The conductivity of a semiconductor sharply increases when 400 nm (or shorter) wavelength light is shone upon it. This is consistent with what energy gap for this material?
A)
5.2 eV
B)
3.6 eV
C)
3.1 eV
D)
1.0 eV
E)
0.45 eV
C
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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
From Appendix 1, Table 5, the Stephan-Boltzmann constant
From Table 11.3, the hemispherical emissivity of the spheres will be approximated by that for
oxidized aluminum at 310 K: ? ? = ??1 ???2 = 0.11
A person is declared to be dead upon the irreversible cessation of spontaneous body functions: brain activity, or blood circulation and respiration. However, only about 1% of a person's cells have to die in order for all of these things to happen. How can someone be dead when 99% of his or her cells are still alive?
What will be an ideal response?
Two identical loudspeakers that are 5.00 m apart and face toward each other are driven in phase by the same oscillator at a frequency of 875 Hz. The speed of sound in the room is 344 m/s
If you start out standing midway between the speakers, find the shortest distance you can walk toward either speaker in order to hear a minimum of sound. A) 0.197 m B) 0.295 m C) 0.0983 m D) 0.393 m E) 0.590 m
In straight-line motion, if a moving object slows down, the direction of the acceleration is ______________ the direction of the velocity
Fill in the blank(s) with correct word