1. Air and Air Pressure 6C

2. Properties of Air
Air is a mixture of elements, compounds and molecules that has mass, volume, and therefore density.
The more molecules we put in a given volume of air, the more mass it will have. So this air will be more dense.

3. Pressure Pressure is a force that acts over a certain area.
Liquids and gases are fluids. Fluids are any material that is able to flow. Fluids exert pressure because of the motion of their particles.
Pressure will always move from a high pressure to a low pressure area. The pressure will always try to equalize. You see this when you get a hole in your bicycle tire.

4. Inflating example
Air inside a ball pushes against the sides. The more air we put in a ball, the more the molecules push. So the pressure increases as we add air.

5. Pressure Formula Pressure = force/area
In the SI system, we will measure force in Newtons and area in m2. This will give us pressure in Pascals(N/m2).
In our system we measure force in pounds and area in square inches (in2). This gives us pressure in pounds per square inch (psi).

6. Review If we increase the force, what should happen to the pressure?
It should increase.
If we increase the area over which we apply a force, what should happen to the pressure?
It should decrease!

7. Air Pressure
Air pressure is the result of the column of air that is above you.
There is so much air above you that at sea level you have 14.7 lbs/in2 pushing on you.

8. Why are we not crushed by air pressure?
Air pressure is equal in all directions.
So air pushes equally in all sides of us. The forces are balanced!

9. Measuring Air Pressure
We use barometers to measure air pressure.
Mercury barometer: a glass tube sealed at the top partially filled with mercury.
Air pressure pushing on the mercury in the dish causes the mercury to rise and fall in the tube.

10. Aneroid Barometers
Aneroid Barometer: Works without liquid. Has an airtight metal can that is sensitive to changes in air pressure. This chamber is connected to a dial.

11. Units of Air Pressure
TV weather stations and aviation use inches of mercury.
Meteorologists use millibars, an SI unit. A unit of pressure equal to one-thousandth of a bar or 100 pascals, most commonly used to measure atmospheric pressure.

12. Altitude and Pressure
Altitude (or elevation) is the distance above sea level.
The higher ones altitude, the less air there is above you. So there will be less air pressure the higher one goes.

13. Pressure analogy
Imagine a stack of books. The bottom book feels all the weight of the all the books stacked above it, while the second one up feels slightly less weight. The higher in the stack, the less pressure one would feel.

14. Altitude and Density
As the air pressure decreases, the density of the air decreases. The air particles are not squashed together as tightly the higher one goes.
The air at sea level and at 6km has the same 21% oxygen, but at 6km there are fewer molecules, so you take in less oxygen with each breath.

15. Madgeburg Hemispheres
Air pressure is surprisingly strong. If we have two hollow metal spheres full of air, the pressures are equal so they will easily separate.

16. Madgeburg Hemispheres
But if we remove the air from the hollow spheres, there will be much lower pressure on the inside. The higher pressure outside will push in on the spheres. If the spheres are small, with a diameter of 4 inches, it will take over 180 pounds of force to pull them apart.

17. Pressure Demo
If we attached half of the sphere to the ceiling and removed the air, Mr. H would be able to hang from it.
This is also why the soda can was so easily crushed by the air pressure in the room.