Transcript Gases
Gases Yet another state of matter Gases • Gases are compressible (unlike liquids or solids) • Gases do not hold their own shape • Gases are less dense than solids or liquids • Gases DO have mass and weight The atmosphere • The atmosphere is made up of a combination of gases – Mostly nitrogen, then oxygen • Sun’s light gives energy to make the molecules in atmosphere bounce around • Earth’s gravity keeps molecules from flying off into space • Why doesn’t the moon have an atmosphere? More Atmosphere • The atmosphere is NOT uniformly dense • Air is densest closest to the surface of the earth • Gets less dense as you go up – 50 % of the atmosphere is below 5.6 km – 75 % of the atmosphere is below 11 km – 99 % of the atmosphere is below 30 km To give perspective on how thick the atmosphere is… • Draw the circle thing on the board Weight of Air • Remember, AIR HAS WEIGHT • At sea level, 1 m3 of air has a mass of ~1.2 kg and a weight of 11.7 N • What is the mass of air in this room? – Assume the room is 5m X 5m X 3m and that we are at sea level • Which weighs more? A grapefruit or the mass of the air in a refrigerator? Atmospheric Pressure • Atmospheric pressure is due to the weight of the air above us • Why don’t we feel it? • Because we are in it all the time. • Does a bag of water have weight? • Now submerge that bag of water in the pool. Does it feel like it has weight there? No. For the same reason, we don’t feel the weight of air. • Now set up the demonstration. More Atmospheric Pressure • Density of air changes with temperature • At sea level at 300 celsius (~900 F), air has a density of 1.16 kg/m3. This means that 1 m3 of air has a mass of 1.16 kg. • Imagine a bamboo pole extending upwards from the earth’s surface to 30km. If the pole has a cross-sectional area of 1cm2, the air in the pole would have a mass of ~1 kg. • There are 10,000 cm2 in 1m2, so there is a mass of roughly 10,000 kg of air above a patch of ground of 1m2. So what does all this mean? • • • • It means that air pressure is STRONG. Or as they say in Boston, “wicked strong”. Let’s do a demonstration. Okay, that should have been impressive. Why did the can get crushed? • So why don’t we get crushed by atmospheric pressure? Implications of atmospheric pressure • What is a barometer? • A barometer measures atmospheric pressure. See page 294. • Why do we care about variations in atmospheric pressure? Drinking out of a straw • What is happening when you drink from a straw? • See figure 20.8 on page 295. Why can the girl on the left drink from the star but the girl on the right can’t? Boyle’s Law • Think about the air inside a balloon. We can consider the molecules to act like tiny pingpong balls as they bounce around inside. • So in this case, what is pressure, exactly? Why does a balloon stay inflated? What forces are pushing against the walls of the balloon? • What happens if we add more air? Boyle’s Law continued • Boyle’s Law states a relationship between the pressure of a gas and its volume. • Let’s say you have a particular volume of gas at a particular pressure. If you keep the temperature of the gas the same and change either the pressure or the volume, the gas will obey this relationship: • P1V1 = P2V2 • In other words, the product of the pressure and volume remains constant. • As volume goes down, pressure goes up and vice versa. Buoyancy of air • Remember all that buoyancy stuff for liquids? It applies to gases, too. • A blimp hovers at a particular height off the ground for the same reason a fish hovers at a given depth of water. • An object is buoyed up by a force equal to the air it displaces. • The reason we don’t see this happen as often is because it is harder to find an object with a density less than the density of air. – What are some examples? Bernoulli’s Principle • When the speed of a fluid increases, pressure decreases