What do quasars look like?
a) Fuzzy, extended patches of light.
b) Large concentrations of dark matter.
c) Individual stars in the Milky Way.
d) Glowing gas clouds.
e) Planetary nebulae
Answer: c) Individual stars in the Milky Way.
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A composite refrigerator wall is composed of 5 cm of corkboard sandwiched between a 1.2 cm thick layer of oak and a 0.8 mm thick layer of aluminum lining on the inner surface. The average convective heat transfer coefficients at the interior and exterior wall are 11 and 8 W/(m2 K), respectively. (a) Draw the thermal circuit. (b) Calculate the individual resistances of the components of this composite wall and the resistances at the surfaces. (c) Calculate the overall heat transfer coefficient through the wall. (d) For an air temperature of -1 0C inside the refrigerator and 32°C outside, calculate the rate of heat transfer per unit area through the wall.
GIVEN
• Refrigerator wall: oak, corkboard, and aluminum
• Thicknesses
? Oak (Lo) = 1.2 cm= 1.2*10-2 m
? Corkboard (Lc) = 5 cm= 5*10-2 m
? Aluminum (La) = 0.8 mm = 8*10-4 m
• Convective heat transfer coefficients
? Interior ( ci h ) = 12 W/(m2 K)
? Exterior ( co h ) = 8 W/(m2 K)
• Air temperature
? Inside (Ti) = -1°C
? Outside (To) = 32°C
FIND
(a) Draw the thermal circuit (b) The individual resistances (c) Overall heat transfer coefficient (U) (d) Rate of heat transfer per unit area (q/A)
ASSUMPTIONS
• One dimensional, steady state heat transfer
• Constant thermal conductivities
• Contact resistance between the different materials is negligible
SKETCH
PROPERTIES AND CONSTANTS
Oak (ko)= 0.19 W/(m K) at 20°C
Corkboard (kc) = 0.0415 W/(m K) at 20°C
Aluminum (ka) = 236 W/(m K) at 0°C
60. mi/h is approximately
A) 20. m/s. B) 120. m/s. C) 30. m/s. D) 60. m/s. E) 30. km/h.
Young children that need glasses are rarely prescribed bifocals. Why?
a. They are almost always nearsighted. b. They are almost always farsighted. c. They have considerable accommodation power. d. Parents don't go for the additional expense.
Which of the following types of radiation has the least ability to penetrate matter?
a. Alpha particles b. Gamma rays c. X-rays d. Beta particles