BALLOONS ON A SPOOL
INTRODUCTION
This activity
definitely brings with it the element of surprise. Very few parents,
children, or teachers predict the observed behavior. We
originally performed this activity as a demonstration using a three-way
connector between balloons and mouth. However, as a hands-on activity,
the method required more dexterity and breath control than young children
had. We later read about a similar activity using a clothespin and
plastic tubing in Janice VanCleave's book1
and another method using a thread spool between the balloons.2
We have found that the latter method works well with pairs of children
performing the activity hands-on rather than as a demonstration.
This activity is appropriate for elementary school children in grades 3-6,
especially to introduce concepts about polymers and surface tension or
stretch. PURPOSE The purpose of this activity is
to determine how the size of a balloon affects the air pressure and the
surface tension or stretch or contracting pull in the balloon. MATERIALS (for 2 participants)
PROCEDURE
EXPLANATION
Unlike some authors, we believe that the observed behavior is not the result of differences in air pressure inside the balloon. We argue below that these internal air pressures are equal. The behavior is instead due to the larger surface tension or stretch or contracting pull of the smaller balloon.
The unexpected result of air flowing from the smaller balloon (deflating the smaller balloon) to the larger balloon (inflating the larger balloon) can be understood as follows.
Air pressure inside each of the two balloons can be thought of as balancing two effects from the outside: (1) external air pressure (often called atmospheric pressure) pushing inward on the balloon from the outside and (2) internal pressure developed because of the surface tension or contracting pull of the rubber in the balloon. For each balloon (and even for an automobile tire) the internal air pressure must equal the sum of the external air pressure plus the contracting pull caused by the surface tension or degree of stretch of the balloon.
Consider first the situation when at least one twist-tie is still in place and the balloons are not open to each other. In both balloons the internal air pressure must equal the sum of the external air pressure plus the contracting pull caused by the surface tension or stretch of the balloon. If that were not true, one of the balloons would either expand or contract. Therefore, the air pressures inside the two balloons must be the same, and we can neglect that factor in trying to understand our results.
Consider next the situation after both twist-ties have been removed, and ask the following question. In the process of blowing up a balloon, at what point in terms of size of the balloon is blowing it up most difficult - when you are just starting to blow up the balloon, or when the balloon is already at least half full? Most people would give the former answer - when you are first starting to blow up the balloon. That is the time at which your ears are most likely to pop or to hurt. That is the time at which you decide to stretch the balloon more before trying to blow it up. That is the time at which some people cannot blow up a balloon at all! Once you get past that point, anybody can blow the balloon up farther. The answer indicates that the surface tension or stretch of the balloon is greater when it is small than when it is large. That larger surface tension or stretch or contracting pull causes the smaller balloon to force its air (but not quite all of it) into the larger balloon as we observed.
ADDITIONAL SUGGESTIONS New balloons are required for each child for health and sanitation reasons because each child holds the balloon to the mouth and blows up a balloon. The same balloon can then be used by the same child for The Invincible Balloon or for Balloon Stretch. Thus, this activity can easily be the first one in a series of balloon activities.
REFERENCE
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