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HANDS-ON PRESSURE: Are You Stronger than Air?

The Magdeburg Sphere

Diandra Leslie-Pelecky Edited by Anne Starace

Abstract

Air pressure is something we often take for granted because it’s everywhere all the time. This activity provides a graphic illustration of exactly how strong air pressure can be. If there is air inside and outside an object, the air pressure will be similar. If air is removed from the inside, there is only the air pressure from the outside with no pressure inside. Find out if you are stronger than air! Keywords: air pressure, force, differential pressure, Magdeburg sphere Funded by the National Science Foundation and the University of Nebraska

Hands-On Pressure: The Magdeburg Sphere- - V 2.0 Copyright the Board of Regents of the University of Nebraska 2002

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Content Standards

K 1 2 3 4 5 6 7 8 1.2.1 4.2.1 8.2.1 8.3.2

History & Process Standards K 1 2 3 4 5 6 7 8 8.8.3

Skills Used/Developed:


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TABLE OF CONTENTS I. OBJECTIVES...............................................................................................................................4 II. SAFETY......................................................................................................................................4 III. LEVEL, TIME REQUIRED AND NUMBER OF PARTICIPANTS.......................................4 IV. REQUIREMENTS ...................................................................................................................5

A. Materials .................................................................................................................................5 B. Facilities..................................................................................................................................5

V. INTRODUCTION .....................................................................................................................5 VI. PROCEDURE...........................................................................................................................9

A. SETUP ....................................................................................................................................9 B. ACTIVITY............................................................................................................................10 C. CLEANUP ............................................................................................................................11 D. VARIATIONS......................................................................................................................11

VI. FREQUENTLY ASKED QUESTIONS.................................................................................12 VII. TROUBLE SHOOTING .......................................................................................................13 VIII. HANDOUT MASTERS ......................................................................................................13 IX. REFERENCES: ......................................................................................................................13


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I. OBJECTIVES Students will: -understand the cause of air pressure. -participate in an activity that will demonstrate to them the strength of air pressure.

II. SAFETY • Make sure that when the audience volunteers pull on the spheres that they don't pull so hard

that they might fall. Every once in a while, the spheres will pull apart. This is especially true when using the hand pump, because the vacuum obtained using this method is not as good as that using the electrical pump.

• Keep audience away from the pump - the motor can get very warm. There is a belt guard on

the pump to ensure that no one can catch fingers in the belt. Never use a pump without a belt guard.

III. LEVEL, TIME REQUIRED AND NUMBER OF PARTICIPANTS

A. LEVEL The activity can be adjusted for K-5 and 6-12.

B. TIME REQURED 10 - 15 minutes.

C. NUMBER OF PARTICIPANTS Groups of 3 - 7 work best. Note that the Pressure Presentation is available for larger groups.


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IV. REQUIREMENTS A. Materials

Magdeburg Sphere (in the ‘Hands-On Pressure box) Vacuum pump (electrical or mechanical) Illustrations

B. Facilities Electrical outlet Space about 6’ by 4’, clear of any breakable or sharp objects

V. INTRODUCTION We often take air pressure for granted, because it’s always present; this leads us to not appreciate just how strong air pressure can be. This demonstration will provide a graphic illustration of its strength. Air pressure is due to the motion of air molecules, which are constantly bumping around. When an air molecule bumps into something, it exerts a force on that thing. We define pressure to be the ratio of force to area, or

P FA

=

Forceoutwardfrom insidethe eye

Force fromair

molecules

Figure 1: The normal state for your eyes: pressure from inside the eye pushed out and pressure from the air molecules outside pushes in.

Forceoutwardfrom insidethe eye

No forcefrom air

molecules

Figure 2: If there are no air molecules, the pressure from the inside of the eye is not counteracted. The result is that your eyes would bug out of your head!


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The force of the air molecules on things (such as you!) thus creates an air pressure. Science fiction movies like to place their heroes and heroines in danger by having them find themselves outside in space without a space suit. Perhaps the best example of this was in the Arnold Schwarzenegger movie, “Total Recall,” which showed a dream sequence of Arnold being outside on the planet Mars. The lack of air pressure did yucky things to him, such as pulling his eyeballs out of their sockets. This is because your eyes exert a pressure outwards that balances the air pressure they feel, as shown in Figure 1. If you took away all of the air, the eyes would be pushing outward and there would be nothing pushing back! Figure 2 shows why Arnold’s eyes bugged out. We call the absence of air molecules a vacuum. Vacuums are found in many places, including in the refrigeration system in your car and in your thermos bottle. But how do you remove the air molecules? In the early days of science, people didn’t have the ability to eliminate air pressure. Otto von Guericke (1602-1686) was a physicist born in the town of Magdeburg Germany. Guericke built the first vacuum pump in 1650 and used it to study vacuums. He showed that sound cannot travel in a vacuum, that candles will not burn in a vacuum and how strong air pressure is. The Magdeburg Hemispheres were named after his home town. The original hemispheres were built for a demonstration for the Emperor Ferdinand III. Guericke placed the two hemispheres together and removed the air from inside them. This leads to the situation shown in Figure 3: there are air molecules pushing on the outside of the sphere, but there are no molecules pushing on the inside of the sphere. This leads to a net force pushing the hemispheres together. This force is entirely due to air pressure. If you try to pull the spheres apart, you’ll find that you can’t!

In fact, Guericke used two teams of horses to pull the evacuated hemispheres apart and even they couldn't do the job. This indicates that the force is pretty large. Air pressure is 101 kPa. (kPa = kiloPascal.) The Magdeburg hemispheres have a diameter of 4 inches (which is 0.1 meter). One hemisphere, therefore has an area:

[ ]A r=12

4 2π

where r is the radius of the sphere (which

in this case is 0.05 m). Plugging in the numbers, we would get:

Vacuum

Figure 3: The Magdeburg Sphere in its evacuated state.


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( )( )[ ]A m

A

=

=

12

4 314 05 2. .

.0157 m2

We also have to convert Pascals to a more familiar unit:

1 Pa = 1 Nm2

where N stands for ‘Newton’, which is the unit for Force in the metric system. The prefix ‘kilo’ means 1000, so 1 kPa = 1000 Pa = 1000 N/m2

P FA

F PA

=

=

So we can now calculate the force that is exerted on one of the hemispheres:

F PA

F

F

=

=

=

=

(101 kPa)(.0157 m

(101 kPa)1000

1 kPa(.0157 m

F 1590 N

2

Nm 22

)

)

To give you an idea of how much force that is, 1590 N would correspond to 357 pounds (1 lb = 4.45 N). This means that you could hang 357 pounds from the sphere without pulling it apart!


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When the air is let back in, the spheres can be easily separated, because now there is air pressure on both sides of the sphere, so the net force on each hemisphere is zero, as shown in Figure 4.

Variations This experiment can be performed with two plungers, dent pullers, suction cups, etc. Some of the equipment is difficult to find. The principles are the same: you have to have two halves of something that you can put together and remove the air from the space between them.

Open

Figure 4: when the valve is opened, air molecules rush inward. Now there is pressure both inside and outside the walls of the sphere, so the net force on the sphere is zero.


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VI. PROCEDURE A. SETUP • The hemispheres have a rubber ring, which must fit between the two halves. The ring must

be clean and free of any small particles that will disrupt the seal. Do not use vacuum grease - if you use too much, the grease gets in the way of the seal.

• If using the electrical pump, make sure you have an outlet nearby. Check the pump hoses to make sure that they will attach properly to the valve of the spheres. (i.e. the diameter of the hole in the hose must be comparable to the diameter of the valve.)

• Clear an area about 6 feet long and 4 feet wide to let people try to separate the hemispheres. Make sure that there is nothing sharp or breakable in the immediate vicinity.

• Become familiar with the valve on the sphere - make sure you know which way to turn it to open or close it.

Assembled vacuum pump:


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Vacuum pump pieces (besides white “body”)

B. ACTIVITY • Allow the audience to look at the hemispheres to make sure that there’s no glue or other

tricks. • If using the electrical pump:

• Put hemispheres together with the rubber ring in between. Make sure the valve is open. Connect the hemispheres to the vacuum pump. Evacuate until the dial on the vacuum pump reads near 30.

• Close the valve before turning off the pump and removing the hose. • If using the hand pump

• Put the hemispheres together with the rubber ring in between. Attach the tubing to the spheres. Make sure the valve is open.

• Pump furiously. The vacuum achieved by this technique is not as good as that found using the electrical pump. You might want to have an audience member do the pumping.


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• Ask for two volunteers to try to separate the two halves. Sometimes, the sphere will pull apart if the seal wasn’t very good, so make sure that you have other people behind each of your pullers to catch them if they should go flying backward.

• Don’t let the volunteers twist the spheres - this can rupture the seal. • After everyone has tried to get the spheres apart, open the valve and let the air back into the

sphere. Continue to hold the spheres as if they were still strongly sealed. Ask for a volunteer to help you get them apart. They will come apart easily.

C. CLEANUP • Make sure that the rubber seal doesn’t get lost.

D. VARIATIONS

This demonstration can be performed with two plungers, dent pullers, suction cups, etc. You can even sometimes do it with the palms of your hands. Place the two objects opposite each other, as shown in Figure 5. Then push them together, being careful to match the edges. If you’re doing it right, you will be pushing all of the air out of the space between the plungers, as shown in Figure 6. This is the same as the Magdeburg hemispheres: there is air pushing from the outside, but no air molecules on the inside to push back. Note, though, that this variation doesn’t work as well as using a vacuum pump, so be extra careful when letting kids pull on the ends: in some cases, they will be able to yank the objects apart.

Figure 5: A variation involving plungers Air molecules willbe pushed out here

Figure 6: Pushing the air molecules out of the space between the plungers.


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VII. FREQUENTLY ASKED QUESTIONS What is that rubber thing? Students are frequently convinced that the rubber gasket seal is, in fact, a trick element that secretly holds the two hemispheres together. You can explain its purpose as follows:

If I put two pieces of metal together, even though they look very smooth, there are still gaps between the pieces. If you were able to looking at the metal on the same scale as the size of an atom, it would look something like Figure 7. The holes are small enough that you can’t see them with just your eyes, but they are big enough that atoms can escape. The rubber gasket is flexible, so that when you put it between the two surfaces of the metal, it fills in the holes, as shown in Figure 8. When you twist the hemispheres, you are displacing the rubber seal and allowing air molecules to enter the sphere, which in turn creates a force outward. When people can pull the spheres apart, make sure that they aren’t twisting while pulling.

Air molecules can get in through the holes!

Figure 7: What the two surfaces of a metal would look like if you could look at them on the length scale of an atom. Atoms and molecules can fit through the gaps.

The rubber gasket seals the holes so thatair molecules can’t get in or out

Figure 8: Sealing holes using the rubber gasket.


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VIII. TROUBLE SHOOTING Problem: Spheres do not hold together 1) The vacuum is not strong enough. Try the following:

a) check the seal to make sure that it isn’t dirty b) make sure that the valve on the sphere is open when you’re pumping c) pump for a longer period of time - watch the gauge on the front of the electrical

pump -- it should read about 30 mm for a good strong vacuum

IX. HANDOUT MASTERS Graphics are stored in the file Magdeburg power point.

X. REFERENCES For more information about air pressure: http://kids.mtpe.hq.nasa.gov/archive/air_pressure/ Magdeburg Hemispheres are available from a number of scientific education companies: VWR/Sargent Welch (1-800-SARGENT) WL1509A (about $55) You can buy vacuum pumps from the same places, but they are expensive. You can use a hand-operated pump, such as the Nalgene model, available from: VWR/Sargent Welch (1-800-SARGENT) WLS-71480-C (about $50-$80) This pump has a gauge on it, which really helps you know when you’ve pumped enough.


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