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# Atmospheric Pressure: Air Force per Area (page 2)

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Author: Janice VanCleave

### Try New Approaches

Heating a gas causes its molecules to speed up, thus increasing the KE of the gas. How does this affect air pressure? Determine the effect that heating has on the air pressure by repeating the experiment using warm tap water in the shoe box. Science Fair Hint: Take photographs to indicate the size, the shape, and the position of the balloons at the start and finish of the experiment.

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1. Earth's atmosphere is held in place around Earth by gravity. Atmospheric pressure, which is created by air molecules hitting and bouncing off of surfaces, including each other, keeps gravity from pulling all of the air molecules to Earth's surface. Gravity pulls the atmosphere downward, and air pressure pushes the atmosphere upward. Does this mean that atmospheric pressure is only directed upward against the pull of gravity? Design an experiment to determine the direction of atmospheric pressure. One way is to use a manometer (an instrument used to measure the pressure of fluids). See Appendix 3 for instructions on how to make a manometer. Test atmospheric pressure by holding one end of the manometer tube in different directions: right, left, up, and down. The water levels in the tubes will be even if the pressure on both sides is equal.
2. The weight of the column of air above an area of Earth at a lower elevation produces a greater atmospheric pressure than at a higher elevation. This is because as the column of air increases in height, its weight increases, causing the air molecules near Earth's surface to have greater density. As the density of air increases, the number of molecules of air hitting against a surface increases. Design an experiment to prove that atmospheric pressure is the same no matter where it is measured as long as the density of the air is the same. The manometer could be used to compare the atmospheric pressure of different air samples. One way is to stand the manometer outdoors, in a spot where only the atmosphere is above the instrument. Cut a notch out of one side of the lid of a box, such as a large shoe box. Set the box next to the manometer for one or more minutes so that the box will fill with the same air around the instrument. Insert the end of the manometer into the box and secure it to the inside of the box with tape. Close the box so the manometer tube fits in the cutout of the lid. Use tape to cover the opening in the lid around the tube. Compare the water levels in the tubes. Repeat with air collected from other places. Remember that temperature affects the density of air. If possible, repeat this experiment at different elevations, such as on a mountain and at sea level.
3. A diagram can be used to show how weight (measurement of the gravity force) and air pressure affect the structure of Earth's atmosphere. Show a 1-m-square column of the atmosphere made of blocks of an equal number of air molecules. Extending from Earth's surface, one block of air would be stacked on another. The air pressure of the moving air molecules of the bottom block would support the weight of all the blocks above it. The weight of the air blocks would tightly compress the air molecules in the bottom block, thus the bottom block would be more compressed (pushed together). Thus the density of this block would be greater than any above it. Near Earth's surface, the density of air is greatest, and atmospheric pressure is about 14.6 psi (pounds per square inch), which is about 100,000 Pa (pascals). At 3.4 miles (5.5 km) above Earth's surface, the pressure is about half that at Earth's surface; at 6.8 miles (11 km), the pressure is about a fourth that at Earth's surface. For more about the relationships among gravity, density, and air pressure, see Louis A Bloomfield, How Things Work: The Physics ofEveryday Life (New York: Wiley, 1997), pp. 123-127.

### Get the Facts

1. A siphon is generally a tube used to make liquid flow from a higher level to a lower level. What effect does atmospheric pressure have on the flow of liquid in a siphon? For information, see Lewis Carroll Einstein, Thinking Physics (San Francisco: Insight Press, 1995), pp. 204-205.
2. A barometer is an instrument used to measure atmospheric pressure. Italian mathematician and physicist Evangelista Torricelli (1608- 1647) discovered the principle of a barometer in 1643. How can a barometer be made? How does it work? For information, see Janice VanCleave's A+ Projects in Earth Science (New York: Wiley, 1999), pp. 187-192.
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