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Unformatted text preview: 20 Heat and the First Law of Thermodynamics CHAPTER OUTLINE 20.1 Heat and Internal Energy 20.2 Specifi c Heat and Calorimetry 20.3 Latent Heat 20.4 Work and Heat in Thermodynamic Processes 20.5 The First Law of Thermodynamics 20.6 Some Applications of the First Law of Thermodynamics 20.7 Energy Transfer Mechanisms ANSWERS TO QUESTIONS Q20.1 Temperature is a measure of molecular motion. Heat is energy in the process of being transferred between objects by random molecular collisions. Internal energy is an objects energy of random molecular motion and molecular interaction. *Q20.2 With a specifi c heat half as large, the T is twice as great in the ethyl alcohol. Answer (c). Q20.3 Heat is energy being transferred, not energy contained in an object. Further, a low-temperature object with large mass, or an object made of a material with high specifi c heat, can contain more internal energy than a higher-temperature object. *Q20.4 We think of the product mc T in each case, with c = 1 for water and about 0.5 for beryllium. For (a) we have 1 1 6 = 6. For (b), 2 1 3 = 6. For (c), 2 1 3 = 6. For (d), 2(0.5)3 = 3. For (e), a large quantity of energy input is required to melt the ice. Then we have e > a = b = c > d. Q20.5 There are three properties to consider here: thermal conductivity, specifi c heat, and mass. With dry aluminum, the thermal conductivity of aluminum is much greater than that of (dry) skin. This means that the internal energy in the aluminum can more readily be transferred to the atmosphere than to your fi ngers. In essence, your skin acts as a thermal insulator to some degree (pun intended). If the aluminum is wet, it can wet the outer layer of your skin to make it into a good conductor of heat; then more internal energy from the aluminum can get into you. Further, the water itself, with additional mass and with a relatively large specifi c heat compared to aluminum, can be a signifi cant source of extra energy to burn you. In practical terms, when you let go of a hot, dry piece of aluminum foil, the heat transfer immediately ends. When you let go of a hot and wet piece of aluminum foil, the hot water sticks to your skin, continuing the heat transfer, and resulting in more energy transfer to you! Q20.6 Write 1 000 1 1 3 1 000 kg 4 186 J kg C C kg m J 3 ( )( ) = ( ) V . kg C C ( )( ) 1 to fi nd V = 3 2 10 3 3 . m . *Q20.7 Answer (a). Do a few trials with water at different original temperatures and choose the one where room temperature is halfway between the original and the fi nal temperature of the water. Then you can reasonably assume that the contents of the calorimeter gained and lost equal quantities of heat to the surroundings, for net transfer zero. James Joule did it like this in his basement in London....
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