1. Chapter 20: Gases

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

3. 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

4. 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

5. Earth’s Atmosphere

6. 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

7. 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.

8. 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

9. 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!

11. 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.

13. 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

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

16. 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

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

18. 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.

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