Physics Textbook: The Atmosphere PDF

C H A P T E R 14 GASES 291 P hysics is firmly established in the educational main- stream in southern Sweden. Contributing to that has been the teaching effort of Per Olof Zetterberg at Lund University. Per Olof, known simply as P.O., contends that a student’s first univers...

C H A P T E R 14 GASES 291 P hysics is firmly established in the educational main- stream in southern Sweden. Contributing to that has been the teaching effort of Per Olof Zetterberg at Lund University. Per Olof, known simply as P.O., contends that a student’s first university course in physics should be a pleasurable experience. That’s right, pleasurable. By being pleasurable, his course has inspired many students to major in physics who may not otherwise have. P.O. presents phys- ics to beginning students as “fascination first; rigor later— when they are prepared.” P.O. is joined by his son Johan, also a PhD physicist, a recent addition to the Lund Uni- versity faculty. This father-and-son team, playfully pulling on the Magdeburg hemispheres as shown in the chapter- opener photo, are joined by Johan’s fiancée, physicist Sara Blomberg, together making an impressive Lund team that and outside of Sweden. Their motto is “Have Physics, will elevates public appreciation of physics. The team further travel!” With their dedication to physics education, helped promotes this appreciation by presenting physics demon- by their good energy, the Lund team contributes to physics strations enhanced by laser light activity to communities in being alive and well in Scandinavia. 14.1 The Atmosphere The thickness of our atmosphere is determined by two competing factors: the That’s right. Ninety-nine percent kinetic energy of its molecules, which tends to spread the molecules apart, of Earth’s atmosphere is below an and gravity, which tends to hold them near Earth. If Earth’s gravity were altitude of 30 km (only 0.5% of somehow turned off, atmospheric molecules would dissipate and disappear. Earth’s radius). Or if gravity acted but the molecules moved too slowly to form a gas (as might occur on a remote, cold planet), our “atmosphere” would be a liquid or solid layer, just so much more matter lying on the ground. There would be nothing to breathe. The –55°C Ozone layer 40 km atmosphere keeps us alive and warm, and without it, we would perish within minutes. But our atmosphere is a happy compromise between ener- –55°C getic molecules that tend to fly away and gravity that holds 99% of the atmosphere 30 km them back. Without solar energy, air molecules would lie on is below here Earth’s surface the way popcorn settles at the bottom of a pop- corn popper. But, if heat is added to the popcorn and to the atmospheric gases, both bumble their way up to higher alti- 20 km 90% of the atmosphere tudes. Pieces of popcorn in a popper attain speeds of a few is below here kilometers per hour and reach altitudes up to a meter or two; molecules in the air move at speeds of about 1600 kilometers –36°C 10 km per hour and bumble up to many kilometers in altitude. For- Cirrus clouds Mt. Troposphere tunately, there is an energizing Sun, there is gravity, and Earth Everest Cumulus 50% of the has an atmosphere. clouds Mt. Whitney atmosphere The exact height of the atmosphere has no real meaning –2°C is below 5.6 km because the air gets progressively thinner and thinner the higher one travels upward. Eventually, it thins out to emptiness in interplan- F I G U R E 1 4. 1 The atmosphere. Air is more com- etary space. Even in the vacuous regions of interplanetary space, however, there pressed at sea level than at higher is a gas density of about one molecule per cubic centimeter. This is primar- altitudes. Like feathers in a huge ily hydrogen, the most plentiful element in the universe. About 50% of the pile, what’s at the bottom is more atmosphere is below an altitude of 5.6 km (18,000 ft), 75% is below 11 km squashed than what’s nearer the top. 292 PA R T T w O P R O P E R T I E S O F M AT T E R (36,000 ft), 90% is below 18 km (60,000 ft), and 99% is below about 30 km (100,000 ft) (Figure 14.1). A detailed description of the atmosphere can be gases as well as liquids flow; found on various Web sites. hence, both are called fluids. A gas expands indefinitely and fills all the space available to it. Only when the quantity of gas is very large, such CHECK POINT as in the atmosphere of a planet or why do your ears sometimes pop when you change altitude—say, moving a star, do gravitational forces limit in a skyscraper elevator or descending in an airplane? the size or shape of a gas. chEck yOur answEr A change in altitude means a change in air pressure, as discussed in the next section, and this causes a temporary imbalance in the pressures on the two sides of your eardrum. 14.2 Atmospheric Pressure To vacuum pump We live at the bottom of an ocean of air. The atmosphere, much like the water in a lake, exerts pressure. For example, when the air pressure inside a cylinder like the one shown in Figure 14.2 is reduced, there is an upward force on the piston from the air outside. This force is great enough to lift a heavy weight. If the inside diameter of the cylinder is 10 cm or more, a person can be suspended by this force. Contrary to common thought, what the experiment of Figure 14.2 does not show is a “force of suction.” If we say there is a force of suction, then we assume that a vacuum can exert a force. But what is a vacuum? It is an absence of matter; it is a condition of nothingness. How can nothing exert a force? The piston holding FIGURE 14.2 the weight up in Figure 14.2 is not sucked upward. The piston is pushed by the Is the piston that supports the load weight of the atmosphere. pulled up or pushed up? Just as water pressure is caused by the weight of water, atmospheric pres- sure is caused by the weight of air. We have adapted so completely to the invisible air that we don’t feel it, and we sometimes forget that it has weight. Perhaps a fish “forgets” about the weight of water in the same way. The reason Many deep-sea creatures experi- we don’t feel this weight crushing against our bodies is that the pressure inside ence enormous water pressures our bodies balances the pressure of the surrounding air. There is no net force on their bodies, but they suffer no ill effects. As for us at the bottom for us to sense. of Earth’s atmosphere, no net At sea level, 1 m3 of air has a mass of about 1.25 kg. So the air in your kid sis- force or strain is exerted on them ter’s small bedroom weighs about as much as she does! The density of air decreases because the pressures inside their with altitude. At 10 km, for example, 1 m3 of air has a mass of about 0.4 kg. To bodies match the surrounding fluid pressure. For many creatures, but not all, problems occur when they change depth too suddenly. Scuba divers, for example, who make the mistake of rising to the surface too quickly experience pain and possible death from rapid decom- pression—a condition known as the bends. (Scuba is an acronym for Self-Contained Underwater Breath- FIGURE 14.3 ing Apparatus.) Marine biologists You don’t notice the weight of a bag of water while are looking for ways to bring you’re submerged in water. Similarly, you aren’t depth-sensitive deep-sea creatures aware of the weight of air while you are submerged to the surface without killing them. in an “ocean” of air.  C H A P T E R 14 GASES 293 compensate for this, airplanes are pressurized; the additional air needed to fully pres- surize a modern jumbo jet, for example, is more than 1000 kg. Air is heavy if you have enough of it. If your kid sister doesn’t believe that air has weight, you can show her why she falsely perceives the air to be weight-free. If you hand her a plastic bag of water, she’ll tell you that it has weight. But, if you hand her the same bag of water while she’s submerged in a swimming pool, she won’t feel its weight. That’s because she and the bag are surrounded by water. Likewise with the air that is all around us. Consider the mass of air in an upright 30-km-tall bamboo pole that has an in- side cross-sectional area of 1 cm2. If the density of the air inside the pole matches the density of the air outside, the mass of enclosed air would be about 1 kg. The weight of this much air is about 10 N. So the air pressure at the bottom of the bamboo pole would be about 10 N per square centimeter (10 N/cm2). Of course, the same is true without the bamboo pole. There are 10,000 cm2 in 1 m2, so a column of air 1 m2 in cross-section that extends up through the atmosphere has a FIGURE 14.4 mass of about 10,000 kg. The weight of this air is about 100,000 N (105 N). This The mass of air that would occupy weight produces a pressure of 100,000 N/m2—or, equivalently, 100,000 pascals or a bamboo pole that extends 30 km 100 kilopascals. To be more exact, the average atmospheric pressure at sea level is up—to the “top” of the atmos- 101.3 kilopascals (101.3 kPa).1 phere—is about 1 kg. This air weighs The pressure of the atmosphere is not uniform. Besides altitude variations, atmos- about 10 N. pheric pressure varies from one locality to the next, and from day to day. This leads to moving weather fronts and storms that shape our weather. When a high-pressure system approaches, you can expect cooler temperatures and clear skies. When a low- pressure system approaches, expect warmer weather, rain, and storms. Measurement of changing air pressure is important to meteorologists when predicting the weather. SCREENCAST: atmospheric Pressure FIGURE 14.6 The weight of air bearing down on FIGURE 14.5 a 1-m2 surface at sea level is about Ann Brandon fascinates her students when she 100,000 N. In other words, atmos- rides on a cushion of air blown through a hole in pheric pressure is about 105 N/m2, or the middle of this jumbo air puck. about 100 kPa. CHECK POINT 1. about how many kilograms of air occupy a classroom that has a 200-m2 floor area and a 4-m-high ceiling? (assume a chilly temperature of 10°c.) VIDEO: air has weight 2. why doesn’t the pressure of the atmosphere break windows? 1 The pascal (1 N/m 2) is the SI unit of pressure. The average pressure at sea level (101.3 kPa) is often called 1 atmosphere. In British units, the average atmospheric pressure at sea level is 14.7 lb/in 2. VIDEO: air is Matter