An AC power generator produces 27 A (rms) at 4730 V. The energy is transmitted through a long-distance power line, which has a total resistance of 173 ?. What percentage of the power delivered by the generator is dissipated as heat in the long-distance line if a step-up transformer is not used?
a. 1%
b. 20%
c. 31%
d. 99%
e. 98%
d
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A 10-A current flows through a wire for 2.0 min. (e = 1.60 × 10-19 C)
(a) How much charge has passed through this wire? (b) How many electrons have passed any point in the wire? What will be an ideal response?
At greater distances from the sun, planetary surface temperatures increase
Indicate whether the statement is true or false
A sphere 20 cm in diameter containing liquid air (–140°C) is covered with 5-cm-thick glass wool (50 kg/m3 density) with an emissivity of 0.8. Estimate the rate of heat transfer to the liquid air from the surrounding air at 20°C by convection and radiation. How would you reduce the heat transfer?
GIVEN
• A sphere containing liquid air covered with glass wool
• Sphere diameter (Ds) = 20 cm = 0.2 m
• Liquid air temperature (Ta) = – 140°C = 133 K
• Surrounding air temperature (T?) = 20°C = 293 K
• Insulation thickness (s) = 5 cm = 0.05 m
• Insulation emissivity (?) = 0.8
FIND
• Rate of heat transfer from liquid air to surrounding air
(q)
• How can this be reduced?
ASSUMPTIONS
• Steady state conditions
• The surroundings behave as a black body enclosure at T?
• Surrounding air is still
• Thermal resistance of the convection inside the sphere and of the container wall are negligible
SKETCH
PROPERTIES AND CONSTANTS
the Stephan-Boltzmann constant (?) = 5.67 × 10–8 W/(m2 K4).
the thermal conductivity of glass wool (ki) = 0.037 W/(m K)
A truck is traveling down a steady slope such that for each 1 m the truck goes forward along the slope it goes down 0.04 m (we call this a 4% grade). Imagine that the truck’s brakes fail. What is the approximate increase in the truck’s speed after 30 s, assuming that the engine is not used and there is little drag or other friction? 
22 (mi/h)/s.]
A. 0.9 mi/h B. 11 mi/h C. 26 mi/h D. 53 mi/h E. 660 mi/h F. Other (specify)