ch13 - CHAPTER 13 THE TRANSFER OF HEAT CONCEPTUAL QUESTIONS...

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CHAPTER 13 THE TRANSFER OF HEAT CONCEPTUAL QUESTIONS ______________________________________________________________________________________ ______ 2 . REASONING AND SOLUTION A heavy drape, hung close to a cold window, reduces heat loss through the window by interfering with the process of convection. Without the drape, convection currents bring the warm air of the room into contact with the cold window. With the drape, convection currents are less prominent, and less room air is circulated directly past the cold surface of the window. ______________________________________________________________________________________ ______ 5 . REASONING AND SOLUTION A piece of Styrofoam and a piece of wood are sandwiched together to form a layered slab. The two pieces have the same thickness and cross-sectional area. The exposed surfaces have constant temperatures. The temperature of the exposed Styrofoam surface is greater than the temperature of the exposed wood surface. The rate of heat flow through either layer can be determined from Equation 13.1: Q / t = kA T / L , where k is the thermal conductivity of the layer, A and L are the cross-sectional area and thickness of the layer, respectively, and T is the temperature difference between the ends of the layer. Since heat is not trapped within the sandwich, the rate at which heat flows through the sandwich, Q / t , must be uniform throughout both layers. Therefore, ( kA T / L ) Styrofoam = ( kA T / L ) wood . Since both layers have the same cross- sectional area and thickness, A and L are the same for both layers. Therefore, k Styrofoam T Styrofoam = k wood T wood . From Table 13.1, we see that the thermal conductivity of Styrofoam is less than the thermal conductivity of wood; therefore, the temperature difference between the two ends of the wood layer must be smaller than the temperature difference between the two ends of the Styrofoam layer. From this, we can conclude that the temperature at the Styrofoam-wood interface must be closer to the lower temperature of the exposed wood surface. ______________________________________________________________________________________ ______ 9
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ch13 - CHAPTER 13 THE TRANSFER OF HEAT CONCEPTUAL QUESTIONS...

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