University of Minnesota
School of Physics & Astronomy

Physics Force

Air Pressure

Vocabulary

air pressure: Pressure exerted by the weight of air on an object in that air. There will be discussion about this when we describe the reason for the plunger being held onto a piece of clear plexiglass, when the rubber playground softball base is used to lift a chair using air pressure, when we use the Magdeburg disks being held together by air pressure and when we crush a 55-gallon drum using air pressure.

atmospheric pressure: This is also the pressure caused by air, but usually thought of as the normal value or standard value of 14.7 pounds per square inch. The atmospheric pressure at the place where we are on the face of the earth is usually less than this because we are above sea level, and hence have less "weight" pushing down on us. Again, used with the same activities as air pressure.

cavity: A hollowed out or empty space enclosed by some form of walls, in this case the walls will be formed by the rubber softball base and the top of the chair it will be lifting.

condensation: The changing of a gas into a liquid. In this case we will be discussing the condensation of water vapor into liquid water when we discuss the reasons for the 55-gallon drum being collapsed. When water vapor condenses to liquid water, it takes up considerably less space, and this is very important in reducing the pressure inside the drum.

force: Simply a push or a pull. We will be talking about force as we describe the force caused by air pressure on the plunger and plexiglass, chair and rubber softball base, the two Magdeburg disks, and the force exerted on the 55-gallon drum.

high pressure: A space where the pressure is more pounds per square inch than a neighboring space. We will be discussing the idea that the pressure is higher outside the 55-gallon drum than inside the drum.

low pressure: A space where the pressure is fewer pounds per square inch than a neighboring space. We will use this term along with the term high pressure in order to describe the resulting force that will tend to push something one way or the other.

Magdeburg: A city in Germany where a scientist first demonstrated the large amount of air pressure that we all live in. It was done by removing the air from between two large hemispheres that were sealed air tight and then two teams of horses were engaged to try to pull them apart. Having not been able to do it with the horses, the scientist then walked over to the hemispheres, opened a valve to let air back into the cavity between the hemispheres and they fell apart. We will be doing a similar type of activity using plexiglass disks which we will call Magdeburg Disks.

molecular motion: The name given to the intrinsic property of all matter that is at normal temperature where the molecules of that matter are in a constant state of motion. We will be using this concept to help describe what causes air pressure. The molecules of air are forced downward by gravity, and also are in a constant state of movement, thus causing an enormous number of collisions in one second with the outsides of the plunger and plexiglass, the Magdeburg disks, the chair and the rubber base and the 55-gallon drum. When taken all together, these collisions cause the pressure against these objects.

pressure: In the context of our show, it will be the number of pounds per square inch exerted by gaseous molecules. When the number of pounds per square inch are multiplied by the number of square inches on an object, it is possible to determine the total force, the push on the object.

steam: The gaseous state of water when it is heated above 100º Celsius at standard atmospheric pressure. When we heat the water in the 55-gallon drum, it will boil and turn to steam.

vacuum: A space where there is no pressure, usually caused by the lack of any molecules. A totally empty space is another way of describing it. We will be talking about the partial vacuum produced by several situations which we will create. Thus, in our partial vacuum, there will be very low pressure but perhaps not a totally empty space.

vapor: The gaseous state of a substance that is, under normal conditions, a liquid but the gas is not at a temperature above its boiling point. Thus, the moisture in the air (humidity) can be called water vapor. There will be water vapor in the area above the water in the 55-gallon drum before we seal it off and then cool it. This water vapor will condense, causing less space to be taken up by the water molecules. Consequently the barrel will have a partial vacuum in it, causing a large difference in pressure from inside to outside.

volume: In the context of this show, it can be thought of as space defined by an enclosure of some kind. It is 3 dimensional much like a box, but not necessarily having square corners. One of the volumes we will be working with is in the form of a cylinder-the 55 gallon drum. In the chair lift, we will have a volume of irregular shape, we will call it a cavity.

 

Plunger and Plexiglass

 

What to look for:

A common toilet plunger will be used to stick to a clear piece of plexiglass. The clear plexiglass shows that there is nothing in the plunger, but it is "stuck" to the plexiglass.

 

How it happens

The reason this happens is because the air is forced out from between the plunger and plexiglass and when the plunger tries to straighten up again, there is a low-pressure area created under the plunger, and the higher atmospheric pressure pushes the plunger onto the plexiglass.

Thus, the plexiglass and plunger are squeezed together by atmospheric pressure. The usual term used is that the plunger is held to the plexiglass with "suction". There is no such thing as suction, only a lowering of the pressure in one place or another.

When your vacuum cleaner cleans up dust or dirt, it does not "suck up" anything. It creates a lower pressure (vacuum) in the hose, and the higher atmospheric pressure pushes the dust or dirt into the hose and on into the container in your cleaner that accepts the dirt and dust.

 

Chair Lift

 

One of the common misconceptions about air pressure is that it acts in the direction we call down. This is partly because of the explanation of air pressure as the weight of the air (14.6 pounds on average at sea level) above any square inch of earth surface. And weight acts downward with the pull of gravity. For a full understanding you must add the concept that once a fluid (gases are fluids) is under pressure the fluid than exerts this pressure in all directions including up. It may be better to think of air pressure as the result of all the little pushes of the air molecules bouncing off every square inch of surface the air has contact with. It may be difficult to comprehend, but there are over a sextillion (1021) such collisions for each square inch of surface each second at normal air pressure.

When you pull on the trigger (plunger) of a squirt gun you pull towards yourself yet the water shoots in the opposite direction due to this phenomenon of fluids.

If you decorate a cake using the old frosting in a bag technique you did not have to squeeze the bag towards the opening. Just put the frosting under pressure and it will ooze out the opening no matter what the direction.

 

What to look for:

The demonstrator will bring out a small stool and a soft-rubber baseball base from a play ground with a handle in it. He will place the base on the top of the stool, and lift up on the handle and the stool will be lifted off the floor easily. He will then show that the base is not glued to the stool by lifting the corner of the base and the stool will fall to the floor.

 

How it happens:

The stool and the base are smooth and will seal out any atmosphere from getting between them. When the demonstrator lifts on the handle, there is a small cavity formed because of the flexibility of the rubber base. There is very low pressure in this cavity, thus allowing the greater pressure of the atmosphere to push the rubber base and the top of the stool together hard enough to lift the stool from the floor.

When the demonstrator lifts the corner of the rubber base, he allows air between the stool and base, equalizing the pressure with the atmosphere, and the cavity is no longer a low pressure area.

FOSS Connection: Air & Weather - Activity 2: Air Explorations

Magdeberg Swing

What to look for:

The demonstrator will take two disks of clear heavy plastic with a gasket btween them and draw the air out from between them using his mouth and lungs. He will then hook it to a tall tower and hang from them, with just the pressure difference on the disks' insides and outsides. He will then open a valve and let the air back into the space between the disks, and they will fall apart.

 

How it happens: 

The average man can, using his mouth and lung power, draw a small volume of air down to about one half as much pressure as normal atmospheric pressure. This means approximately 7 to 8 pounds per square inch. The normal atmosphere has about 15 pounds per square inch. Now with the area of the two disks being approximately 70 to 80 square inches, the difference in pressure produces about 500 pounds of force tending to hold the two disks together. Thus, he would have to weigh about 500 pounds to pull the two disks apart using his weight!

 

Magdeburg Hemispheres

 

What to look for:

The demonstrators will bring out two quite large clear plastic disks, approximately 12 inches in diameter, with heavy rope attached to each of them. These disks have had the air removed from between them by use of a vacuum pump. The demonstrators will try to pull the disks apart, they cannot. They will then enlist the help of about 20 students to try to help them pull the disks apart. They will find that even with the strength of all the students put together, they will not be able to pull them apart. The demonstrators will then open a valve and the air rushing back between the disks will be heard. The disks will then fall apart on their own.

How it happens:

The vacuum pump will reduce the pressure between the disks to a very low value, less than one pound per square inch. This means there is a difference of over 14 pounds per square inch of pressure tending to push the disks together. The area of the disks is about 115 square inches. Multiply this by approximately 14.2 pounds per square inch, and you realize it would take over 1,600 pounds of force to pull the disks apart! Because of the way forces behave, each student would have to pull with over 160 pounds to pull them apart. This is very difficult even for adults when pulling horizontally.

The valve is then opened to let air back in between the disks to raise the pressure to atmospheric pressure. As soon as the pressure between the disks is equal to the pressure on the outside of the disks, they no longer have any force pushing them together and they fall apart.

 

Boiling Flask
When water is boiled in a boiling flask the air in the flask is replaced with steam and the air is flushed out through a glass tube and rubber stopper in the mouth of the flask. As the boiling water turns to steam it takes up about 10 times the volume it did when it was liquid water. The pressure remains the same as it is in contact with the outside air. Using a pair of gloves the flask is pick up and turned over, the glass tube in the top is placed into a container of water. As the steam begins to cool and condense, the pressure in the flask decreases and becomes less than normal room air pressure. The room air pressure then pushes water up into the flask much like a drinking straw when you lower the pressure in the straw as you drink lemon aid.

 

Materials

Boiling flask, one hole stopper, glass tube, gloves, beaker, ring stand, ring, burner, wire gauze, test tube clamp, water.

Instructions

Place about 20 ml of water in the boiling flask and clamp the flask to the ring stand. Put the ring in place with the wire gauze between the ring and the flask. The glass tubing should be put through the one hole stopper until it protrudes a little over half way into the bulb of the flask.

Clamp the flask in place and light the burner and allow the water to boil. After the steam has been coming out of the glass tube for several minutes, turn off the burner. Using the gloves, unclamp the flask and tubing and turn it upside down placing the glass tube in a beaker of water. Allow the flask to cool, as the steam condenses, there is very little steam left in the flask so the pressure goes down. The room air pressure then pushes the water in the beaker up into the flask.

Crush the Barrel

 

What to look for:

The demonstrators will bring out a large, metal 55-gallon drum. The same thing you might see dispensing fuel oil or pesticides on a farm. It will be standing vertically on a stand, with heat being applied to it. The demonstrators will turn off the heat, thread in a plug in the opening on the top, and begin to spray water on the barrel with spray bottles. After a period of a couple of minutes the barrel will crush to a smaller size, bending and twisting the metal quite profoundly. It will do this in a split second, with a rather loud sound.

 

How it happens:

This is the finale in a series of demonstrations designed to demonstrate the incredible pressure of the atmosphere. The barrel has about 3 liters of water in it, which has been boiling for a few minutes. This boiling causes the air inside to be forced out and replaced with hot water vapor. There will be steam coming out the opening at the top as it comes out on stage which shows most, if not all, of the air has been forced out of the barrel.

When the demonstrator removes the heat and caps the opening, it begins the process of cooling the barrel and its contents. This causes the steam in the barrel to condense into water, thus significantly reducing the volume of the contents. When this takes place, the pressure is lowered inside the barrel, and now the atmospheric pressure outside the barrel will be much greater than the pressure inside. It turns out that the barrel can withstand about 9 pounds per square inch difference, but beyond that it is simply too much pressure for the metal to withstand and it collapses.

Because the barrel is round and reinforced with ribs along its outside, it builds up quite a lot of tension and when some imperfection in the roundness occurs, the barrel collapses in a hurry.

There are several tons of force exerted by the pressure on the barrel that act to crush it, and the integrity of the steel is overcome . It is an implosion rather than an explosion, and the sound is quite distinctive.

FOSS Connection: Air & Weather - Activity 2: Air Explorations

Grade 4 Weather Unit

 

When you get back:

To demonstrate the pressure in the atmosphere, students can be given two straws, a glass soda bottle, some water and some modeling clay (pliestocene).

1. put water in the soda bottle until it is half full.

2. Put two straws in the bottle so that the ends are under the surface of the water. seal up the mouth of the bottle with the clay so that no air can enter the bottle or leave outside the two straws. Be sure the student can access one straw while the end of the other can be covered with a finger. Straws with a flexible section built into them will work best for this activity.

3. Now make some predictions as to what will happen when they try to sip some water through the straw with the second one left open, and with the second one closed up by holding a finger over the end of it. See diagram below:

When a student sips water, what she does is to lower the pressure in the straw she is sipping on. Then the atmospheric pressure will push down on the surface of the water, and since pressure is transmitted equally in all directions, it will push the water up the straw and into her mouth. When the students try to sip the water with the second straw covered, it does not allow the atmosphere to push down on the surface of the water and push it up the straw they are trying to sip through.

If the students blow into the straw, it will force the water up the second straw. You might ask why this happens and discuss the results. Ask why there are bubbles at the bottom of the second straw when they sip through their straw.

 

Cup and Balloon

 

According to Boyle's law, if you increase the volume of a confined gas, its pressure will decrease assuming you hold the temperature very close to constant or exactly constant and allow no more gas molecules to enter the confined space. Conversly, if you increase the pressure on an enclosed gas, its volume will decrease. In this demonstration, you will be able to use this concept to "fasten" a paper cup to the outside of a balloon by increasing the volume of an enclosed gas without adding any gas to the enclosed chamber.

When you shoot a dart with a rubber cup on the end of it against a smooth flat surface, the air inside the cup is forced out as it is flattened against the surface. Then the rubber cup, because of its elastic qualities tries to push away from the surface to again form the cup shape and it increases the volume under the cup, between the cup and the flat surface. This increase in volume is enough to lower the pressure under the cup and it is held onto the flat surface by the now higher atmospheric pressure. We say the dart "sticks" to the surface.

 

Materials:

paper or plastic drinking cup, round rubber balloon.

 

Instructions:

Blow up a balloon to less than half its potential size and close off the opening. Then place the mouth of a paper cup against the side of the balloon and hold it there while you blow up the balloon the rest of the way. You will find the cup becomes "fastened" to the surface of the balloon and will actually hold on fairly tightly until you force the cup to "let loose" of the balloon. Because of the curvature of the surface of the balloon, when it is not blown up all the way, you place the mouth of the cup against its surface, and the balloon extends a small distance into the cup. Then, as you blow up the balloon farther, its curvature becomes such that the volume of the air trapped inside the cup becomes larger, thus lowering the pressure inside the cup. The pressure inside the cup is now less than atmospheric pressure, and the atmosphere outside the cup "pushes the cup" onto the balloon. This situation is diagrammed below
Note that the dark area in the diagram at the left is smaller than that in the diagram on the right where the balloon has been blown up further, owing to the difference in the curvature of the balloon. This increase in volume decreases the pressure inside the cup, thus making the atmospheric pressure larger and it pushes the cup onto the balloon.

 

 

 

Boiling Flask
When water is boiled in a boiling flask the air in the flask is replaced with steam and the air is flushed out through a glass tube and rubber stopper in the mouth of the flask. As the boiling water turns to steam it takes up about 10 times the volume it did when it was liquid water. The pressure remains the same as it is in contact with the outside air. Using a pair of gloves the flask is pick up and turned over, the glass tube in the top is placed into a container of water. As the steam begins to cool and condense, the pressure in the flask decreases and becomes less than normal room air pressure. The room air pressure then pushes water up into the flask much like a drinking straw when you lower the pressure in the straw as you drink lemon aid.

 

Materials

Boiling flask, one hole stopper, glass tube, gloves, beaker, ring stand, ring, burner, wire gauze, test tube clamp, water.

 

Instructions

Place about 20 ml of water in the boiling flask and clamp the flask to the ring stand. Put the ring in place with the wire gauze between the ring and the flask. The glass tubing should be put through the one hole stopper until it protrudes a little over half way into the bulb of the flask. Clamp the flask in place and light the burner and allow the water to boil. After the steam has been coming out of the glass tube for several minutes, turn off the burner. Using the gloves, unclamp the flask and tubing and turn it upside down placing the glass tube in a beaker of water. Allow the flask to cool, as the steam condenses, there is very little steam left in the flask so the pressure goes down. The room air pressure then pushes the water in the beaker up into the flask.

 

 

Crush the Pop Can

(Air Pressure)  

The force of air pressure can be shown simply by boiling water in an aluminum pop can and plunging it top down into cold water. Air pressure is about 15 pounds per square inch so the total pressure on a pop can could be above 200 pounds.

 

Materials

Pop can, burner/hot plate, gloves, water, container larger than a pop can.

 

Instructions

Place a small amount of water in the pop can (20 ml-30 ml) and bring to a vigorous boil. When a good head of steam is coming out of the top, pick the can up, flip it over, and plunge it into the cool water top down. The water will seal the opening of the can and cause the water vapor in the can to condense. As the steam inside the can condenses the pressure inside the can will decrease dramatically. The can will crush due to the higher air pressure outside the can. The can will be hot enough to burn, so don't forget to wear the gloves.

FOSS Connection: Air & Weather - Activity 2: Air Explorations

Grade 4 Weather Unit

 

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