The Caliper

Fun with Balloon Pressure

A new experiment idea for your Gas Pressure Sensor!

Do you know how the pressure inside a balloon changes as it is blown up? The more air you blow in, the higher the pressure, right? Not exactly. We have found that many students (and sometimes teachers!) have misconceptions when it comes to the pressure inside a balloon. Here is one way it can be measured:

Assemble the apparatus shown here. We use the rubber stopper that comes with our Gas Pressure Sensor. Pull the two tapered valve stems out of the large end of the stopper and insert them into the small end. Attach the balloon to the large end of the stopper. Connect the Gas Pressure Sensor to one of the stems. The figure shows the end of a bicycle pump connected to the other valve, but you can inflate the balloon by blowing directly into this valve as well. Begin data collection, then start blowing into the valve (or pumping with the bicycle pump).

Graph of pressure as balloon is inflated

Our results are shown in this graph. Notice that the pressure increases quickly at first, but once the initial stretching of the balloon occurs, the pressure drops back down. As the balloon gets larger and tighter, the pressure begins to rise again until it eventually pops. (If you are blowing into the balloon, you may not be able to blow hard enough to pop it.)

An interesting demo can be done using this information. Two balloons, one fully inflated and one partially inflated, are connected via a tube with a valve in it, as demonstrated by Dave Vernier at right. When the valve is opened, most people expect both balloons to reach the same size. Instead, the small balloon gets smaller. Why? The graph provides the answer. The smaller balloon is behaving like our balloon did in the 7- to 8-second section of the graph. The larger balloon is like the 15- to 20-second section. The smaller balloon, with a higher pressure, will force its air into the larger balloon.