Lesson What's Hot and What's Not?

Learning Objectives

After this lesson, students should be able to:

• Define the terms conduction, convection and radiation in the context of heat transfer.
• Describe everyday examples of ways people try to cause or prevent heating and cooling by conduction, convection and radiation.
• Give examples of materials that serve well for heating by conduction, convection and radiation.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

International Technology and Engineering Educators Association - Technology

State Standards

Introduction/Motivation

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 power plants. 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 in the Lesson Background section to introduce the fundamental principles of heat transfer. As you present this material, or immediately following it, let students work in teams of four to conduct their own simple demonstrations of the three types of heat transfer. For these demonstrations, have ready a birthday candle poked into the bottom of an upside-down paper cup for each team, as well as the other materials mentioned in the demo descriptions below.

Conduction Demo: 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 soon heats up. The heat then spreads by conduction to the fingers of the person holding it. If you provide metal rods of various lengths, students could 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.

Convection Demo: If 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 grocery stores) upside down about an inch above the flame. After a few moments, the smoke accumulates under the pie plate and then begins to escape out around its edges, where it curls 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 no breeze, the bug-coil demonstration is well suited for outdoors.)

Radiation Demo: To see the effects of radiation, have students 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, continue.) We encounter heating and cooling devices everyday in our homes, workplaces and modes of transportation. Can you think of some examples? (Give many students the chance to volunteer examples and explain the roles that conduction, convection and radiation play in them. Following are examples that students may come up with or examples 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 operates when the car is not moving but the engine is so hot that radiation alone is not adequate for cooling.

• Similarly, wood stoves heat our rooms by conduction, convection and radiation. The fire inside the stove heats the cast iron surfaces of the stove, which then radiate 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 was 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 in homes or cars, 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 placed into the walls and roofs of buildings reduces heat loss by convection. Being thick, fluffy stuff, air cannot move easily through all its mixed up layers of fibers. 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. Insulation performs the same role when it traps the cool, air-conditioned air inside a house away from the hot exterior air on the other side.

• Similar to insulation, 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.

What might be some examples of cooling and heating "mechanisms" that occur in nature? What do animals do if they need to cool off or warm up? (Ask for student suggestions. Some examples are described below.)

• 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 cannot make their own seek out the shade of trees, plants, rocks, etc.

• Mammals that live in cold climates have thick, insulating fur. Many mammals that live where in regions with distinct seasons grow thicker fur during the winter and shed the excess insulating material during the summer when it is no longer needed.

• 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.
• Cumulus clouds form when warm air at the Earth's surface rises very high into the atmosphere through convection.

(Once students' understanding of the heat transfer concepts are verified, proceed to introduce and conduct the associated activity Hot Cans and Cold Cans where stduents apply the concepts of conduction, convection and radiation as they work in teams to solve two challenges. .)

Lesson Background and Concepts for Teachers

Heat Transfer by Conduction

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 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, molecules are less densely packed together, and they must 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 when water freezes—and therefore becomes solid—the molecules have slowed so much that instead of bouncing off one another, they are only vibrating in place. (They 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 start this process.

Some materials are better conductors than others. Metals are especially good, but glass and ceramics are not bad either. Plastic and wood are relatively poor conductors, which is why wooden spoons are good for cooking and saucepans have plastic handles. 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 its pot warms up.

Heat Transfer by Convection

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, in which heat is transferred from molecule to molecule, in convection heat is transferred by bulk flow.

Students have most likely already seen 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. A common small holiday chime rings 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, locate the candle somewhere along the circumference of the jar. Light the candle and 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 heat soup in a pot on the stove, we do not usually just wait until conduction and convection have done their things before we start eating the soup. Instead, we speed up the process by stirring the soup as it heats. The stirring motion of a spoon helps to move the warmed 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 (without stirring). In forced convection, we use some mechanical device to move the liquid, thereby moving the heat, too.

Heat Transfer by Radiation

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 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.

Associated Activities

• Hot Cans and Cold Cans - Using only common, everyday materials, students design and test methods to cause soda cans filled with warm water to cool as much as possible in 30 minutes, while maintaining the warm temperature of an identical water-filled soda can for the same duration.

  Watch this activity on YouTube

Vocabulary/Definitions

conduction: 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.

convection: 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.

radiation: The transfer of heat energy by waves of visible or infrared light moving through space.

Assessment

Heat Transfer Examples: Ask students to define and give examples of heat transfer by means of conduction, convection, and radiation.

More Heat Transfer Examples: Ask students to describe ways people try to cause or prevent heating and cooling by conduction, convection and radiation in everyday life.

Heat Transfer Materials Examples: 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.

Lesson Extension Activities

Have students design and conduct simple experiments 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).

Have students design and conduct experiments to test the insulating abilities of materials designed to be insulators, such as fiberglass building insulation (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.

Have students 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.

Copyright

Contributors

Supporting Program

Acknowledgements

This content was developed by the MUSIC (Math Understanding through Science Integrated with Curriculum) Program in the Pratt School of Engineering at Duke University under National Science Foundation GK-12 grant no. DGE 0338262. However, these contents do not necessarily represent the policies of the NSF, and you should not assume endorsement by the federal government.

This lesson was originally published, in slightly modified form, by Duke University's Center for Inquiry Based Learning (CIBL). Visit the http://www.ciblearning.org/ website for information about CIBL and other resources for K-12 science and math teachers.