The Movement of Air Molecules

The movement of air molecules from one place to another is due to differences in energy of the air in each area. More energetic air molecules are warmer, faster and less dense than less energetic air molecules. In response to changes in temperature, air molecules move faster and spread apart, or move slower and get closer together.

In this project, you will demonstrate how convection currents are produced as a result of differences in temperature. You will determine why air pollution increases during an air condition called inversion. You will demonstrate the effect that temperature has on the density of air and use the results to explain why cold air is more dense than warm air. You will also model a convection cell to learn how updrafts and downdrafts are produced.

Getting Started

Purpose: To demonstrate how convection currents are produced.

Materials

• 2-inch (5-cm) piece of mosquito coil and a coil stand (found where camping supplies are sold)
• matches
• two I-quart (I-liter) glass jars with equal-size mouths
• 3 × 5-inch (7.5 × I2.5-cm) index card
• desk lamp
• 9 × I2-inch (22.5 × 30-cm) sheet of black construction paper
• transparent tape

Note: This experiment requires access to a freezer.

Procedure

CAUTION: Get adult approval to handle the burning mosquito coil and candle in this chapter.

1. Follow the package instructions to place the coil in the mosquito coil stand and to ignite the end of the coil. This should be done on an outdoor table.



2. Invert one of the jars and stand it over the burning coil for 2 to 3 minutes or until the jar is filled with smoke.
3. Lift the jar and cover its mouth with the index card.
4. Take the jar indoors and place it on a table near a desk lamp.
5. Place the second, empty jar in the freezer for 2 minutes.
6. Remove the jar from the freezer. Invert the cold jar and stand it mouth to mouth on the smoke-filled jar, with the index card between them.
7. Stand the black paper behind the jars and secure it to the bottom jar with tape (see Figure 26.1).
8. Slightly lift the top jar, remove the index card, and place the jar mouths together. Do this with as little movement to the bottom jar as possible.
9. Position the lamp so that the smoke is as visible as possible.
10. Observe the contents of the two jars.

Results

The smoke in the bottom jar rises and fills the top jar.

Why?

The air in the bottom jar is warmer than the air in the top jar. Warm-air molecules have more energy and move around faster than do less energetic cold-air molecules. The speedy warm-air molecules tend to move away from each other. So warm air, with its molecules spaced farther apart, is less dense (compact) and lighter than cold air with its molecules closer together. While in the freezer, the open jar filled with cold, dense air. When the card was removed, the heavy cold air sank and the light warm air rose. The smoke was lifted with the rising warm air. This up-and-down movement of air due to differences in temperature is called convection currents.

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