Everyone who teaches physics knows very well that sometimes a simple device or experiment can help to make a concept clear. In this paper, inspired by "The Jumping Pencil"1 by Martin Gardner, I will discuss a simple demonstration device that can be used to start the study of air pressure.
Punch a hole (2–3 mm diameter) near the bottom of an empty, hard plastic bottle. In my country plastic bottles made from polyethylene terephthalate (PET) are widely used for colas and mineral water and are available in two sizes, 0.30 L and 1.5 L. Choose a bottle that has reasonably stiff sides. Place a small balloon in the bottle and spread its neck over the bottle's mouth as shown in Fig 1. Inflate the balloon by blowing into it, and then seal the hole with a small piece of transparent tape. Now you have a bottle with a balloon inside it that, remarkably, remains inflated!
Figure 1. Here are three experiments you can perform with this simple device:2,3
1. Put the bottle on the table in front of your students and ask them why the balloon does not deflate, despite the fact that the mouth of the balloon is open to atmospheric pressure. You can pass the bottle around so that your students can examine it themselves. My experience shows that they will not be able to give the answer at once. After some brainstorming you can show what happens when you remove the tape.
Now put your finger on the hole and ask the class if it is possible to inflate the balloon.
2. Ask your students if it is possible to inflate a balloon without blowing into its open end. After hearing their answers, keep the mouth of the bottle toward them and suck air out of the hole. If your bottle is sufficiently rigid,4 the balloon will inflate inside the bottle (Fig. 2.).
3. The third experiment that you can do is to show that the bottle/balloon system can act as a simple vacuum pump. Inflate the balloon (still inside the bottle) by blowing into it. Seal the hole with your finger and immerse the bottle in a water-filled bucket so that the mouth of the bottle is above the water surface and the hole is just beneath it. Now remove your finger from the hole. The balloon begins to deflate and water enters the bottle. The water level in the bottle is higher than that in the bucket, showing that the balloon maintains a pressure inside the bottle that is less than atmospheric.
Figure 2.