Creative Learning With Compressed Air

4th grade students use pneumatic launchers to learn about the use of compressed air as a means of storing energy

Purpose and Overview: The purpose of this activity is to teach students that compressed gas can be used as a means of storing energy. The students will construct a pneumatic (air powered) launcher. They will compress air in a pipe which will propel a projectile out of the pipe. Most younger students associate moving or propeling something with touching it. This activity can teach them that they can transfer their energy of motion without having to actually touch the object they wish to move.

Materials Required: Per group of 3-4 students

1 12’’ long piece of ½’’ pipe preferably transparent. The transparent pipe will help to illustrate that they can propel the projectile without actually touching it.

2 small pieces of 5/8’’ diameter soft foam. One piece to act as a projectile, one piece to act as the plunger

2 12’’ long ½’’ diameter dowel rods. Make sure your dowel rod fits in the pipe. Most pipes and dowel rods are either slightly smaller or slightly larger than ½’’. Choose the largest size dowel rod that fits into the pipe without having to force it in.  You can use only one per group if you need to. A different end can be used for the two different purposes.

Masking tape

Optional Materials: Per group of 3-4 students

1 protractor

2 12’’ pieces of 2”x4” or 1”x4” wood with a hole (1/2” to 5/8” in diameter) drilled in one end

A square base

1 1” long piece of ½” pipe

1 4”-6” long ½” diameter dowel rod (length of this piece depends on thickness of wood chosen. This piece should be slightly longer than the width of the two pieces of wood plus 1”)

Wood glue or other strong glue

Overview:
When gas is compressed in an insulated environment, the pressure increases as a result of the decrease in volume. This increase in pressure results in a force that resists the compression. As a result, when you compress air, you are doing work on the air. This type of work is known as p-V work.

In this activity, your students will create a device that stores the energy created by doing p-V work on air in a cylinder. This energy will be stored as compressed air. Since the only way to see the energy is to see its effects, the compressed air will be used to launch a small piece of foam as a projectile.

Preparation:
Cut your materials into the correct sizes. Make sure you have enough to split your class into groups of 3-4 students plus enough for one additional launcher. Use the extra parts to build one complete model that you can use as a demonstration. You should probably also cut some extra pieces in case your students mess up and need new materials.

To build your own launcher:

1. Tape one of the small pieces of foam to the end of one of the long dowel rods. This is now the plunger.
2. If you are using the optional materials, you can create a base for your launcher. You make the base by first sliding the small piece of pipe through the small dowel rod. Then slide one end of the dowel rod through one of the 12” pieces of wood and the other end through the other piece. The small piece of pipe should be in the center. Then glue the two pieces of wood to the square base. Tape the 12” long piece of pipe and tape it to the small piece of pipe. Your long pipe should now rotate freely. Attach the protractor to the side of one of the 12” long pieces of wood in order to determine the angle at which you are launching your projectile.
3. Whether or not you build the stand, you can now test your launcher. Use the long dowel rod that is not the plunger and push the remaining small piece of foam into the one end of the pipe. Make sure that it seals that end of the pipe fairly well. Now slide your plunger into the other side of the pipe and slide it back and forth within the pipe. Make sure you do not touch your projectile.
4. If it does not work, check to make sure that your plunger is creating a good seal. At the point where the foam meets the dowel rod, add some masking tape to increase the diameter. Make sure you do not add too much because then you will not be able to slide the plunger into the pipe.

Activity Instructions

1. Divide your class into groups of 3-4 students
2. Give each group a set of materials. Point out which item is supposed to be used as the projectile. If you are using the optional materials, hold onto them for now.
3. Instruct each group to construct something that will launch the projectile out of the pipe.
4. Most if not all of your groups should be at a loss. If any groups have something, allow them to demonstrate what they have.
5. Now teach them about the idea behind p-V work. Tell them that it takes work to launch the projectile and they need to find a way of storing their p-V work until they have created enough to launch the projectile. You can use the example of a hand held bicycle pump to illustrate the idea of compressing air by changing the volume.
6. Now have the students attempt to build the launcher again. Students with a natural inclination towards engineering will quickly realize how to apply what you just instructed them towards creating the launcher.
7. If any groups are still having trouble building their own launcher, try to give them hints and lead them along. If they still cannot build one, then demonstrate with your pre-built launcher.
8. If you are building the base, wait until each of the groups are relatively done building their launcher. Give each of them the necessary materials and instruct them how to build it using the instructions given above.
9. Have each of the groups demonstrate their completed launcher.

Troubleshooting
“The plunger is getting stuck in the pipe and I can’t push it back and forth.”

• You probably got some of the glue from the tape on the pipe. First try replacing the tape. If it is still stuck you need to replace the pipe.
  

“I can’t build up any pressure in the pipe”

• Make sure both the projectile and the plunger are creating enough of a seal on with the pipe. If they are not, you can tape around the circumference in order to make a better seal. Do not use Duct tape or other tapes that use a lot of glue.
  

“Instead of pushing the projectile out, my projectile gets sucked in”

• You are actually creating a negative gauge pressure in the pipe by pulling out air when you pull back with the plunger. Try replacing both the pipe and the tape on the plunger. It is likely that your plunger is getting stuck on the pipe somewhere.

Remember, this should be fun for both you and your students. Be creative and think of outside the box. There are many ways that you can customize this activity to your class, so feel free to experiment. This is an outline, not a set of rules. Have fun.

There is another way to demonstrate a similar principle. The activity involves constructing a pneumatic cannon and using it to show the relationship between the pressure of the compressed air and the distance that the object is propelled. In order to do this activity it is necessary to have launcher with which you can vary the pressure. Although we do not provide instructions on how to construct a pneumatic cannon, you can easily find them by doing an internet search for "pneumatic potato cannon" You should be able to find several different designs. We do not recomend building a regular potato cannon since the explosion required is not in any way controlable. We also want to let you know that any kind of cannon that uses as much pressure as is used in these types of cannons can be extremely dangerous. The project above does not use pressures of any great magnitude so there is no danger with that. If you decide to do this activity, you should do so with the help of someone who has some experience with potato launchers and should take the necessary safety precautions.

Here is a student worksheet that outlines the project (not including construction of a pneumatic cannon)

Transformation of Energy Through Pressurized Air

In this activity, you will explore the transformation of potential energy stored in pressurized gas to kinetic energy of a projectile.

When gas is compressed in a chamber, it gains the potential energy to do work. As the pressurized gas is released, it transfers the stored energy to the projectile by exerting a force on it. This energy is converted into kinetic energy as the projectile accelerates.

Now, let’s first see how pressure affects the distance the projectile is propelled. Set the air cannon at a 45-degree angle and fire it while varying the pressure. Record your results in the chart below. Graph your results.