A thermometer registers a change in temperature of 100°F. What change in temperature does this correspond to on the Kelvin Scale?
a. 453
b. 328
c. 180
d. 55.6
e. 24.5
D
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A certain ideal gas has a temperature 300 K and a pressure 5.0 × 104 Pa. The molecules have a mean free path of 4.0 × 10?7 m. If the temperature is raised to 350 K and the pressure is reduced to 1.0 × 104 Pa the mean free path is then:
A. 6.9 × 10?8 m B. 9.3 × 10?8 m C. 3.3 × 10?7 m D. 1.7 × 10?6 m E. 2.3 × 10?6 m
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 power line with a resistance of 2 ohms carries a current of 80 A. The power dissipated in the line is
A) 40 W. B) 160 W. C) 320 W. D) 12,800 W. E) none of the above
What is the sum of 1123 and 10.3 written with the correct number of significant figures?
A) 1133 B) 1133.3 C) 1133.3000 D) 1.1 × 103 E) 1.13 × 103