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Unformatted text preview: 20 CHAPTER OUTLINE 20.1 Heat and Internal Energy 20.2 Specific 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 Heat and the First Law of Thermodynamics 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 The T is twice as great in the ethyl alcohol. Q20.3 The final equilibrium temperature will show no significant increase over the initial temperature of the water. Q20.4 Some water may boil away. You would have to very precisely measure how much, and very quickly measure the temperature of the steam; it is not necessarily 100 C. Q20.5 The fingers are wetted to create a layer of steam between the fingers and the molten lead. The steam acts as an insulator and can prevent or delay serious burns. The molten lead demonstration is dangerous, and we do not recommend it. Q20.6 Heat is energy being transferred, not energy contained in an object. Further, a large-mass object, or an object made of a material with high specific heat, can contain more internal energy than a higher- temperature object. Q20.7 There are three properties to consider here: thermal conductivity, specific 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 fingers. 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 specific heat compared to aluminum, can be a significant 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! Q20.8 Write 1 000 1 1 3 1 000 1 kg 4186 J kg C C kg m J kg C C 3 = b g a f e j b g a f V . to find V = 3 2 10 3 3 . m . 575 576 Heat and the First Law of Thermodynamics Q20.9 The large amount of energy stored in concrete during the day as the sun falls on it is released at night, resulting in an higher average evening temperature than the countryside. The cool air in the surrounding countryside exerts a buoyant force on the warmer air in the city, pushing it upward and moving into the city in the process. Thus, evening breezes tend to blow from country to city....
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