg. ° C, and the heat loss from the water during heating can be neglected. The time it takes to heat the water to the desired temperature is approximately:
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thermo_exam1_sol - ENGRD 221 Thermodynamics (Prof. N....

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