Semiconductors: In a good insulator, the highest energy band containing electrons, called the valence band, is
A. only partially filled.
B. completely filled.
C. completely empty.
Answer: B
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A satellite of mass 500 kg orbits the Earth with a period of 6000 s. The Earth has a mass of 5.98 × 1024 kg. (a) Calculate the magnitude of the Earth's gravitational force on the satellite
(b) Determine the altitude of the satellite above the Earth's surface.
Only two forces act on a 3.0-kg mass. One of the forces is 9.0 N east, and the other is 8.0 N in the direction of 62° north of west. What is the magnitude of the acceleration of the mass?
a. 2.0 m/s2 b. 2.4 m/s2 c. 3.3 m/s2 d. 2.9 m/s2 e. 5.7 m/s2
Two concentric spheres 0.2 m and 0.3-m in diameter, with the space between them evacuated, are to be used to store liquid air (133 K). If the surfaces of the spheres have been flashed with aluminum and the liquid air has a talent heat of vaporization of 209 kJ/kg, determine the number of kilograms of liquid air evaporated per hour.
GIVEN
• Two concentric spheres with the space between them evacuated and liquid air in the inner sphere
• Diameters ? D1 = 0.2 m ? D2 = 0.3 m
• Liquid air temperature (Ta) = 133 K
• Room temperature (T?) = 293 K
• Surfaces of the spheres have been flashed with aluminum
• Heat of vaporization of liquid air (hfg) = 209 kJ/kg = 209,000 J/kg
FIND
• The number of kilograms of liquid air evaporated per hour ( )m
ASSUMPTIONS
• Steady state
• Convective thermal resistance between the liquid air and interior sphere is negligible
• Thermal resistance of the sphere walls is negligible
• Natural convection on the exterior is negligible
• The room behaves as a blackbody enclosure
• The thickness of the sphere walls in negligible
SKETCH
PROPERTIES AND CONSTANTS
the Stephan-Boltzmann constant (?) = 5.67 × 10–8 W/(m2 K4)
the hemispherical emissivity of the spheres will be approximated by that for oxidized aluminum at 310 K: ? = ?1 ? 2 = 0.11
The statement "The albedo of the near side of the Moon is 0.12" implies that the Moon:
A) absorbs only 12 percent of the light that hits it. B) reflects only 12 percent of the light that hits it. C) orbits Earth with an inclination of 12°. D) revolves around the Sun with an inclination of 12°.