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sm_pdf_chapter11 - Chapter 11 Energy in Thermal Processes...

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Chapter 11 Energy in Thermal Processes Quick Quizzes 1. (a) Water, glass, iron. Because it has the highest specific heat ( ) 4186 J kg C ⋅° , water has the smallest change in temperature. Glass is next ( ) 837 J kg C , and iron ( ) 448 J kg C is last. (b) Iron, glass, water. For a given temperature increase, the energy transfer by heat is proportional to the specific heat. 2. (b). The slopes are proportional to the reciprocal of the specific heat, so larger specific heat results in a smaller slope, meaning more energy to achieve a given change in temperature. 3. (c). The blanket acts as a thermal insulator, slowing the transfer of energy by heat from the air into the cube. 4. (b). The rate of energy transfer by conduction through a rod is proportional to the difference in the temperatures of the ends of the rod. When the rods are in parallel, each rod experiences the full difference in the temperatures of the two regions. If the rods are connected in series, neither rod will experience the full temperature difference between the two regions, and hence neither will conduct energy as rapidly as it did in the parallel connection. 5. (a) 4. The From Stefan’s law, the power radiated from an object at absolute temperature T is proportional to the surface area of that object. Star A has twice the radius and four times the surface area of star B. (b) 16. From Stefan’s law, the power radiated from an object having surface area A is proportional to the fourth power of the absolute temperature. Thus, () ( ) 4 44 AB B 22 1 6 Ae T AeT σ === P B P . (c) 64. When star A has both twice the radius and twice the absolute temperature of star B, the ratio of the radiated powers is ( )( ) 24 4 AA BB A 42 4 B B B 41 22 6 4 RT AeT σπ == = = P P = 401

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402 CHAPTER 11 Answers to Even Numbered Conceptual Questions 2. In winter the produce is protected from freezing. The specific heat of Earth is so high that soil freezes only to a depth of a few inches in temperate regions. Throughout the year the temperature will stay nearly constant day and night. Factors to be considered are the insulating properties of the soil, the absence of a path for energy to be radiated away from or to the vegetables, and the hindrance of the formation of convection currents in the small, enclosed space. 4. The high thermal capacity of the barrel of water and its high heat of fusion mean that a large amount of energy would have to leak out of the cellar before the water and produce froze solid. Evaporation of the water keeps the relative humidity high to protect foodstuffs from drying out. 6. Yes, if you know the specific heat of zinc and copper, you can determine the relative fraction of each by heating a known weight of pennies to a specific initial temperature, say 100° C, then dump them into a known quantity of water, at say 20° C. The equation for conservation of energy will be ( ) ( ) ( ) Cu Zn 1 c 100 C 20 C pennies water water mx c x T m cT ⋅+− ° −= °   The equilibrium temperature, T , and the masses will be measured. The specific heats are known, so the fraction of metal that is copper, x , can be computed.
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sm_pdf_chapter11 - Chapter 11 Energy in Thermal Processes...

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