What is the spectral type of a main sequence star with a luminosity 100 times greater than the sun?
A) G B) A C) M D) O
B
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Characteristic x-rays are the result of
a. outer electron transitions. b. inner electron transitions. c. nuclear electron states. d. buckytubes.
A 6.0-J pulse of light from a laser has what momentum?
a. Light does not have momentum because it doesn't have mass. b. 3.0 × 10^9 kg·m/s c. 6.7 × 10^–9 kg·m/s d. 2.0 × 10^–8 kg·m/s
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)
Our nearest stellar neighbor is a little less than 1 parsec away
Indicate whether the statement is true or false