Thermal Conduction

Transfer of Vibrational Energy

The term heat is often used very loosely. It is commonly said that hot objects possess more heat than do cold objects but technically the energy in the hot object is not heat, it is thermal energy. Heat is the movement of thermal energy from one object to another as a result of differences in temperature. The process by which heat is transferred from one particle to another due to collisions of the particles is called thermal conduction or conduction.

In this project, you will determine the effects of distance, time, cross-sectional area, and types of materials and temperature on thermal conduction. You will also compare the efficiency of different thermal insulators.

Getting Started

Purpose: To determine how distance affects thermal conduction.

Materials

• lemon-size piece of modeling clay
• one small birthday candle
• metal cookie sheet or baking pan
• 1 small paper clip
• permanent marker
• metric ruler
• margarine
• 18-by-30-inch (45-by-75-cm) piece of aluminum foil
• transparent tape
• kitchen matches

Procedure

1. Use a grape-size piece of clay to stand the candle on the metal cookie sheet, as shown in Figure 30.1.
2. Unbend the paper clip to form a metal wire that is as straight as possible.
3. Use the marker and the ruler to make marks on the wire at 2 cm, 4 cm, and 6 cm starting from one end.
4. Use the remaining clay to form a cylindrical holder for the wire that is about 3 inches (7.5 cm) tall. You should be able to stick one end of the wire into the side of the clay holder so that when the wire is held parallel to the table the free end of the wire will be positioned above the candlewick (in the top of the flame when the wick is lit, as shown in Figure 46.1).
5. Place balls of margarine about ¹⁄₂ inch (1.25 cm) in diameter on the marks on the wire.



6. Fold the aluminum foil in half placing the long sides together. Then connect the ends with tape, forming a circle about 4 inches (10 cm) high.
7. Place the circle of aluminum around the candle, wire, and cylindrical clay holder. This will shield drafts from the materials.
8. Use a match to light the candle.
9. With the end of the wire in the tip of the flame, observe the samples of margarine. Make note of the order in which the samples show any sign of melting. When all of the samples show signs of melting, blow out the flame.

Results

The samples of margarine melt in order of placement from the flame, with the first margarine sample closest to the flame showing the first signs of melting with the sample farthest from the flame melting last.

Why?

Thermal conduction is the movement of heat through a substance from a region of high temperature to a region of lower temperature. Conduction occurs in fluids when their moving molecules collide. But conduction is more commonly the method of heat transfer through a solid material without the movement of the solid material itself. Materials through which heat flows readily are called thermal conductors or conductors. Metals, such as the paper clip in this experiment, are particularly good conductors of heat because of a high concentration of free electrons (electrons in some solids, particularly metals, that are attracted relatively equally to all nearby atoms and thus are not tightly bound to a single site and are relatively free to move through the solid), which transfer heat when they collide with the atoms of the metal. The measure of the ability of a material to conduct heat is called thermal conductivity.

If a substance is heated, such as the wire in this investigation, the heat is conducted from the heated end to the cooler end. The fact that the margarine sample closest to the heated end of the wire melts first, followed by the second sample, and then the third, shows that it takes time for the heat to be conducted from the hotter end of the wire to the cooler end. Thus the amount of heat conducted through the metal wire from the warmer end to the cooler end is proportional to the time during which conduction has been taking place.

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