According to their rotation curves, most spiral galaxies:
A) rotate as a solid body.
B) rotate counterclockwise.
C) have decreasing rotational speeds with increasing distance.
D) contain dark matter.
E) contain dark energy.
D
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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 point charge q = +1 ?C is located at the origin. What is the flux of the electric field of this charge through a square whose corners are (x, y, z) = (1, 1, 1), (-1, 1, 1), (-1, 1, -1), and (1, 1, -1)?
A) 11.3 × 104 Nm2/C B) 0.5 × 104 Nm2/C C) 0 D) 1.0 × 104 Nm2/C E) 1.9 × 104 Nm2/C
The isotope, tritium, has a half-life of 12.3 years. Assume we have 10 kg of the substance. How much tritium will be left after 30 years?
a. 0.20 kg b. 1.8 kg c. 0.18 kg d. 1.7 kg e. 4.1 kg
Approximately what core temperature is required before hydrogen fusion can begin in a star?
A) 10 million K B) 10,000 K C) 10 trillion K D) 10 billion K E) 1 billion K