Act5_sol - 1/16/05 Activity 5 Solutions: Thermal Energy,...

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1/16/05 1 Activity 5 Solutions: Thermal Energy, the Microscopic Picture 5.1 How Is Temperature Related to Molecular Motion? 1) Temperature Your instructor will discuss molecular motion and temperature. a) At a particular temperature, do all of the molecules move at the same speed? No, there is a range of speeds. b) How does the average speed of molecules at a higher temperature differ from their average speed at a lower temperature? On average, molecules at a higher temperature have a faster speed than molecules at a lower temperature. c) Watch the demonstration of diffusion of food coloring in beakers of warm and cold water. Explain the differences in the diffusion rates. Food coloring spreads (or diffuses) more quickly in warm water. This demonstrates that the warm water molecules, on average, are moving faster than cold water molecules. 2) Evaporative Cooling What happens to the temperature of a liquid as it evaporates? Your instructor will demonstrate a cool tube (a cryophorus tube) that contains water. a) What happens when one end of the tube is cooled with liquid nitrogen? Water in the cool tube’s bulb freezes. b) The middle of the tube remains near room temperature. Explain why the water in the tube’s bulb freezes. This shows that a liquid cools as it evaporates. The faster moving water molecules leave the water surface in the upper bulb and form water vapor in the tube. The slower moving molecules remain in the liquid in the upper bulb. Thus, the average speed of the liquid molecules is reduced and the remaining liquid is cooler. The faster molecules in the upper bulb that have evaporated move throughout the cool tube. Those molecules in the end of the tube cooled by liquid nitrogen condense and freeze there and are trapped. The faster molecules continue to evaporate and leave the upper bulb, until the remaining molecules in the bulb have so little energy that the liquid in the bulb freezes. c) Group Discussion Question: When you put a pot of water on the stove to heat to boiling, why does the water come to a boil faster if you put a lid on the pot?
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1/16/05 2 3) The Dippy Duck The dippy duck contains liquid freon, which evaporates easily at room temperature. Wet the head of the duck and place the cup of water in front of the duck’s head. Explain what happens to the dippy duck in terms of evaporative cooling and the center of mass of the duck. Water evaporates from the duck’s head, cooling it. Cooling the freon gas inside the duck’s head causes the gas molecules to condense. This reduces the volume of the gas and liquid freon flows up the tube from the duck’s tail to fill the space previously taken up by the gas. As the freon moves up, the center of mass is raised until the duck topples over into the cup of water. While the duck’s head
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Act5_sol - 1/16/05 Activity 5 Solutions: Thermal Energy,...

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