Sample HW12 - PHYS 21800 This print-out should have 7 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. Serway CP 11 19 001 10.0 points A student drops two metallic objects into a 20 g steel container holding 298 g of water at 20 ◦ C. One object is a 415 g cube of copper that is initially at 81 ◦ C, and the other is a chunk of aluminum that is initially at 5 ◦ C. To the student’s surprise, the water reaches a final temperature of 20◦ C, precisely where it started. What was the mass of the aluminum chunk? Assume the specific heat of aluminum is 900 J/kg ·◦ C and of copper 387 J/kg ·◦ C. Serway CP 11 27 002 10.0 points A 20 g block of ice is cooled to −64◦ C. It is added to 558 g of water in an 100 g copper calorimeter at a temperature of 21◦ C. Find the final temperature. The specific heat of copper is 387 J/kg · ◦ C and of ice is 2090 J/kg · ◦ C . The latent heat of fusion of water is 3.33 × 105 J/kg and its specific heat is 4186 J/kg · ◦ C . Serway CP 11 48 003 10.0 points A 1.13 m2 solar collector collects radiation from the Sun and focuses it on 320 g of water that is initially at 22.1 ◦ C. The average intensity of radiation arriving from the Sun at Earth’s surface at this location is 558 W/m2 , and we assume that this is collected with 100% efficiency. Find the time required for the collector to raise the temperature of the water to 100◦ C. The heat capacity of water is 4186 J/kg ·◦ C . Correct answer: 2.75818 min. 1 Serway CP 11 36 004 10.0 points A box with a total surface area of 1.52 m2 and a wall thickness of 2.37 cm is made of an insulating material. A 8.3 W electric heater inside the box maintains the inside temperature at 20.3◦ C above the outside temperature. Find the thermal conductivity of the insulating material. Heat Loss Through Glass 005 10.0 points A Thermopane window of area 8.9 m2 is constructed of two layers of glass, each 2.7 mm thick separated by an air space of 5 mm. If the inside is at 12 ◦ C and the outside is at −28 ◦ C, what is the heat loss through the window? The thermal conductivity of glass is 0.8 W/m ·◦ C and of air is 0.0234 W/m ·◦ C . Answer must be in kilowatts. Skin Temperature 006 10.0 points A child has a temperature of 101◦ F . If her total skin area is 76 m2 , find the energy loss per second due to radiation, assuming the emissivity is 1. Assume the room temperature is 70◦ F. The Stephan-Boltzmann constant is 5.6696 × 10−8 W/m2 · K4 . Convert deg F Correct answer: 8241.37 W. to kelvins! Skin Blackbody Emission 007 10.0 points The temperature of your skin is approximately 35◦ C. What is the wavelength at which the peak occurs in the radiation emitted from your skin? Answer must be in nanometers.

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