Sausages, balloons and bubbles

There are some interesting demonstrations using air, where the scientific principles may be too difficult to explain to your pupils. However, if their interest is stimulated and they continue to study science at a higher level, they may have that “Eureka!” moment in the future.

Blow up that sausage 1 - Make a long sausage out of a bin bag. Cut across the bottom and down one side. Fold in half (long sides together) and join along the edges with glue, stitching or stapling. Tie a knot in one end or close with an elastic band.

Ask if anyone thinks they can blow up the bag with one breath. Your volunteer will probably put his or her mouth close to the neck of the bag and blow. The pupil will not fill the bag in this way!

Figure 4 - Air collected when bag is close

However, if you hold the bag open in front of you, say 30 cm from your face, and blow, you will find that much more air enters the bag than was contained in your lungs. The scientific principle involved is called the Bernoulli Effect. A fast moving stream of air is surrounded by an area of low atmospheric pressure. In fact, the faster the stream of air moves, the more the air pressure of the moving air drops. When you blow into the bag, higher pressure air in the atmosphere forces its way into the area of low pressure created by the stream of air from your lungs. In other words, air in the atmosphere is drawn into the long bag at the same time as you are blowing into the bag.

Figure 5 - Air collected when bag is further away

For ready-made bags or for more scientific information see this link on the web. This activity can explain how some weather systems work.

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