Air Pressure

Pressure Guide

Webelos Scientist

Activity Badge

C.A. Hughes

Department of Physics

University of Central Oklahoma

This demonstration outline was originally prepared for use by the Department of Physics at the University of Central Oklahoma as an outreach tool to teach physical intuition about pressure to elementary and middle school science students. As such, it contains equipment (e.g. vacuum pumps and Pascal’s vases) that may not be available to typical Cub Scout leaders. These demonstrations may be skipped or brought into the program by asking the Scouts "What do you think would happen if...". Alternatively, it may be possible to borrow these from area secondary schools or colleges.

Air Pressure/Bernoulli’s Principle Demonstrations

Materials Needed

Demo 1	2 heavy boards - board 1 has 1 nail in it, board 2 has no nails in it
Demo 2	one board with many nails equally spaced (about one inch apart), one board with 4 nails equally spaced, several balloons (9 inch diam)
Demo 3	piece of paper, ruler, pencil, and 15 lbs of weight
Demo 4	balloon and bathroom scale
Demo 5	paper or plastic cup, water, index card or small piece of light card board
Demo 6	empty coke bottle, water, index card or small piece of light card board
Demo 7	suction cup
Demo 8	paper drink cup, something to punch a hole (like a Phillips screwdriver)
Demo 9	Pascal’s vases, water
Demo 10	big marshmallow
Demo 11	vacuum chamber, balloon
Demo 12	vacuum chamber, marshmallow
Demo 13	total recall film clip, TV, VCR
Demo 14	hot plate, aluminum soda can, water, tablespoon, dish, ice
Demo 15	glass, balloon
Demo 16	20 oz soda bottle, balloon, something to punch a hole (like a Phillips head screwdriver).
Demo 17	Cartesian diver
Demo 18	8 ½ x 11 sheet of paper
Demo 19	ping pong ball, hair drier
Demo 20	ping pong ball, string, and faucet; or piece of paper and hair drier
Demo 21	8 ½ x 11 sheet of paper cut in half lengthwise, tape, pencil or wooden stick
Demo 22	Nothing

What is "Pressure?"

Need to understand "Force" first:

A Force is anything that causes a push or pull.

e.g. Gravity is a force that pulls you against the surface of the Earth (this gives you your weight)

e.g. The floor pushes up against you when you walk on it

Force is different than pressure:

Pressure is the amount of force spread out over a surface.

We can see the difference with a couple of examples :

Demo 1
Here are two basically identical boxes. Box 1 has a small nail glued to the bottom so that it protrudes downward. Box 2 has the same nail taped to the top. Which of these boxes would you prefer to have resting on your hand? *ANS: the one without the nail* But why? They both have exactly the same weight! *ANS: because in box 1, that weight is applied at a single point (BIG pressure) while in box 2 it is spread over a bigger surface (LOW pressure)*

Demo 2
Now look at these two boards. Board 1 has many small nails attached to it. Board 2 has a single nail attached to it. Push a balloon on each box with equal force. Which balloon will pop first? *ANS: the one with a single nail. The force is spread out over a wider area when there are many nails.*

So pressure is force spread over an area:

If a force is spread over a large area, pressure is smaller.

If a force is spread over a small area, pressure is bigger.

What is "atmospheric pressure?"

The air around us exerts a force on us due to the weight of the air on top of us.

Air doesn’t seem to weigh very much to us because it is a gas. Remember though that the atmosphere is about 20 miles thick so there’s 20 miles of air stacked up over your head.

The air actually gets thinner as you go up, so most of the weight comes from the air in the first few miles above the earth.

The actual atmospheric pressure works out to 14.7 lbs/square inch (14.7 psi) at the surface of the earth (at sea level).

What does 14.7 psi mean?

Demo 3
Draw a 1inch by 1 inch square on a piece of paper. Directly over this 1 inch x 1 inch square there are 14.7 pounds of air. [Show how much weight there is in 14.7 lbs]

That’s a lot of force. Each 1x1 inch square patch on top of your head feels 14.7 lbs of force. If you add it all up, you have almost 100 lbs of force pushing down on you from the air over your head!

Demo 4
What happens to an object that has 14.7 pounds over each 1 inch square surface? {Push down on a balloon (you can set it on a bathroom scale to show the total force)} *Ans: It squishes!*

So, with all of this crushing pressure on top of us, how come we don’t squish like a balloon?

Before we answer this question, we should probably show that this 14.7 psi pressure really is there and is exerting a force over each square patch of area...

Demo 5

Fill a paper cup to the brim with water.

Now place a piece of cardboard over the top of the cup and invert it.

Think about what might happen:

If there is no atmospheric pressure, the weight of the water should push down against the cardboard causing water to go all over the place.
If there is atmospheric pressure, it should exert a bigger force on the cardboard than the water does and the cardboard should "stick" to the bottom of the glass.

Ans: There is atmospheric pressure and the cardboard sticks to the bottom of the cup.

Demo 6
Repeat the above experiment with a coke bottle.

Demo 7
Take a small suction cup (1 in by 1 in is good…that’s about the size used to hang window ornaments with). Why is it hard to lift the suction cup up? *Ans: Atmospheric pressure.*

Now back to the question: So how come you don’t squish out like a balloon when 14.7 psi pushes down on your head?

Even though the air causing the pressure is above you, the air pushes with equal pressure on all sides of your body. This is known as "Pascal’s Law" and is true for any fluid (a fluid is anything that "flows" which can be any gas or liquid).

The pressure in any fluid is the same at the same horizontal level of the fluid. So atmospheric pressure can change as you go up and down (as you would in a plane) but is always the same when you are the same distance below the top of the atmosphere.

Demo 8

Here is another example of Pascal’s Law with a different kind of fluid: water.

Take a paper cup and fill it with water. Now punch holes on opposite sides of the cup. The water should stream out equally on both sides. This indicates that the pressure of the water is the same on both sides of the cup.

Punch two more holes at higher levels. The streams won’t be as strong because the pressure is smaller at higher horizontal levels in the fluid.

If you stay at one horizontal level, the pressure is the same in the fluid in all directions. That’s Pascal’s Law.

Demo 9
Get a set of Pascal’s vases. Fill one of the container containers. To what level will the others fill? Ans: to the same horizontal level.

Your whole body occupies pretty much the same position in the atmosphere, so you will feel pretty much the same pressure pushing in on you from your head to your toes. You only see a significant difference in air pressure if you climb a mountain.

So Pascal’s Law tells you why you don’t get squished downward like a balloon. But, if you take an object and squish it on all sides with the same pressure, something else can happen:

Demo 10
Take a marshmallow and push it inward on all sides. What happens? *Ans: It compresses.*

14.7 lbs pushing inward over each square inch of your body is a great deal of force. How come you don’t get squished up like a marshmallow? In fact, how come a marshmallow doesn’t get squished up like a marshmallow when the atmosphere pushes in on it? How come we don’t seem to feel anything at all?

Two reasons:

First, all objects will push outward with a pressure of 14.7 psi while the air pushes inward. As the air pushes in on our heads and our sides, our heads and sides push back out the same way and we don’t feel like we’re being squished. A marshmallow pushes outward with 14.7 psi and so the inward pressure is balanced by an outward pressure.

Second, an object’s structure might keep it from collapsing. A marshmallow will get squished if you push on it, but a penny will not. The penny has a stiffer structure that can withstand an increase in pressure while a penny cannot.

Is there any way to see the outward push that objects exert?

One way to see the outward pressure that an object exerts is to get rid of the air around them. With the air gone, what should an outward push cause the object to do?

Ans: It should expand.

Demo 11
We’ll use a vacuum chamber to do this. Put a balloon in the vacuum chamber and remove the air. What happens? *Ans: The balloon expands*

Demo 12
Now put a marshmallow in the vacuum chamber and remove the air. What happens? *Ans: The marshmallow expands*

Demo 13
Show a clip from the movie "Total Recall" where Arnold S. is trapped on the surface of Mars. What happens to Arnold when he hits the surface of Mars where the atmosphere has a smaller pressure?

Now you should be able to answer two questions:

Question 1: What happens to an astronaut in outer space if they don’t have a pressurized spacesuit?

Ans: They explode.

Question 2: What happens to somebody at the bottom of an ocean if they don’t have a pressurized suit?

Ans: They squish.

At the bottom of a swimming pool, the increase in pressure isn’t enough to squish you but is enough to allow you to feel the pressure on your ears. Sometimes they "pop."

Now here is an experiment that shows the squishing effect of the atmosphere.

Demo 14
Put exactly one tablespoon of water in an empty aluminum soda can. Heat to boiling (not just steaming…boiling) Remove the can and invert it upside down in cold ice water. What happens? As water boiled it turned into steam. Steam pushes air outside of can. When can of steam is cooled, steam turns back into tiny amount of water leaving a vacuum in the can. Atmospheric pressure crushes can.

Does air always have a pressure of 14.7 PSI?

Plain old normal atmospheric pressure has a pressure of 14.7 PSI. What if we squish this air into a smaller volume?

Demo 15
Take a glass and put an empty balloon in it. Now blow the balloon up. As you blow air into it, the balloon can’t expand once it fills the volume of the glass. The pressure keeps getting bigger. If it weren’t for its rigid walls, the glass itself would shatter.

Demo 16

Punch a small hole in the balloon of an empty 20 oz plastic soda bottle. Put a balloon in the bottle so that the end you blow the balloon up with is pulled out and over the mouthpiece of the bottle. Now blow up the balloon. What happens?

Ans: The balloon blows up until it hits the walls of the container, just as in the last demo.

Now let all the air back out, put a finger over the hole in the bottom of the bottle, and try and blow up the balloon again. What happens this time?

Ans: It is impossible to blow the balloon up.

Demo 17
Cartesian Diver A straw or medicine bottle is placed inside of a glass container. It has just enough air in it that it just barely floats. If you squeeze on the outside of the container, the rubber end or the straw collapses, compressing the air, causing the density to get smaller and the stopper sinks.

All the stuff we’ve done so far has had to do with air that stood still.

WHAT HAPPENS IF THE AIR IS MOVING?

Bernoulli’s Principle: Moving air has a smaller pressure than stationary air AND the faster it moves, the smaller its pressure.

Demo 18

Simplest example of Bernoulli’s Principle

Hold a piece of paper by one end. Hold the edge horizontally so that the rest of the paper bows and hangs downward. Now blow down along the surface of the paper.

What happens?

You expect the paper to get pushed by your breath (as if you blew directly into it--- like a sail). Instead, as you blow along its surface, the faster moving air produces a smaller pressure than the still air below and the paper rises. That’s Bernoulli’s principle.

Demo 19
Hair drier and ping pong ball. Turn on a hair drier (needs a circular nozzle and a fairly fast air flow). Place a ping pong ball in the middle of the air stream and release it. What happens? Now, with the ball suspended in the air stream, tilt the nozzle. Does the ball stay in the air stream? Why?

Demo 20
Ping pong ball on a string Hold a ping pong ball attached to a string near a stream of water (like out of faucet) or an air stream (like a hair drier). You could also use a sheet of paper. What happens? The ball (or paper) is pulled towards the moving water (or air).

Demo 21
Airplanes lift for two reasons: the wing can tilt creating a sail effect and the air travels a greater distance over the top of the wing (making it move faster) producing a bernoulli lift. Make a sample airplane wing out of a piece of paper.

Demo 22
House in tornado or hurricane (this is a thought experiment)