Electric power lines require convective cooling from the surrounding air to prevent excessive temperatures in the wire. If a 1 inch diameter line is wrapped with nylon to increase heat transfer with the surroundings, how much nylon can be wrapped around the wire before it begins to act as an insulator? The convective heat transfer coefficient is 5 Btu/hr?ft^2 ?ºF.
What will be an ideal response?
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What causes a mirage?
a. A layer of cold air close to the ground. b. The physiological effects of water deprivation. c. A layer of warm air close to the ground. d. A "sandwich" of layers of warm, cold, and warm air. e. Extremely hot air all the way from the ground to the top of the atmosphere.
You throw a rock horizontally off a cliff with a speed of 20 m/s and no significant air resistance. After 2.0 s, the magnitude of the velocity of the rock is closest to
A) 28 m/s B) 20 m/s C) 40 m/s D) 37 m/s
It is found that lead from 2-mm-thick old paint on a certain wood surface has permeated to 1000 ppm at a depth of 2 mm below the outer wood surface. If the paint contains 20% (by mass) lead and was applied 20 years before the lead content to the wood was measured, estimate the diffusivity of the lead into the wood if it is found that the paint has lost 50% of the original amount of the lead. Assume that the lead did not diffuse into the air. Wood may be approximated as cellulose, C6 H10 O5, with molecular weight of 116, lead has molecular weight of 207, and the paint density may be taken as 1020 kg/m^3 when new.
What will be an ideal response?
Using the information estimate the ambient air temperature that could cause frostbite on a calm day on the ski slopes.
GIVEN
• Skier’s skin of thickness (L) = 3 mm = 0.003 m exposed to cold air
• Inner surface temperature of skin (Tsi) = 35°C
• Thermal conductivity of skin (k) = 0.35 W/(m K)
• Convective heat transfer coefficient in still air ( ch ) = 20 W/(m2 K)
• Frostbite occurs at an outer skin surface temperature (Tso) = 10°C FIND
• The ambient air temperature (T?) that could cause frostbite ASSUMPTIONS
• Steady state conditions prevail
• One dimensional conduction occurs through the skin
• Radiative loss (or gain from sunshine) is negligible
SKETCH
