SM_chapter20 - 20 Heat and the First Law of Thermodynamics...

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20 Heat and the First Law of Thermodynamics 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 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 object’s energy of random molecular motion and molecular interaction. Q20.2 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 find V = × 3 2 10 3 3 . m . *Q20.3 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. *Q20.4 With a specific heat half as large, the Δ T is twice as great in the ethyl alcohol. Answer (c). Q20.5 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 specific heat, can contain more internal energy than a higher-temperature object. Q20.6 If the system is isolated, no energy enters or leaves the system by heat, work, or other transfer proces- ses. Within the system energy can change from one form to another, but since energy is conserved these transformations cannot affect the total amount of energy. The total energy is constant. *Q20.7 (i) Answer (d). (ii) Answer (d). Internal energy and temperature both increase by minuscule amounts due to the work input. Q20.8 The steam locomotive engine is one perfect example of turning internal energy into mechanical energy. Liquid water is heated past the point of vaporization. Through a controlled mechanical process, the expanding water vapor is allowed to push a piston. The translational kinetic energy of the piston is usually turned into rotational kinetic energy of the drive wheel. Q20.9 The sunlight hitting the peaks warms the air immediately around them. This air, which is slightly warmer and less dense than the surrounding air, rises, as it is buoyed up by cooler air from the val- ley below. The air from the valley flows up toward the sunny peaks, creating the morning breeze. *Q20.10 Yes, wrap the blanket around the ice chest. The insulation will slow the transfer of heat from the exterior to the interior. Explain to your little sister that her winter coat helps to keep the outdoors cold to the same extent that it helps to keep her warm. If that is too advanced, promise her a really cold can of Dr. Pepper at the picnic.
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