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 m 3 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 30 0 celsius (~900 F), air has a density of 1.16 kg/m 3. This means that 1 m 3 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 1cm 2, the air in the pole would have a mass of ~1 kg. There are 10,000 cm 2 in 1m 2, so there is a mass of roughly 10,000 kg of air above a patch of ground of 1m 2.

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 ping- pong 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: P 1 V 1 = P 2 V 2 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