THERMAL CONDUCTION. placed in a different position. Can you explain why the matches go out?

Transcription

1 THERMAL CONDUCTION NAME(S) Pour about 250 ml of water into a 500 ml beaker, and begin heating the beaker on a hot plate. The beaker of water will be used in a later activity. Activity #1 A Parlor Trick Place a wooden match between two prongs of a metal fork as shown here. Light the match (by using another, already lit match) and see how long the match held by the fork will burn Did the match remain lit until all the wood was burned? Try several different matches, one at a time, each one placed in a different position. Can you explain why the matches go out? Heat Flow by Conduction You have already investigated two ways in which thermal energy can flow from one place to another. In Unit 1 you saw that if a fluid was unevenly heated, then convection currents were created in the fluid. Convection currents provide a way for thermal energy to move from one part of a fluid to another as the fluid itself begins to move. Light is another way in which thermal energy can move from one location to another. This process is called radiation. A simple example of thermal energy transfer via radiation occurs when you turn on a light bulb such as the ones you used during the activities on electricity. The electric current caused the filament in the bulb to heat up. As the filament became very hot, it began to glow and give off light (i.e. radiation). This light, in turn, can warm up objects as the objects absorb the light energy and turn it back into thermal energy. The heating of the earth by the sun is an example of thermal energy transfer via radiation. On the sun, matter is converted into radiation (light) via nuclear reactions as very hot hydrogen atoms are fused together to form helium atoms. The resulting radiation travels across space until it is absorbed by another object (in this case the earth) and is converted into thermal energy. A third way in which thermal energy can by transferred is via conduction. Conduction occurs via direct contact between heated particles. The reason the match in Activity #1 kept going out was that the metal particles making up the fork were conducting heat away from the match so quickly that the temperature of the match became too low to support the burning. In this process, the relatively hot particles making up the match transferred their thermal energy to the fork particles as the particles collided with each other. As the fork particles gained energy from the match particles, the temperature of the fork increased and the temperature of the match decreased as a consequence of the conservation of energy. Since metal is very good at transferring thermal energy throughout UNIT 4 ENERGY AND INTERACTIONS IV - 53 CALVIN COLLEGE

2 its entire volume, the match continued to decrease in temperature until the flame went out. A demonstration of the conduction of thermal energy in a metal is provided in the next activity. Activity #2 Thermal Conductivity in Different Materials In Unit 2, Section 7 (Temperature) you moved about the room feeling objects with your skin, and you attempted to rank the objects according to which felt hottest and coldest. If you did not do this activity, take a few minutes now to feel at least six different (non-living) objects in the room. Place your cheek against the objects, because cheeks are particularly sensitive to temperature variations. (Do not attempt to feel the beaker which is being heated, or the area immediately around the hot plate, as the temperatures in this area have been artificially increased.) 1. Do all of the objects in the room seem to feel the same temperature? As part of the activity on Temperature, you actually measured the temperature of the various objects in the room. (If you did not do this, then take a few minutes to measure the temperatures of the objects you just felt.) 2. Are all of the objects in the room about the same temperature? 3. Are the objects in the room warmer or cooler than normal skin temperature? (If you do not know, take the temperature of your skin.) 4. Based on the temperature of your skin and the temperatures of the objects in the room, would you say that the objects in the room tend to remove thermal energy from you when you touch them, or do they tend to transfer thermal energy to you when they are touched? 5. Which types of objects in the room (i.e., consider the kinds of materials they are made from) conduct thermal energy away from you the fastest? The slowest? UNIT 4 ENERGY AND INTERACTIONS IV - 54 CALVIN COLLEGE

3 Place a metal and a wooden spoon of about the same size into the beaker of water which you have been heating. (If spoons are not available, any two metal and wooden objects will work if they are of about the same size and shape.) 6. Feel the handles of the spoons over a period of time, and determine which of them conducts thermal energy better. You should also press a thermometer against the handles to measure the temperatures to be sure that you are not fooled by what you feel. What are your conclusions? Are they consistent with what you discovered when you felt the temperatures of the objects in the room? The ability of a substance to conduct thermal energy depends on the particles making up the substance and how those particles interact with each other. In general, solids conduct thermal energy better than liquids, which in turn conduct better than gases. Air, therefore, is a very poor conductor of thermal energy. Since air also does not absorb light very well, the main mechanism for thermal energy transfer in air is via convection currents. This explains why warm air and cold air masses do not mix well and come to equilibrium. Instead, the warm air tends to rise up over the cold air, often causing rain. Since water also does not conduct thermal energy nor absorb light well, most thermal mixing in water occurs through convection as well. The water at the bottom of the oceans and deep lakes remains a constant 4 C because it is at this temperature that water is the densest. This cold water will not rise up to mix with warmer water due to its high density, and it is too far below the surface to absorb light energy. The warmer water above does not conduct thermal energy well enough to cause the temperature of the lower water to rise. Although solids usually conduct thermal energy better than liquids and gases, variation in the ability of different solids to conduct thermal energy is enormous. In general, one finds that solids which are good electrical conductors are also good thermal conductors. Thus, metal is found to be a good thermal conductor, but wood and plastic are not. The very best thermal insulators are those solids which do not allow thermal transfer via conduction or radiation. (Of course convection can not occur in a solid because a solid can not flow.) One way to make a good thermal insulator is to find a solid which reflects light well (to inhibit thermal transfer via radiation) and which contains trapped pockets of air within the solid (since air conducts so poorly). An example of this would by white styrofoam, which contains enclosed pores filled with air. A good thermal insulator is equally good at keeping hot substances hot and cold substances cold. That is because in either case the insulator is inhibiting the flow of thermal energy either into UNIT 4 ENERGY AND INTERACTIONS IV - 55 CALVIN COLLEGE

4 or out of the substance. In the days before refrigeration, the pioneers used saw dust in order to store ice throughout the summer. They would cut large blocks of ice from frozen lakes, and then pack these blocks a few inches apart in wooden sheds. The space between and above the blocks was filled with sawdust, and then additional layers of ice and sawdust were added. The ice would remain frozen for their use throughout the entire summer. UNIT 4 ENERGY AND INTERACTIONS IV - 56 CALVIN COLLEGE

5 Conduction Homework 1. Can thermal conduction occur in outer space (a vacuum) between two objects that are not touching? 2. A few days after a snowfall, it is common to see that some house roofs are still covered with snow while others have lost most of their snow. a. Which of these conditions indicates a roof which is properly insulated? b. Which will keep the inside of a house better insulated: a roof covered with snow, or a roof bare of snow? 3. Consider what it feels like to be immersed in air at a temperature of 55 F, or to be immersed in water at a temperature of 55 F. Which would feel colder to you? Why? 4. Elementary school students enjoy having contests to see who can keep an ice cube from melting over the longest period of time. What are some strategies you might use to win such a contest? 5. A thermos is an amazing thing. You can fill it with ice water, and the water will remain cold for many hours. Or, you can fill it with soup, and the soup will stay hot for hours. How does a thermos know whether it is supposed to keep something hot or cold? Why does it never get mixed up? 6. There is a new device which has been marketed recently. It allows you to defrost meat very quickly, and yet it uses no energy. You simply place the meat on top of the device, and the meat really does defrost much quicker than usual. How does the device work? (Hint: the device contains no moving parts and consumes no energy. It is simply a large block of aluminum.) 7. Fred wishes to tell his wife that he loves her, but she is on the far side of a very crowded room. Fred considers three different ways he might get his message to his wife: i. He can yell to her from across the room ii. He can fight his way through the crowd until he reaches her side, and then he can whisper in her ear. iii. He can give his message to the person he is closest to, who in turn will tell another person, who will tell another person, and so on until the message reaches his wife. Disregarding questions of proper etiquette at this point, consider Fred s three methods of speaking to his wife, and explain which method most closely resembles the transfer of thermal energy via conduction, which is most like convection, and which is most like the transfer of thermal energy via radiation. 8. Why does a thick walled container insulate better against thermal conduction than a thin walled container? 9. Pewter is an alloy of tin and lead, and many people considered it to be quite attractive. Fancy containers are often made from pewter, but rarely do you see coffee or tea cups made from this material. Why do you think that is? UNIT 4 ENERGY AND INTERACTIONS IV - 57 CALVIN COLLEGE

6 10. If you wish to bring home a gallon of ice cream from the store in the summer without it melting, would it be better to wrap the ice cream in your winter coat or just to leave it in the plastic bag from the store? Explain your answer. 11. a. Does wrapping a potato in aluminum foil increase or decrease the baking time needed? Explain why. b. Some people attempt to make potatoes bake more quickly by inserting nails into the potatoes while they are in the oven. Explain how this works. UNIT 4 ENERGY AND INTERACTIONS IV - 58 CALVIN COLLEGE