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# Atmospheric Pressure: Air Force per Area

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Author: Janice VanCleave

A fluid is any substance, a gas or liquid, that flows and offers little resistance to a change in its shape when under pressure. Earth's atmosphere (the blanket of gases surrounding Earth) is made up of air, which is a fluid. Air has mass and weight, but no fixed size or shape. It is compressible, which means that the particles that make up air can be pushed closer together. The closer the particles are pushed together, the greater their density (amount of particles in a given volume). Pressure is the force exerted on a unit area of a surface. The measure of the force that results from the collision of air molecules in Earth's atmosphere on a specific area is called atmospheric pressure.

In this project, you will determine the affect of temperature on air pressure. You will also use a manometer to determine the direction of atmospheric pressure, and you will compare air pressures at different layers of Earth's atmosphere.

### Getting Started

Purpose: To determine how a decrease in temperature affects air pressure.

### Materials

• 1 plastic shoe box
• cold tap water
• 12 or more ice cubes
• black permanent marker
• 2 empty I-liter plastic soda bottles
• two 9-inch (22.S-cm) balloons
• timer
• S-ounce (ISO-ml) paper cup

### Procedure

1. Fill the box half full with cold tap water.
2. Add the ice cubes to the water.
3. Use the marker to label the bottles A and B.
4. Place one of the balloons over the mouth of each bottle.
5. Stand bottle A next to the shoe box and lay bottle B on its side inside the box.
6. Use the paper cup to dip and pour the chilled water over the surface of bottle B for two or more minutes.
7. Observe the balloons on both bottles periodically, noting any changes in their size, shape, or position.

### Results

There are no noticeable changes in the balloon on bottle A The balloon on bottle B may be slightly less inflated, but basically there is no noticeable change in its size and shape. However, the balloon was partially moved inside bottle B.

### Why?

Temperature is how hot or cold an object is, which is determined by the average kinetic energy (KE) of the particles of the object When bottle B was chilled with the cold water, the temperature of the air inside the bottle decreased; thus the kinetic energy of the air particles inside the bottle decreased. With lower kinetic energy, the air particles move slower and there are fewer collisions of the air molecules on the inside surface of the bottle and balloon. Thus the pressure (the measure of the force exerted on a specific area) of the air inside bottle B decreased.

The number of air molecules inside the two bottles is the same. The temperature of the air outside bottles A and B is relatively the same, with a slight decrease of air temperature near bottle B due to the icy water. Thus the atmospheric pressure outside each of the bottles is relatively the same. Atmospheric pressure is the measure of the force of air on a specific area resulting from the collision of gas molecules in Earth's atmosphere on that area; also called barometric pressure. The atmospheric pressure outside bottle A and the pressure of the air inside the bottle are relatively the same; thus no change in the balloon is observed. But the atmospheric pressure outside bottle B is greater than the pressure of the air inside bottle B; thus the air molecules outside the bottle that collide with the surface of the balloon on bottle B push the balloon inside the bottle but the pressure is not enough to inflate the balloon.

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