At what rate is thermal energy generated in the 5-? resistor when ? = 24 V?
a.
13 W
b.
3.2 W
c.
23 W
d.
39 W
e.
51 W
b
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When a positive charge is released and moves along an electric field line, it moves to a position of
a. lower potential and lower potential energy. b. lower potential and higher potential energy. c. higher potential and lower potential energy. d. higher potential and higher potential energy. e. greater magnitude of the electric field.
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
The greenhouse effect occurs in the
A) troposphere. B) stratosphere. C) lithosphere.
Main-sequence stars many times the mass of the Sun produce their energy by the set of nuclear reactions called the CNO cycle
a. True b. False Indicate whether the statement is true or false