Sample Test Chp1 4

MECH 371 Thermodynamics I Fall 1998 TEST #1 Name:................................................. Note:

Open book, closed notes

One of the answers given for each question is the right answer, rounded to some appropriate number of significant digits (usually 2 or 3). In the unlikely event that all the answers are wrong (due to an error in my solution), ignore the answers and simply write down your answer next to the question. Take the gravitational acceleration to be g = 9.8 m/s2, the density and specific heat of liquid water to be 1000 kg/m 3 and 4.18 kJ/kg.°C, and the specific heats of air to be cv = 0.718 kJ/kg.°C and cp = 1.0 kJ/kg.°C

15% 1 The atmospheric pressures at the top and the bottom of a building are read by a barometer to be 96.0 and 98.0 kPa. If the density of air is 1.2 kg/m3, the height of the building is (a) 14 m

(b) 17 m

(c) 142 m

(d) 170 m

(e) 210 m

15% 2 A room is filled with saturated steam at 100°C. Now a 5-kg bowlind ball at 25°C is brought to the room. Heat is transferred to the ball from the steam, and the temperature of the ball rises to 100°C while some steam condenses on the ball as it loses heat (but it still remains at 100°C). The specific heat of the ball can be taken to be 1.8 kJ/kg.°C. The mass of steam that condensed during this process is (a) 80 g

(b) 128 g

(c) 299 g

(d) 351 g

(e) 405 g

15% 3 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 (a) 4.6 min

(b) 5.6 min

(c) 6.7 min

(d) 9.0 min

(e) 11.4 min

15% 4 A 500-m3 rigid tank is filled with saturated liquid-vapor mixture of water at 200 kPa. If 20% of the mass is liquid and the 80% of the mass is vapor, the total mass in the tank is (a) 705 kg

(b) 500 kg

(c) 258 kg

(d) 635 kg

(e) 2809 kg

10% 5 Consider a sealed can that is filled with refrigerant-134a. The contents of the can is at the room temperature of 25°C. Now a leak developes, and the pressure in the can drops to the local atmospheric pressure of 100 kPa. The temperature of the refrigerant in the can is expected to drop to (rounded to the nearest integer) (a) 0°C

(b) -26°C

(c) -18°C

(d) 3°C

(e) 25°C

15% 6 The roof of an electrically heated house is 6 m long, 8 m wide, and 0.25 m thick. It is made of a flat layer of concrete whose thermal conductivity is 0.8 W/m.°C. During a certain winter night, the temperatures of the inner and outer surfaces of the roof are measured to be 15°C and 4°C, respectively. The average rate of heat loss through the roof that night was (a) 35 W

(b) 422 W

(c) 3379 W

(d) 2246 W

(e) 1690 W

15% 7 A 2-m3 rigid tank contains nitrogen gas at 500 kPa and 300 K. Now the gas is compressed isothermally to a volume of 0.1 m3. The work done on the gas during this compression process is (a) 950 kJ

(b) 0 kJ

(c) 1610 kJ

(d) 2996 kJ

(e) 562 kJ

Extra Credit (25 points) 10% 8 An apple loses 0.9 kJ of heat as it cools per °C drop in its temperature. The amount of heat loss from the apple per °F drop in its temperature is (a) 0.25 kJ

(b) 0.50 kJ

(c) 1.00 kJ

(d) 1.62 kJ

(e) 0.81 kJ

15% 9 A container equipped with a resistance heater and a mixer is initially filled with 3 kg of saturated water vapor at 100° C. Now the heater and the mixer are turned on; the steam is compressed, and there is heat loss to the surrounding air. At the end of the process, the temperature and pressure of steam in the container are measured to be 300°C and 0.5 MPa. The net energy transfer to the steam during this process is (a) 200 kJ

(b) 359 kJ

(c) 561 kJ

(d) 889 kJ

(e) 1568 kJ

AFTER GRADING: Answer Key: 1d, 2c, 3b, 4a, 5b, 6e, 7d, 8b, 9d Grade range: 15-125 (Note that there is 25 points of extra credit). Average: 82 1/3 of the class finished the test early (This was a 50 minute test). The test was given in 3 versions to prevent cheating (the same questions, but the order of questions/answers was different.) The test covers material from Chaps. 1-4 of Cengel/Boles. Percentages of Correct Answers: 1. 2. 3. 4. 5. 6. 7. 8. 9.

91% 52 83 70 70 65 52 61 48