In some cases, we have seen extrasolar planets pass in front of their stars
Indicate whether the statement is true or false
TRUE
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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
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
You are heating two chunks of metal of the same size and shape. One is made of lead and is heated to 400 K. The other is made of iron and is heated to 350 K. Which of the following statements apply to the thermal radiation the two chunks emit?
A) The two spectra peak at the same wavelength. B) The spectra cannot be compared because they come from different materials. C) The lead spectrum peak is at a shorter wavelength than the iron spectrum peak. D) The lead spectrum peak is at a longer wavelength than the iron spectrum peak.
A ball is thrown vertically upwards at 29.4 m/s. For its complete trip (up and back down to the starting position), its average speed is:
a. 14.7 m/s. c. 4.90 m/s. b. 9.80 m/s. d. not given.
A 5.0 cm focal length lens is used as a magnifier. Assuming a near point of 25 cm, about what magnification is attained?
A. 25 B. 10 C. 60 D. 1.2 E. no value given is within 2 of an appropriate answer