exam1-thermo-b

# exam1-thermo-b - Problems(1 pt each You can choose 6 out of...

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Problems: (1 pt. each. You can choose 6 out of 7, or lowest score will be dropped if you complete all of them). USE SI UNITS TO EXPRESS YOUR RESULTS 1) To ensure a perfectly tight fit, aluminum bolts used in airplane construction are made thicker than the holes, and they are cooled down with dry ice before introducing them. If the diameter of the hole is 20mm: what is the diameter of the bolt at 20 ° C, if the two diameters are the same when the bolt is cooled down to -78 ° C (the temperature of the dry ice)? α =24 x 10 -6 ° C -1 (We consider that the diameter of the hole does not change with temperature) 2) An air cooled motorcycle engine loses significant amount of heat through thermal radiation according to the Stefan-Boltzmann equation. Assume that the ambient temperature is 27 ° C. Suppose the engine generates 15hp (11kW) of power and, due to several deep surface fins, has a surface area of A=0.50m 2 . A shiny engine has an emissivity e=0.050, whereas an engine that is painted black has emissivity e=0.95. Determine the equilibrium temperatures for the shiny engine and the black engine. (Assume that radiation is the only mode by which heat is dissipated from the engine). Stefan-Boltzmann constant σ = 5.67x10 -8 W/ m 2 K 4 3) Consider a calorimeter with mass m=100g and specific heat 0.21cal/g ° C, containing 1kg of water at 75 ° C. We drop a mass of iron (C=0.113 cal/g ° C) at 180 ° C. Calculate the mass of iron needed to vaporize 10g of water. (C for water = 1 cal/g

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