Chapter 20: Gases

What are two characteristics of gases? Gases take the shape of their container Gases fill all available space-No definite volume

The Atmosphere Held in place by Earth’s gravity Energized by the sun (otherwise would be matter on the ground) No definite surface (unlike the ocean) Density is not uniform at all depths (unlike a liquid) Air is more compressed at sea level than higher altitudes

The atmosphere is like a huge pile of leaves! Our atmosphere is thin! 99% of it is below 30 kilometers! Relative to the size of the world, our atmosphere is like a thin film of condensation on a billiard ball

Earth’s Atmosphere

Atmospheric Pressure Weight of the air At sea level, 1 cubic meter of air at 20 degrees Celsius has a mass of 1.2 kg/cubic meter

Why doesn’t the pressure of the atmosphere break windows? The pressure of the atmosphere doesn’t normally break windows because it’s pressing on both sides equally. There is no net force on the windows.

Review The Atmosphere: Gets energy from the sun and gravity holds it in place. No definite surface More compressed at sea level, thins with increases in altitude Overhead

Barometers Measures atmospheric pressure A barometer “balances” when the weight of liquid inside he tube exerts the same pressure as atmosphere outside of it. Mercury barometer-76 cm tall Water barometer-10.3 meters!

Aneroid Barometers Aneroid means “without liquid” Small metal box with most of the air removed. Slightly flexible lid is hooked to a spring and lever system.

Boyle’s Law At a constant temperature, the pressure times the volume of an enclosed gas is constant. If one increases, the other decreases. pressure x volume = different pressure x different volume

Practical Applications/Examples of Boyle’s Law Balloon Airtight pump Tire tubes Inflatable balls

Buoyancy of Air Archimedes’ Principle: An object surrounded by air is buoyed up by a force equal to the weight of the air displaced. Cubic meter of air = 1.2 kg = 12 N Dirigibles (blimps) are like big fish in water! Yellow box questions on page 299

Bernoulli’s Principle When the speed of a fluid increases, pressure drops (decreases). Applies to steady fluid flow, not turbulence

Some Questions What happens to atmospheric pressure on a windy day? During hurricanes and tornadoes? Where is atmospheric pressure greater, at head or shoulder level? How would a barometer reading vary while going up and down in an elevator of a tall building? Does air have weight? T/F The earth’s atmosphere extends upwards for hundreds of kilometers.

Applications/Examples of Bernoulli’s Principle Lift for airplanes and jets Curve Balls Shower Curtains Hydrofoils Bicycle Racing