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E234 THERMODYNAMICS
Exam 1 – February 9, 2009 105 points total; Maximum grade is 100 Data:
water = 1000 kg/m3 oil = 0.9 g/cm3 mustard = 1.1 kg/liter g = 9.81 m/s2
kplaster = 0.79 W/m.K
kinsulation = 0.037 W/m.K
kbrick = 0.7 W/m.K
1. 20 A 2liter container is filled with 500 g of olive oil, 300 g of vinegar and 100 cm of 3 mustard. Find the average specific volume and density of the masses when you exclude the mass and volume of the air. Assume vinegar has the same density as water.
Average specific volume in SI units:
Average density in g/cm3 and kg/m3:
V = moil/oil + mvin/vin + Vmustard = 500/0.9 + 300/1 + 100 = 955 cm3
m = moil + mvin + Vmustardmustard = 500 + 300 + 100x1.1 = 910 g v = 0.955/0.910 = 1.049 L/kg = 1.049x103 m3/kg vkg/m3 2. 20Rocky's base metabolic rate is 100 W. He weighs 80 kg. This morning, he climbed stairs 25cm high at the rate of 140 steps per minute for 30 minutes on a Stairmaster. Assume
potential energy is expended climbing up and no energy is used climbing down. a. How much energy did he burn (in J and in kWh)? Et =Total energy spent = (number of steps in 30 min) x (potential energy per steps) + metabolic Et = (140x30)x(80x9.81x0.25) + 100x30x60 = 824 + 180 = 1004 kJ = 0.278 kWh
b. What was his total average power (in W) during the exercise? Power = 1004/1800 = 0.558 kW =558 W
3. 15A large blower in a chip processing plant keeps the air inside at 10 cm water pressure relative to the hallway at atmospheric pressure. a. What is the gage pressure in the room in kPa Pgage = 10 cm water =gh = 1000x9.81x0.10 = 981 Pa Spring 2009 b. What is the net force on a door measuring 1.9 m by 1.1 m?
F =PA =981x1.9x1.1 = 2050 N or about 200 kg!
4. 30A cold room is made of walls of total area 100 m . The wall is made up of three layers: 2 1cm thick plaster board, 15cm thick glasswool and 5cm thick brick to the outside at 15oC. The heat transfer coefficient for the inside wall surface is 15 W/m2.K and for the outside wall
surface 40 W/m2.K. What is the power needed of the refrigerator to keep the room at –20oC?
Determine the overall thermal resistance: 5. 20Consider a bicycle pump. You plug the outlet and push the piston in two different ways: a) very rapidly and b) very slowly. The pump is made of a metal of high thermal conductivity. For which process can the walls be considered to be diathermal? Adiabatic? Explain clearly your thinking. The gas will heat up as it is compressed
a.
Very rapidly
If it is compressed very rapidly, the heat produced will not have time to escape. The walls can be considered to be adiabatic since they do not act to remove the heat. The process is essentially
adiabatic. A thermometer in the gas would register an increase in temperature.
b. Very slowly If it is compressed very slowly, the heat produced will have time to escape and the temperature of the gas will remain essentially constant so that the walls can be considered to be diathermal. A thermometer in the gas would register a constant temperature. Spring 2009 ...
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This note was uploaded on 05/25/2010 for the course E234 E234 taught by Professor Gallois during the Spring '09 term at Stevens.
 Spring '09
 GALLOIS

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