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Wind: Classroom Activities

Temperature Effects on Air


Goal

To develop a better understanding of the effects of temperature on air density and air flow.

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Background Information

When air is warmed, it expands and becomes less dense. As the air becomes less dense, its air pressure decreases. This occurs because molecules in warm air have greater kinetic energy (energy of motion) than in cold air. As the molecules move faster they spread out. As the molecules move farther apart the volume of the air increases and its density decreases. When air is cooled, its molecules move closer together. The air contracts and becomes denser. As air becomes denser, it contains more mass per unit area and exerts greater pressure. Therefore, cooler, denser parcels of air have a higher air pressure than warmer, less dense parcels of air. As air warms it becomes relatively less dense than the cooler surrounding air and rises as a result.

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Activity Overview

Students examine how the density of air changes with air temperature. They observe that the volume of a balloon at room temperature is greater than the volume of the same balloon when it is cooled in a freezer. Then, they inflate a plastic bag with warm air. They observe how the plastic bag rises toward the ceiling when it is released.

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Materials and Equipment

Large round balloons, string, access to a freezer or ice-filled cooler, dry cleaning plastic clothes bags (large gown size), masking tape, hair dryer, measuring tape

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Activity

  1. Ask students to inflate a balloon. Have them to tie a piece of string around the widest part of the balloon. Be sure that they make the string tight enough so it will not slip off easily, but not so tight that it is squeezing the balloon.
  2. Have students place the balloon in a freezer (or in an ice-filled cooler). After 15 minutes, ask them to remove the balloon from the freezer.
  3. Have students record their observations.
    Students should observe a decrease in the volume of the balloon after it has been removed from the freezer or cooler. They should notice that the string falls off after the balloon has been in the freezer, indicating that the volume is less than when the balloon was at room temperature.
  4. Have students complete the following:
    1. Assume that no air escaped from the balloon. What does the fit of the string around the chilled balloon tell you about the density of air in it?
      You may need to remind students that density is mass per unit volume. Before the balloon was cooled, the string fit snugly around it. Cooling the air in the balloon reduced its volume. Because no air escaped from the balloon, it can be reasonably inferred that the density must have increased, i.e. same mass, smaller volume.
    2. How does air temperature affect the density of air?
      As air temperature decreases the density of the air increases.
  5. Provide students a plastic dry cleaning clothes bag. Ask them to use a small piece of masking tape to seal the hole at the top end of the bag. Instruct them to tape the bottom of the bag so that it has an opening of about 15 centimeters.
  6. Have students use a hair dryer set on low to inflate the plastic bag.
  7. After the bag is inflated tell students to release the bag.
  8. Have students record their observations.
    Students should observe that the plastic bag rises when it is released.
  9. Ask students to explain their observations. Tell them to refer to the density of air in their explanations.
    After inflating the bag with the hair dryer, the air in the plastic bag is warmer and therefore less dense than the surrounding air. The surrounding, more dense, cooler air moves below the warmer air and forces the plastic bag to rise.

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