Week 15

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E.20 The wine's thermal energy is providing the latent heat of melting necessary to transform ice into water. 21. Why is it that you can put a metal pot filled with ...
Homework 15 11. Why are black steam radiators better at heating a room than radiators that have been painted white or silver? E.11 Black radiators are very efficient at transferring heat via radiation. White or silver radiators radiate heat less well and aren't as effective at heating their surroundings. 12. The space shuttle generates thermal energy during its operation in earth orbit. How is it able to get rid of that thermal energy as heat in an airless environment? E.12 The space shuttle radiates its excess heat away into cold, dark space. 13. Some air fresheners are solid materials that have strong odors. If you leave these air fresheners out, they slowly disappear. What happens to the solid material? E.13 The solid material sublimes and its molecules become gaseous. 14. Frozen vegetables will “freeze-dry” if they’re left in cold, dry air. How can water molecules leave the frozen vegetables? E.14 The water molecules sublime from the frozen vegetables, going directly from the solid phase to the gaseous phase. 15. If you remove ice cubes from the freezer with wet hands, the cubes often freeze to your fingers. How can the ice freeze the water on your hands? Shouldn’t they melt instead? E.15 The ice is colder than its melting temperature and must warm up before it starts melting. 16. On a bitter cold day, the snow is light and powdery. This snow doesn’t begin melting immediately when you bring it into a warm room. Why? E.16 The cold snow is well below its melting temperature and it must absorb a moderate amount of heat in order to warms up to that melting temperature. 17. When you put a loaf of bread in a plastic bag, there is still air around the bread. Why doesn’t the bread dry out as quickly in the bag as it does when there’s no bag around it? E.17 The bag traps steam so that the humidity soon rises to 100% so that net evaporation ceases. 18. Even on a very humid day the hot air from your blow dryer can extract moisture from your hair. Why is heated air able to dry your hair when the air around you can’t? E.18 Heating the air lowers its relative humidity. Increasing the temperature stabilizes the gaseous phase of water relative to the liquid phase. 19. If you add a little hot tea to ice water at 0 °C, the mixture will end up at 0 °C so long as some ice remains. Where does the tea’s extra thermal energy go? E.19 It goes into the ice’s latent heat of melting. 20. If you put a warm bottle of wine in a container of ice water, the wine will cool but the ice water won’t become warmer. Where is the wine’s thermal energy going? E.20 The wine’s thermal energy is providing the latent heat of melting necessary to transform ice into water. 21. Why is it that you can put a metal pot filled with water on a red-hot electric stove burner without fear of damaging the pot? E.21 The boiling water will remove heat quickly enough to protect the pot. 22. Why does a pot of water heat up and begin boiling more quickly if you cover it? E.22 Trapping the moisture allows the relative humidity to reach 100% so that net evaporation ceases. 23. Putting a clear plastic sheet over a swimming pool helps keep the water warm during dry weather. Explain. E.23 It prevents evaporation from cooling the water. 24. If you try to cook vegetables in 100 °C air, it takes a long time. But if you cook those same vegetables in 100 °C steam, they cook quickly. Why does the steam transfer so much more heat to the vegetables? E.24 The condensing steam releases its latent heat of evaporation and that enormous heat cooks the vegetables quickly. 25. Why does it take longer to cook pasta properly in boiling water in Denver (the “mile high city”) than it does in New York City? E.25 Water boils at a lower temperature in Denver.

26. You’re floating outside the space station when your fellow astronaut tosses you a cold bottle of mineral water. It’s outside your space suit but you open it anyway. It immediately begins to boil. Why? E.26 With no atmospheric pressure to crush them, any bubbles of steam that form in the cold water grow by evaporation and expand. The water boils even though it’s cold. 27. The molecules of antifreeze dissolve easily in water. Why does adding antifreeze to the water in a car radiator keep the water from freezing in the winter and boiling in the summer? E.27 Anything dissolved in water stabilizes the water so that it has trouble freezing or evaporating. 28. When the outside temperature is –2 °C (29 °F), you can melt the ice on your front walk by sprinkling salt on it. But if you are out of salt, will baking soda do? E.28 Yes, anything that dissolves in water will do. 29. A quality perfume is a mixture of essential oils and has a scent that changes with time and skin temperature. Explain the gradual change of the perfume’s scent with time in terms of evaporation. E.29 Different chemicals leave the liquid perfume at different rates, so some evaporate away earlier than others. Higher skin temperature promotes evaporation, even in the slower leaving chemicals. 30. An icy sidewalk gradually loses its ice even when the weather stays cold. Where does the ice go? E.30 It sublimes and the resulting water vapor is carried off by the air. 31. How would you estimate the temperature of a glowing coal in a fireplace? E.31 From the color of its light; whiter is hotter. 32. Doctors use an infrared camera to look for inflammation, which appears as a hot patch on otherwise cooler skin. What differences would the camera observe at this hot patch? E.32 The warmer skin would emit brighter, shorter wavelength infrared light. 33. The strongest evidence for the Big Bang theory of the origin of the universe is the thermal radiation emitted by that explosion. This radiation has cooled over the years to only 3 K and is now mostly microwaves. Why should 3 K thermal radiation be microwaves? E.33 The cooler the object, the longer the wavelengths of its blackbody spectrum. The spectrum of a very cold object peaks in the microwave portion of the electromagnetic spectrum. 34. You have a roll of papered foil that is shiny on one side and black on the other. You wrap a hot potato in that foil. Which side should be facing outward to keep the potato hot longest? E.34 The shiny side should face outward.

Additional questions: 1. Describe the heat transfer mechanisms of convection. a. Convection occurs when a moving fluid transports heat from a hotter object to a colder object. 2. A resting person generates about 100 Watts of thermal power. Explain where the energy comes from to produce this power. a. From the chemical potential energy stored in the foods we eat. 3. When the temperature is hotter than 98.6° F and you are outside, which direction does heat flow? a. From the air into your body. 4. How does a coat limit heat loss from your body with respect to conduction? a. A coat adds a layer of material that reduces the overall conductivity from your body to the air. It also increases the distance between, which reduces conduction also. 5. How does a coat limit heat loss from your body with respect to convection? a. A coat traps a layer of air between it and your body. This layer of air would normally move away by convection if the coat was not present. It would be replaced by colder air that would have to be heated. Coats prevent the layer of air from moving away.

6. What factors increase heat flow due to conduction? a. The conductivity of the material, great temperature difference, greater surface area, and a small distance between objects. 7. Why do we feel colder on a windy day even if the temperature is the same as a still day? a. We lose heat not only to conduction, but on a windy day convection takes away more heat from our body. 8. Why is fiberglass insulation such a good insulator? a. In the walls of a building, it prevents the air from moving and carrying away heat. The long fibers trap the air.