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Instructor: Dr. Scott A. Yost Office: 307 Nielsen Physics Building Hours: Monday, Wednesday and Friday, 1-2 PM or by appointment Textbook: L.A. Bloomfield, How Things Work, 2nd ed. |
Phone: 974-7852 E-Mail: syost@utk.edu Web Site: http://homework.phys.utk.edu/phys101 |
These are the answers to the even-numbered exercises and problems, and all of the cases assigned in Chapter 6. The answers to the others appear in the back of the textbook.
Exercises
6. Aluminum pans have a higher thermal conductivity and distribute heat more evenly via conduction.
12. The aluminum has a low emissivity and reflects the heat from the nearby element so it doesn't burn the food inside as easily.
22. The dimmer setting causes the bulb to produce less visible light and more infrared light at the same power level, so it reduces the bulb's energy efficiency.
24. The peak frequency of the blackbody spectrum is proportional to the absolute temperature.
30. When the bridge becomes warmer, thermal expansion makes it longer, so it needs room to expand, or it will buckle.
32. The refrigerator must produce a net increase in entropy. It cools the inside by making the outside hotter. On the average, leaving the refrigerator open makes the room warmer to create more entropy.
38. Compressing the air does work on the air, which it stores as thermal energy, raising the temperature.
40. The entropy of the rock decreases, but the entropy of the water increases more, so the net entropy increases.
42. Heat in the room is disordered energy, but electrical power is ordered, so converting heat in the room to electrical power decreases the entropy in the room. This is possible only if the system is not closed, so that the net entropy can be made to increase. A device can use the heat in the room if it exhausts it to a lower temperature location, but it cannot work unless the room is hotter than its surroundings. Eventually, the machine will cool the room to the temperature of its surroundins, and the machine will stop working.
48. The amount of work done by the heat engine is greater if it operates across a higher temperature difference, because more heat is available. That is why hurricanes normally form when the water is warm compared to the air higher in the atmosphere.
Problems
2. The power increases by a factor of 1.43, giving 26,900 W.
4. The power decreases by a factor of 0.2, giving 4 W.
Cases
2a. Convection causes hot water to rise and cold water to sink.
2b. Hot water should be extracted from the top, and cold water should enter the bottom because convection makes hot water rise to the top.
2c. The lower element is where the cold water is coming in, so it must heat the water more to raise its temperature. This makes it age faster.
2d. It is important to keep heat from being lost due to radiation from the outside of the hot water heater.
2e. The heat leaves as thermal energy stored in the hot water being used.3a. Both radiation and convection carry the heat to the food.
3b. Radiation carries most of the heat to the food.
3c. Aluminum foil reduces radiative heating by covering the food with a low emissivity material which does not absorb much electromagnetic energy. When the bottom element is used, this causes the food to be heated more by convection. When the top element is used, it helps to prevent the food from burning due to too much heating from a nearby burner.
3d. The fan increases convection from the burners to the food, so it makes the food receive the energy faster and cook faster with lower energy input. This also makes the food cook more evenly, since they can receive less radiative heat and more convective heat.6a. The evaporator is inside the refrigerator to make the food cold.
6b. The condenser is outside the refrigerator to release heat to the room.
6c. The food will become warm, since the refrigerator can't work if it can't release heat to the room.
6d. The temperature of the room will increase, because it releases more heat to the room than it removes from the food.
6e. More cooling is needed to reach the lower temperature. This requires more entropy to be added to the system, which takes more energy input.8a. Producing energy forever without being recharged would violate the law of energy conservation, or the first law of thermodynamics.
8b. Turning heat entirely into work decreases entropy, and violates the second law of thermodynamics.
8c. At some point, the temperature would be reduced to near absolute zero, at which time there would be no more heat to pump out.
8d. A spontaneously reassembling coffee cup would create order out of disorder spontaneously, decreasing entropy and violating the second law of thermodynamics.
Department of Physics University of Tennessee