heat transfer
Content
NSC 109 Group work
14 March 2007 Answer each of the following questions. You may work together on them if you choose, or consult any reference or tutor. Your written answers are due on Friday, March 16. This worksheet is worth 30 points. The three ways of transferring heat are, in order of efficiency from highest to lowest, conduction, convection and radiation. Use these concepts (and others) to explain each of the following: 1. A vacuum (as you have inside the walls of a thermos bottle) is the best insulator. In a vacuum, there is no matter to carry heat by conduction; nor is there matter that can mix to carry heat by convection. The only remaining means of heat transfer is radiation, which is the least efficient. 2. Air is an excellent insulator, but we can heat buildings with “forced air” in which hot air is blown through ducts. Hot air, introduced to a room and forced to mix with the room air, will rapidly warm the room as it mixes with lower-temperature air and the average air temperature becomes higher than the initial air temperature. This is convection, of a sort. On the other hand, air that cannot move is a good insulator because its molecules are not in contact and cannot easily transfer heat by conduction. 3. Wood is a better insulator than glass. Yet fiberglass batting is commonly used as an insulator in wooden buildings. Why? Fiberglass batting contains a great deal of trapped air. Since trapped air is an excellent insulator, fiberglass batting can be used for efficient insulation. Wood is a better insulator than solid glass because wood, too, contains “empty” air spaces; it is porous. 4. If you turn on an incandescent lamp, you can immediately feel heat from the bulb. However, if you quickly turn the lamp off again, the bulb is not hot to the touch. Explain why you feel heat immediately, but the bulb itself does not immediately get hot. The incandescent lamp gives off energy (including heat energy) by radiation, which travels at the speed of light; so you immediately feel heat when it is turned on. But the bulb itself is mostly made of insulators: it is a glass bulb filled with gas. Insulators don’t warm up very fast, so the bulb itself takes time to get hot. 5. A cube of metal and an equally massive cube of wood are removed from a hot oven at equal temperatures. a. The wood can be (briefly!) safely handled with bare hands, but you need a pair of tongs for the metal. Explain. Wood, being a poor conductor of heat, doesn’t lose heat (to your hands or anything else) as quickly as the metal. b. The metal and wood cubes are dropped onto blocks of ice. The wood melts more ice before coming to 0ºC than the metal. Explain. The amount of ice melted is a function of how much total heat is contained in the block of wood vs. the block of metal (which have the same mass, remember!) This is called “heat capacity,” and it appears that the heat capacity of the wood is greater than that of the metal.
14 March 2007 Answer each of the following questions. You may work together on them if you choose, or consult any reference or tutor. Your written answers are due on Friday, March 16. This worksheet is worth 30 points. The three ways of transferring heat are, in order of efficiency from highest to lowest, conduction, convection and radiation. Use these concepts (and others) to explain each of the following: 1. A vacuum (as you have inside the walls of a thermos bottle) is the best insulator. In a vacuum, there is no matter to carry heat by conduction; nor is there matter that can mix to carry heat by convection. The only remaining means of heat transfer is radiation, which is the least efficient. 2. Air is an excellent insulator, but we can heat buildings with “forced air” in which hot air is blown through ducts. Hot air, introduced to a room and forced to mix with the room air, will rapidly warm the room as it mixes with lower-temperature air and the average air temperature becomes higher than the initial air temperature. This is convection, of a sort. On the other hand, air that cannot move is a good insulator because its molecules are not in contact and cannot easily transfer heat by conduction. 3. Wood is a better insulator than glass. Yet fiberglass batting is commonly used as an insulator in wooden buildings. Why? Fiberglass batting contains a great deal of trapped air. Since trapped air is an excellent insulator, fiberglass batting can be used for efficient insulation. Wood is a better insulator than solid glass because wood, too, contains “empty” air spaces; it is porous. 4. If you turn on an incandescent lamp, you can immediately feel heat from the bulb. However, if you quickly turn the lamp off again, the bulb is not hot to the touch. Explain why you feel heat immediately, but the bulb itself does not immediately get hot. The incandescent lamp gives off energy (including heat energy) by radiation, which travels at the speed of light; so you immediately feel heat when it is turned on. But the bulb itself is mostly made of insulators: it is a glass bulb filled with gas. Insulators don’t warm up very fast, so the bulb itself takes time to get hot. 5. A cube of metal and an equally massive cube of wood are removed from a hot oven at equal temperatures. a. The wood can be (briefly!) safely handled with bare hands, but you need a pair of tongs for the metal. Explain. Wood, being a poor conductor of heat, doesn’t lose heat (to your hands or anything else) as quickly as the metal. b. The metal and wood cubes are dropped onto blocks of ice. The wood melts more ice before coming to 0ºC than the metal. Explain. The amount of ice melted is a function of how much total heat is contained in the block of wood vs. the block of metal (which have the same mass, remember!) This is called “heat capacity,” and it appears that the heat capacity of the wood is greater than that of the metal.
