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Thermal Conduction: Transfer of Vibrational Energy (page 2)

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Author: Janice VanCleave

Try New Approaches

1. How does the cross-sectional area of a metal affect the heat conducted through it? Repeat the experiment using a paper clip with a larger cross-sectional area. Determine if the amount of heat conducted is proportional to or inversely proportional to the cross-sectional area of a material.
2. How does the type of metal affect heat conduction? Repeat the original experiment using different types of wire with the same cross-sectional area, thus the same gauge (a measure of standard size). Wire strippers have gauge sizes that can be used to measure the circumference of wire. Generally a small paper clip is 22-gauge and is made of steel. Use a piece of wire of equal gauge and length but made of different material, such as copper. Be sure to strip away all of the insulation from the wire. Prepare two clay holders and place them on opposite sides of the candle. Position both so they are at the same height and have the same amount of metal in the flame. A larger diameter foil circle will be needed. Compare the time it takes for each margarine sample to melt. For information about thermal conductors, see Mary Jones, Physics (New York: Cambridge University Press, 1997), pp.82-83.

1. Does length of a wire affect heat conduction? Design an experiment to show the effect of length on the thermal conductivity of a conductor. One way is to repeat step 2 in ''Try New Approaches," using two wires of the same gauge but different lengths. When the first sample of margarine shows signs of melting, blowout the candle. Continue to observe the remaining samples of margarine. Use the results to determine if the same amount of heat is transferred from one end of the wire to the other.
2. How is temperature difference between two materials related to heat conduction between them? Design a way to determine if a larger temperature difference causes more or less heat to flow from one material to another. One way is to fill two Styrofoam cups one-fourth full with water. Put hot tap water in cup A and cold tap water in cup B. Measure the temperatures of the hot and the cold water. Place two or more metal washers in the hot water. After 3 minutes, use a spoon to remove the washers from the hot water and place them in cup B, the cup of cold water. At the end of 1 minute, stir the water in cup B, then measure the temperature of the water in the cup. Note the difference between the original temperature of the cold water and the temperature after the warm washers had been in the water for 1 minute. Repeat twice, first using warm water in cup B made warm by mixing equal amounts of hot and cold tap water. Then use water in cup B made cold by adding one or more ice cubes to the water. (Allow the ice to remain in the water for 2 minutes, stir, then remove the ice before adding the washers.) For more information about a material's temperature gradient (the temperature change with distance along a material), see Corine Stockley, Illustrated Dictionary of Physics (London: Usborne, 2000), p. 28.
3. Insulators are materials with a low concentration of free electrons, and thermal insulators are poor conductors of heat. These materials, such as glass, paper, and Styrofoam, depend on the interaction of vibrating atoms and molecules to transfer heat, which is a less efficient method than by movement of free electrons in conductors. Thus less heat is conducted by thermal insulators. Design an experiment to compare the insulating properties of materials used to hold hot liquids. One way is to use cups each made of different insulating materials. Add equal amounts of hot water to each cup. Place a thermometer in each container. Measure the temperature of the water in each container every 2 minutes for 20 minutes or until no further temperature changes occur. Record the temperature in a Temperature/ Insulation Data table like Table 21.1. Use the data to prepare a graph, and record the information for each material on the graph, using a different-color ink for each.

Get the Facts

1. Specific heat of a substance is the amount of energy that must be added to raise the temperature of a unit mass one temperature unit. When 1 calorie of heat is added to 1 gram of water, the water's temperature rises 1° C. What are the specific heats of other materials, such as aluminum? How can specific heat be used to determine the amount of heat transferred through a metal? For information, see "specific heat" in a physics text.
2. Temperature-sensitive nerve endings in your skin can detect differences between the temperature inside and outside your body. You sense an object as feeling cold when heat is transferred from your body to that object. Why do some things that are the same temperature but made of different materials, such as a carpet and a tile floor, feel as if they have different temperatures? For information, see Annabel Craig, Science Encyclopedia (London: Usborne, 1988), p. 14.
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