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Barometric Changes: The Cause and Measurement of Air Pressure

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Author: Janice VanCleave

Atmospheric pressure is the measure of the pressure that the atmosphere exerts on surfaces. Since the atmosphere is composed of air, the term air pressure is sometimes used. Weather forecasters measure atmospheric pressure with barometers and use the term barometric pressure.

In this project, you will make different barometers and use them to measure and compare atmospheric pressure in different places and at different times. You will demonstrate how the impact of air molecules exerts atmospheric pressure. You will show how air with greater density exerts greater atmospheric pressure. You will also learn about the relation between barometric pressure at sea level and at higher altitudes as well as natural barometers.

Getting Started

Purpose: To show how a barometer works.

Materials

• serrated knife (use with adult approval)
• scissors
• two 20-ounce (600-ml) plastic bottles
• one-hole paper punch
• flexible straw
• tap water

Procedure

1. Cut off the top 3 inches (7.5 cm) of one of the bottles. Do this by sawing a small slit in the bottle with the knife, then use the scissors to cut around the bottle. Keep the bottom section.
2. Cover the cut edges of the bottle with tape.
3. Use the paper punch to make a hole about 1 inch (2.5 cm) from the cut edge of the bottle.
4. Insert about 1/2 inch (1.25 cm) of the straw through the hole. The straw should fit snugly in the hole.
5. Stand the cut and uncut bottles side by side. Fill the bottles half full with equal amounts of water.
6. With your fingers over the mouth of the uncut bottle, turn it upside down, then lower it into the cut bottle. The mouth of the bottle should be below the water level in the cut bottle, and the two bottles should fit snugly together. The combined bottles form a barometer.
7. Near the straw, use the marking pen to mark the water lines on both bottles (see Figure 28.1).
8. Blow into the straw and observe the change in the water level in each bottle.
9. Suck air out of the straw, and again observe the change in the water level in each bottle.

Results

Blowing into the straw causes the water to rise above the water line in the top bottle and sink below the line in the bottom bottle. The reverse is true when air is sucked out of the straw: the water sinks below the water line in the top bottle and rises above it in the bottom bottle.

Why?

Atmospheric pressure, also called air pressure, is the force that air exerts on a particular area. It is a measure of the pressure resulting from the force (push or pull exerted on matter) that the total mass of air in an imaginary column exerts on a horizontal area. The greater the mass-—that is, the greater the number of molecules—the greater the air pressure.

Atmospheric pressure is directly related to the density of air, which measures the number of air molecules in a given space. This was demonstrated by the barometer you made in this experiment. A barometer is an instrument used to measure atmospheric pressure. Before you blew air into or sucked air out of the straw, the atmospheric pressure pushing down on the water's surface in the lower bottle was equal to the air pressure pushing down on the water in the upper bottle. Blowing into the straw increased the amount of air in the lower bottle; thus the pressure on the surface of the water in this bottle increased. This increase in pressure forced water from the lower bottle into the upper bottle. The water level in the lower bottle sank, and the level in the upper bottle rose. The reverse happened when air was removed from the lower bottle. The pressure pushing down on the water in the lower bottle decreased, and the pressure in the upper bottle pushed water down into the lower bottle. The water level in the upper bottle sank, and the level in the lower bottle rose.

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