Equilibrium: If the sum of the external forces on an object is zero, then the sum of the external torques on it must also be zero.
A. True
B. False
Answer: B
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A variable resistor has a voltage of 12.0 V placed across it. If the resistance is increased 20%, what happens to the current through it?
A. it increases 20% B. it increases 25% C. it decreases 20% D. it decreases 25% E. it decreases 17%
In a heat exchanger, as shown in accompanying figure, air flows over brass tubes of 1.8- cm-ID and 2.1-cm-OD that contain steam. The convection heat-transfer coefficients on the air and steam sides of the tubes are 70 W/(m2 K) and 210 W/(m2 K), respectively. Calculate the overall heat transfer coefficient for the heat exchanger (a) based on the inner tube area, (b) based on the outer tube area but assume that a fouling factor of 0.00018 (m2 K)/W has developed on the inside of the tube during operation.
GIVEN
• Air flow over brass tube containing steam
• Tube diameters Inside (Di) = 1.8 cm = 0.018 m
Outside (Do) = 2.1 cm = 0.021 m
• Convective heat transfer coefficients Air side h o= 70 W/(m2 K)
Steam side h i= 210 W/(m2 K)
• Fouling factor on the inside of the tube (Rd) = 0.0018 (m2 K)/W
FIND
• The overall heat transfer coefficient for the heat exchanger based on
(a) the inner tube area (Ui) and (b) the outer tube area (Uo)
ASSUMPTIONS
• The heat transfer coefficients are uniform over the transfer surfaces
SKETCH
Thermodynamic Devices: An ideal Carnot engine is operated as a heat pump to heat a room in the winter. The heat pump delivers heat to the room at the rate of 47 kJ per second and maintains the room at a temperature of 293 K when the outside temperature is 237 K. The power requirement to run the heat pump under these operating conditions is closest to
A. 9000 W. B. 7100 W. C. 20,000 W. D. 15,000 W. E. 11,000 W.
Alkali metals have how many electrons past the noble gas configuration?
A. 1 B. 2 C. 3 D. 4 E. 5