practice_exam_1 - Midterm Exam PHYS 1220 Sum09(your name 8...

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Midterm Exam PHYS 1220 Sum09 6/26/2009 __________ (your name) 8 problems You can skip one of the 5 thermodynamics problems. If you work all of the thermodynamic problems, the best 4 grades will count toward the exam grade. You have to work all E&M problems. Exam Tips: Ask questions as early as possible. Full credit only, if the answers are legible, not ambiguous, and the logic is easy to follow and all major steps in a calculation are shown. All multiple choice questions may have more than one correct answer. In order to get full credit, mark all correct answers. You may add short comments to each of your multiple choice answers. In case the answer is wrong, I will give partial credit based on your comment. Make use of the whole exam time. When done solving the problems look for omissions and mistakes.
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1- A long copper rod, 1[m] long and of rectangular cross sectional area (side a has length 4[cm] and side b has length 2[cm]), is insulated to the sides and its ends stick in two water baths. One water bath is at 80°C and the rod is at 40°C when thermal equilibrium has been reached. a) What is the rate of heat flow, dQ/dt, into the second water bath, if its temperature is 20°C? Hint: Use k Cu = 400[W/mK] b) A lightbulb filament is cylindrical. Its radius is [cm] and its length is 2[cm]. How much does the thermal heat radiation of the bulb change when the filament temperature rises from 1000°K to 1200°K, while the temperature of the environment changes from 300°K to 400°K? (Assume that the temperature of the environment rises because of the heat radiation of the filament – the bulb is mounted in a small enclosure.) Hint: Assume that the filament is a perfect blackbody. Leave the value of the Stefan-Boltzmann constant as σ in the equation.
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2- An insulated beaker is kept at temperature 30[°C]. It holds 0.4[kg] of hot water, which is initially at 60[°C]. How many kilograms of ice at -20[°C] must be dropped into the tea to make ‘ice tea’ at 10[°C]? Hint: Use c wat = 4200 []/kgK] c ice = 2100 [J/kgK] L wat-ice = 3 10 3 [J/kg] Assume that the water will first come to an equilibrium with the beaker and then with the ice.
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3- A flask contains H 2 O vapor at 120°C.
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