With the help of simple, teacher-led demonstration activities, students learn the basic physics of heat transfer by means of conduction, convection, and radiation. They also learn about examples of heating and cooling devices, from stove tops to car radiators, that they encounter everyday in their homes, schools, and modes of transportation. Since in our everyday lives there are many times that we want to prevent heat transfer, students also consider ways that conduction, convection, and radiation can be reduced or prevented from occurring.

Engineers encounter problems of warming and cooling liquids in a number of situations. For soda, this usually involves maintaining a cold temperature, but the principles described here are the same. Students approach the problems presented in this activity as engineers, using heat transfer principles to accomplish a goal. heat energy conduction convection radiation In the context of heat transfer, students will be able to define the terms conduction, convection and radiation. Students will be able to describe ways people try to cause or prevent heating and cooling by conduction, convection, and radiation in everyday life. Students will be able give examples of materials that serve well for heating by conduction, convection, and radiation. The concepts involved in heat flow are essential to many topics in science and engineering, ranging from the origins of weather patterns to regulating the rate of nuclear reactions in a power plant. Knowledge of the fundamental principles of heat transfer by conduction, convection, and radiation allows us to understand many of the physical and biological processes we see around us each day, and it also helps us design technological solutions to a wide variety of problems. Use the information below in the Lesson Background and Concepts for Teachers section to introduce the fundamental principles of heat transfer. As you present this material, or immediately following it, you can let students work in teams of four to conduct their own simple demonstrations of the three types of heat transfer. For these demonstrations you will need a birthday candle poked into the bottom of an upside-down paper cup for each team. To experience conduction, have students put one end of a metal rod or spoon directly in the flame or just above it. The end of the metal will soon heat up. The heat will then spread by conduction to the fingers of the student holding it. You could even provide metal rods of various lengths and have students determine how long it takes the heat to travel out to their fingers for the different lengths. Short lengths of copper pipe (donated by a local plumber or plumbing supply store) are ideal, since copper has a very high conductivity. If the students can sit very still without talking, they should be able to see smoke rising from the candle by free convection. This is easier to see if they briefly pinch the flame out, leaving the tip of the wick glowing -- and smoking. They can also try holding an aluminum foil pie pan (obtained from the grocery store) upside down about an inch above the flame. After a few moments they should see the smoke accumulate under the pie plate and then begin to escape out around its edges, where it will curl up toward the ceiling. Be sure to distinguish between the smoke and the heat, however. The smoke allows us to see the movement of the heated air, but it is not the heat itself. (A very smoky alternative to a birthday candle is a "bug coil," but this might set off a smoke detector alarm! If there is no breeze, the bug-coil demonstration is well suited for outdoors.) To see the effects of radiation, students will need to coat the tip of a toothpick with a bit of wax as it melts and runs down the side of the candle. If they let the wax harden for a few moments, they can then move the toothpick slowly toward the flame. They must, however, approach the flame from below, and they should not let the wax tip touch the flame. Rather, when the tip gets a centimeter or two from the flame, they should see the wax begin to melt. Since the tip is not touching the flame, the wax cannot be melting due to conduction. Since the tip is below the flame, it cannot be melting due to convection. Instead, the wax melts because of the heat radiated from the flame. Once students are familiar with the three types of heat transfer, point out that we encounter heating and cooling devices everyday in our homes, workplaces, and modes of transportation. Ask your students to give examples, and see if they can explain the roles conduction, convection, and radiation play in them. The following are examples that students might come up with or ones you can share to get them started. The electronic devices in a desktop computer generate heat. You can feel the heat of the monitor rising out of the vents on its top surface. Processor units usually have small fans to help dissipate their heat by forced convection. If you remove the top of the processor unit, you should be able to see the fan at the back of the unit. Car engines generate a great deal of heat. The radiator, located just behind the grill, uses convection, conduction, and radiation to keep the engine cool. Water is circulated with a pump (forced convection) in pipes that run through the engine block, and heat from the engine is transferred by conduction to the water. The water is carried to the radiator, where it flows through much smaller pipes running past hundreds of small metal folds. Heat is transferred from the water to the metal folds, again by conduction. When the car is in motion, air moves over the surfaces of the radiator, and heat is carried away by convection. When the car is not moving, heat leaves mostly by radiation. All the folds of metal in the radiator create a great deal of surface area from which radiation can occur. Most cars also have a thermostatically controlled fan that will operate when the car is not moving but the engine is so hot that radiation alone will not be adequate for cooling. Similarly, a wood stove heats a room by conduction, convection, and radiation. The fire inside the stove heats the cast iron surfaces of the stove, which then radiate their heat to the rest of the room. Convection currents are also set up when the heated air surrounding the stove rises to the ceiling, drawing the cooler air in the room to the stove where it, too, is then heated. The air next to the wood stove got heated in the first place by conduction. An electric or steam radiator works on the same principle: something hot inside the unit (water or wires carrying electricity) heats the metal surfaces. Radiators, whether they are found in a home or a car, typically are designed to have lots of surface area. This not only lets more room air come into direct contact with the hot radiator for conduction, but the large surface area also provides more radiant surface than would otherwise be available. Insulation put into the walls and roofs of buildings reduces heat loss by convection. Being thick, fluffy stuff, air cannot move easily through all the mixed up layers of fibers it is made of. Instead, the warm air in the house is trapped on one side of the insulation, and the cold air outside the house is kept on the other side. Similarly, clothing keeps us warm primarily by reducing heat loss due to convection, because it prevents the warmth of our bodies from being carried away, especially on a windy day. But clothing also prevents heat loss due to radiation. If you go outside on a cold, clear winter night, your mammalian skin radiates heat to the colder surroundings, and especially into outer space, where it is very cold indeed. Covering your skin with clothes not only provides insulation, but it also blocks and reflects back most of the heat your body radiates. You can also ask students if they can think of examples of heating and cooling devices that occur in nature. Some examples are: Many animals lie down on cool, damp surfaces to help them lose heat by conduction. Others burrow down to where the ground is even cooler. Still others seek out cool water, another good heat conductor, to wade or swim in. A squirrel can hold its tail over its back as if it were a parasol, to block the sun's rays. Any shady area is the result of something getting in the way of the sun's light, which carries heat radiation with it. Animals that can't make their own seek out the shade of trees and plants, rocks, etc. Mammals that live in cold climates have thick, insulating fur. Many mammals that live where there are distinct seasons grow thicker fur during the winter and shed the excess insulating material when it is no longer needed during the summer. Jack rabbits living in the western desert areas have especially tall and narrow ears. They are full of blood vessels that run just beneath the skin. These blood vessels carry heat away from the interior of the rabbit's body and out to the ears, whose large surface areas can cool by radiation and convection. Elephants have exceptionally large ears that can serve as radiators as well. Elephants flap their ears frequently, presumably to add forced convection to their heat loss mechanisms. When their hives get too warm, bees use their wings to fan the hive interior, which is another example of cooling by forced convection. Conduction is easily demonstrated by handing a metal spoon that has been kept in a cool place to a student, and asking him or her to describe how it feels. The student will undoubtedly report that it feels cold. The reason it feels cold is that heat is flowing from the student's warm hand into the cooler spoon. Heat always moves from warm objects to cooler ones, and continues to do so until both objects come to equilibrium at the same temperature. So, if one student holds the spoon long enough, he or she can then hand it to another student, who will report that the spoon feels neither hot nor cold. Conduction works because molecules are always in motion. Consider water molecules, for example. Water is in a liquid state between 0° and 100° C. At room temperature, the individual molecules are constantly bouncing off each other; this motion is driven by the heat present within the system. If we raise the temperature of the water by applying more heat, we can get the molecules to bounce off each other faster and faster, until at 100° they are bouncing so energetically that they can escape the water surface in the form of steam. In this gaseous state the molecules are less densely packed together, and they have to travel further before they collide, but because they have lots of heat energy, they are still moving about very quickly. However, if the water is cooled from room temperature, the molecules move more slowly. At the point where water freezes -- and therefore becomes a solid -- the molecules have slowed so much that instead of bouncing off one another, they are only vibrating in place. (They will continue to do so, but more slowly, until the temperature falls to -273° C. At this point, known as absolute zero, molecular motion ceases altogether, at least in theory.) In heat transfer by conduction, when heat is applied to one end of a solid such as a metal spoon, the molecules comprising that metal begin to vibrate more vigorously than their unheated neighbors. In solids the molecules are very tightly packed together. Therefore, the more energetic vibrations of the heated molecules make them bump against their neighbors, causing them to vibrate more quickly as well. These, in turn, cause their adjacent molecules to vibrate more vigorously, etc., until the heat has been distributed throughout the spoon. When you pick up a metal spoon, heat from your warm hand is enough to get this process going. Some materials are better conductors than others. Metals are especially good, but glass and ceramics aren't bad either. Plastic and wood are relatively poor conductors, which is why wooden spoons are good for cooking and saucepans have plastic handles. You can demonstrate the relative conducting abilities of different materials by placing a large metal spoon in the left hand of a student, while placing a similarly-sized wooden spoon in his or her right hand. Conduction also happens in liquids, and in general, liquids are good conductors. Furthermore, conduction occurs between liquids and solids, which is how soup gets hot soon after the pot it is in does. Conduction in gases is not very efficient because the molecules are so far away from each other, but it can still happen. Heat is more typically transferred through gases, however, by convection. When gases are heated they expand, and so become less dense. The less dense parts, being lighter, rise to the top. Similarly, the cooler, more dense parts, being heavier, move downward. This is the explanation for the well-known phenomenon, "Hot air rises and cold air sinks." In a closed container, convection currents are set up as the rising gas carries heat upwards and the cooler gas is brought closer to the heat source. The currents help distribute the heat throughout the container, and the whole process is driven by density differences. In contrast to conduction, where heat is transferred from molecule to molecule, in convection heat is transferred by bulk flow. Students have probably already seen some examples of convection currents in gases. Smoke rises out of a chimney because of it, and steam rises from the spout of a tea kettle boiling water. There are also small holiday chimes that ring when a pinwheel, mounted over lit candles, turns because of convection. If your classroom has high ceilings, you can probably measure several degrees of temperature difference between the cooler air just above the floor and the warmer air just below the ceiling. Convection in liquids is perhaps less familiar, and a little harder to observe. One demonstration that is fairly effective is to fill a wide jar or beaker (having a diameter of at least 10 cm) with water and place it on a ring stand. Position a candle so that it is just beneath the jar, but is not centered under the jar. Instead the candle should be located somewhere along the circumference of the jar. Light the candle and then carefully place a drop of food coloring as close to the heat source as possible. You can do this with a disposable plastic pipette, which you can then simply leave in the jar. If you try to remove the pipette you will probably create currents in the water that will disperse the food coloring and interfere with the convection pattern you are trying to show. When we're heating soup in a pot on the stove, we don't usually just wait until conduction and convection have done their things before we start eating the soup. Instead, we speed the process up by stirring the soup as it heats. When we do, the spoon helps move the warm liquid next to the walls of the pot away from them, bringing the cooler parts of the liquid to the walls where they, too, can be heated. This is called forced convection, as opposed to the natural, or free convection described above. In forced convection, we use some mechanical device to move the liquid, thereby moving the heat, too. The third type of heat transfer, radiation, is the transfer of heat energy through space by means of both infrared and visible light waves. Although it sounds rather abstract, we are all familiar with it because it is the way energy from the sun warms our planet. Warm objects radiate more heat than they absorb, and the radiation moves outward in all directions. Since it is a form of light, this energy transfer can be interrupted by putting a solid obstacle between the source and the object to be warmed: when you sit in the shade of a tree you no longer feel the radiant warmth of the sun on your skin. Furthermore, dark surfaces can absorb radiation while light-colored surfaces reflect radiated heat. Put sheets of black and white construction paper or pieces of black and white fabric side-by-side on a sunny windowsill, and after a minute or two students can feel the differences in their temperatures. the transfer of heat by molecular motion through a solid or a liquid, from a region of high temperature to a region of lower temperature. the movement of heated molecules of a gas or a liquid from a heat source to another area, due to density differences within the gas or liquid. transfer of heat energy by waves of visible or infrared light moving through space. Hot Cans and Cold Cans After students have conducted the Associated Activity, Hot Cans and Cold Cans, have them prepare line graphs of the temperature data for each of their two cans. The temperature of the water inside the cans should be on the y-axis (since it depended on how long the cans had been cooling) and time should be on the x-axis (since time always proceeds independently of anything else). Have students graph both sets of data on one set of axes, using different symbols or colored lines for each of their cans. They should also graph the temperature data from one of the two control cans on the same set of axes. This way they will have a visual representation that not only compares how their "hot" and "cold" cans behaved, but also compares how well their warming and cooling arrangements performed compared to a can that had had nothing done to it. Then ask each team to make a brief presentation to the class, in which team members present their graphs and summarize the performances of their warming and cooling devices by calculating the temperature change of each during the 30 minutes and comparing these to the temperature changes in the control cans. As part of their presentations, team members should also show their warming and cooling devices or arrangements, and explain how they either tried to take advantage of conduction, convection, and radiation, or, how they tried to eliminate them. Students may have difficulty articulating the roles of conduction, convection, and radiation in their devices, so be prepared to help them understand these roles in their own and other team's devices. Most of the successful warming devices students design will involve insulating their cans with fabric, paper, or some other material. Insulation serves to prevent air from moving over the surfaces of the can, thereby eliminating or greatly reducing its ability to lose heat by convection. We wear sweaters, fleece, and down or fiber filled coats in the winter for the same reason. Another method students might use is to insulate their cans and run a tube from next to the can, through the insulation, and out to the mouth of a student, whose job it is to breathe warm air through the tube for the duration of the experiment. Students using this arrangement are trying to prevent heat loss by convection, while at the same time using forced convection to carry heat to the can. Other students might wrap their cans in dark fabric or paper and set them on a sunny windowsill, taking advantage of radiation. Once a group of students, who had a large, athletic male among its members, had the young man run around the outside of the building for five minutes in order to work up a sweat. Then they had him lie on a table with their can held firmly in his armpit! Their method was a very effective example of warming by conduction. As for cooling devices, students might simply place their cans on the concrete floor in a cool corner of the classroom, and take advantage of conduction (from the warm can to the cool floor) and radiation (from the warm surface of the can to the cooler space surrounding it). They can enhance this method of cooling, however, by fanning the can with a piece of paper for forced convection. If you allow them to use water, they might even wet their can and fan it dry. In this case they are letting evaporative cooling take place, which might merit further discussion. Ask students to define and give examples of heat transfer by means of conduction, convection, and radiation. Ask students to describe ways people try to cause or prevent heating and cooling by conduction, convection, and radiation in everyday life. Ask students to give examples of materials that serve well for heating by conduction, convection, and radiation, and explain why these materials are particularly well suited for the type of heat transfer involved. Students can design and conduct a simple experiment to compare the rate of cooling of a beaker (or soda can) of hot water placed in front of a small electric fan (forced convection) to an identical beaker of hot water that is left alone (natural convection). Students can design and conduct an experiment to test the insulating abilities of materials designed to be insulators, such as fiberglass building insulation (have students wear household rubber gloves to avoid irritation of hands and wrists), feathers from an old pillow, or cut up parts of a winter coat, blanket, or sleeping bag from a thrift store. Students can conduct library and/or Internet research to find further examples of ways in which animals living in cold climates stay warm, and animals living in hot climates stay cool. This lesson and its associated activity were originally published, in slightly modified form, by Duke University's Center for Inquiry Based Learning (CIBL). Please visit the website http://www.biology.duke.edu/cibl for information about CIBL and other resources for K-12 science and math teachers.