Unit #1

Activity 15
You Can’t Vacuum the Moon

Activity Overview

Suction, which is usually thought of as a pulling force, does not exist.

Part A: A classic demonstration is done in which the “suction force” causes two plates or hemispheres to stick together. Students then discuss why they stick together. They then kinesthetically model why air gets "sucked" back in when the valve sealing the two plates is opened.

Part B: More demonstrations are done, each one discussed to explain how the visual result can occur through a pushing force, not the previous false notion of a pulling “suction force”.

Learning Objectives

Students will:

• Describe how several observations of "suction" are really differences in gas pressures (pushing forces), not a pulling force.
• Diagram, on a atomic level, air at sea level and air at high altitude, and identify a relative pressure for these two air samples.
• Demonstrate using a kinesthetic model why "nature abhors a vacuum" and the concept of spatial equilibrium.

Conceptual Prologue

Macro-Micro Connection

When the hot air balloon cools, the pressure inside the balloon decreases, so the outside pressure which is now greater pushes air into the balloon, making it heavier and causing it to sink while equalizing the pressure.

Other macro connections:

• Any situation in which suction seems to be apparent can now be explained properly.
• The expansion of sealed snack bags or the explosion of shampoo in suitcases of airplane passengers can be explained by the changes in air pressure

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Science Concepts

Gases exert a pressure through the collective impacts of their atoms on the surface of an object or container. Because of this, pressure is always a positive value. In other words, gasses can only push on things, never pull on things. The lowest pressure achievable occurs when there are no atoms around, such as in outer space.*

Any time something seems to be pulled by suction, the actual cause must be explained by using pushing forces of gasses. Suction is better defined as a net pushing force in a particular direction due to the differences in two gas pressures.

This can best be understood by looking at several examples which can also be used in class discussion:

• Straws: When you “suck” on a straw, you actually expand the volume of gas on one side of the liquid (the side connected to your mouth). By expanding the volume of the gas inside the straw, you spread the atoms out, resulting in a lowering of pressure inside the straw. It is the outside air pressure that is also pushing on the liquid in the straw which pushes the liquid up the straw. You don’t pull the liquid into your mouth, you lower the internal pressure to allow the outside pressure to push the liquid up the straw.
• Vacuum Cleaners: Vacuum cleaners work by creating a lower pressure just inside the opening which touches the floor. By creating a low pressure inside the machine, higher air pressure in the room pushes its way into the vacuum cleaner, taking the dirt with it. Because there is no, or very little, atmosphere on the moon, you can’t create a lower or higher gas pressure inside and outside the machine, so you can’t “suck” any dirt up from the ground. Nothing happens when you turn on a vacuum cleaner on the moon. (You wouldn’t even hear it because some substance, usually air, is necessary to transmit sound waves.)

*Actually, there are some atoms in outer space, but they are so few and far between that the pressure is almost zero there.

Naive Conceptions

• Suction is a pulling force.

In almost any case where the word suction is used, it is referring to some pulling force. For example, sucking a drink up a straw, using a vacuum to suck up dirt, astronauts being sucked out into space if a hole in their ship occurs. In every one of these cases the cause of the suction is a difference in gas pressure between two regions of space. Gas under higher pressure will push its way to an area of lower pressure. A straw works because the air pressure inside your mouth is less than the air pressure outside which pushes the liquid up the straw. A vacuum cleaner creates a region of low pressure inside so that air under higher pressure will push its way into the vacuum cleaner. Air in a spaceship is under high pressure (compared to outside the ship) and pushes its way out of the hole in the ship. There is no such thing as a pulling force of suction - only pushing forces.

Activity Design and Execution

Major Science Concepts:	• Suction • Gas pressure • Air pressure
Assumed Previous Knowledge:	• That the motion of atoms and molecules is related to their temperature or kinetic energy [energy of motion]. • That pressure is the result of multiple impacts of molecules. • Experience with kinesthetic modeling.
Time:	• Part A: approximately 30 minutes • Part B: approximately 20 minutes • Part C: approximately 30 minutes
Materials:	For the demonstrations: • Metal plates or spheres designed for this demonstration (with a cavity in the middle, a rubber gasket that forms a seal between the plates or spheres and a valve connecting the inside of the chamber formed between them and a vacuum pump). • A vacuum pump. • A bell jar. • Some marshmallows. • A balloon. • Some shaving cream. • All the materials necessary to do the the hydrogen spout demonstration.* For each pair of students: • A computer with Workbench software. *This can be done as an extension. See the extensions section for a description of this demo.
Advanced Preparation: (if any)	• None

Investigative Question: What is suction?

Part A:

1. Connect the vacuum pump to the valve on the metal plates or spheres and turn on the pump. After about 1 minute, close the valve and turn off the vacuum pump. The plates or spheres should be stuck together. You might challenge students to try to pull the plates apart.
2. Tell the students that all you did was use the pump to remove the air from between the plates.
3. Ask them to explain why the plates are stuck together. If someone suggests that they are sucked together, ask them what is pulling them together.
4. Explain that actually, suction as a pulling force does not exist, that anytime they see “suction” they are actually seeing the results of pushing forces of gasses.
5. Ask them to try again, using only pushing forces to explain why the plates are stuck together. Eventually it should come out that the air surrounding us is pushing the plates together.
6. Explain to them that "normal" air pressure is one atmosphere or 15 pounds/square inch, meaning every square inch of surface is feeling a force of 15 pounds on it. We live in this constant pressure, so we are used to it and don’t “feel” it. However, we do feel it when the pressure changes.
7. Ask students if their ears have ever popped and when that has happened.
8. Explain to students that air pressure decreases the higher you go in the atmosphere and ask them to draw two pictures in their notebooks: an atomic level picture of air on the surface, and an atomic level picture of air high in the atmosphere where planes fly.
9. Discuss their pictures which should show air on the ground with atoms more closely spaced than air at high altitudes.
10. Explain to students that "suction" occurs whenever there is a difference in gas pressures and that it is easy to forget about the "normal" air that always surrounds us.

Part B:

1. Do several more demonstrations that display “suction” and explore through discussion how each demonstration works. Have the students write notes about these demos in their notebooks:
   
2. Heat a large open flask and then put a balloon on the top. [Over time the balloon will be pushed into the flask due to the decrease in pressure inside the flask and the constant pressure of the air in the room. The pressure in the flask decreases because the atoms slow down as they cool, causing weaker and less frequent impacts against the walls of the flask and the balloon.]
3. Put a balloon in a bell jar, and turn on the vacuum pump. [The balloon will expand when the pressure is lowered around the balloon. When the balloon is not changing size it looks as if nothing is happening. However, billions of atoms are striking both the inside and outside of the balloon. The pressure from the outside atoms counteracts the pressure from the inside atoms. When the vacuum pump is turned on, atoms from outside of the balloon push their way out of the bell jar, lowering the pressure from atoms on the outside of the balloon. The atoms inside the balloon push the balloon outward until the pressure inside and outside the balloon are again equal.]
4. Put some marshmallows in the bell jar, and turn on the vacuum pump. [If you leave the pump on for long enough, the marshmallows will first expand to a certain point, then start to shrink slowly, and eventually seem to stop shrinking. When you let the air back into the bell jar the marshmallows will shrivel up much smaller than their original size. This occurs because the initial lowering of pressure causes all the tiny gas pockets inside the marshmallow to expand just like the balloon in the previous demonstration. At some point the little gas pockets start to pop, letting air out of the marshmallow so that it can be pumped out by the vacuum pump causing the marshmallows to shrink slightly. When the pressure is returned to normal, there is not as much air inside the marshmallow as before, so it shrinks to a much smaller size. Fewer gas molecules inside the marshmallow mean they will take up less space when the pressure is returned to normal.]
5. Put a beaker inside the bell jar, and turn on the vacuum pump. Ask why the beaker didn’t expand. [There are no gas pockets inside the walls of the glass beaker. Even if there were a significant amount of gas tapped inside the glass, nothing would happen because the glass will not stretch.]

Assessment

Have students write several things in their notebooks:

1. Often, if you pack a half filled bottle of shampoo on a plane trip, the top of the shampoo bottle will pop open. Why does this happen? Why wouldn't this happen if you had a completely full bottle of shampoo?
2. How does a suction cup work? Why does it stick to a window or wall if there is no pulling force of suction?
3. If there is no such thing as a pulling force of suction, then how does a straw work? Think about what must be true about gas pressures in different places and why the gas pressure changes when it does.
4. What if you landed on the moon with someone who is compulsive about keeping things clean and he or she tries to vacuum up all that dust around where the spaceship landed? Why can't you vacuum the moon? Explain how differences in pressure allow a vacuum to work and why this wouldn't work on the moon.
5. When the air inside a hot air balloon cools, the pressure initially decreases. After a short time will the pressure inside and outside the balloon be the same, or different. Explain what happens on an atomic level as the balloon cools. Be sure to talk about the air inside and outside the balloon.

   
   

Internal Notes:
• See computer lab J for mock up of this activity.