What speed is required for a length to contract to 80% of its proper length?
A. 0.80 c
B. 0.60 c
C. 0.40 c
D. 0.20 c
E. 0.50
B. 0.60 c
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An experiment to measure the speed of light uses an apparatus similar to Fizeau's. The distance between the light source and the mirror is 10 m, and the wheel has 800 notches. If the wheel rotates at 9000 rev/s when the light from the source is extinguished, what is the experimental value for c (in m/s)?
a. 2.94 × 10^8 b. 2.92 × 10^8 c. 2.88 × 10^8 d. 2.98 × 10^8 e. 3.01 × 10^8
When a hydrogen atom's electron goes from an energy state of n = 1 to n = 3,
a. the radius of the electron's orbit is unchanged. b. the radius of the electron's orbit is increased. c. the radius of the electron's orbit is decreased. d. the radius of the electron's orbit is infinite.
A semiconductor can conduct electricity
A) only below room temperature. B) never. C) only very much above room temperature. D) at room temperature but conductivity increases with increasing temperature. E) at room temperature but conductivity decreases with increasing temperature.
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)