A Balloon Question

A Physics student blows up two initially identical balloons. However the student blows up one balloon to a larger size than the other. The student then carefully connects the openings of the two balloons together. (See diagram below). The question is, when the two balloons are connected, what happens?

• A. Nothing happens, the two balloons are both surrounded by air, therefore at the same air pressure -- so nothing will happen, the size of the balloons does not change.
• B. The larger balloon has more air in it, so it will decrease in size as it forces air into the smaller of the two balloons, until both are the same size.
• C. The smaller balloon will force air into the larger balloon until nearly all the air from the smaller balloon will be in the larger balloon.
• D. I once saw a clown blow up balloons and make little animals.

See if you picked correctly.

Answer

A Balloon Question - Answer = C

Although it may seem contrary to our intuition, the smaller balloon actually forces air into the larger balloon. The smaller balloon has a smaller radius of curvature, therefore when the balloon's tension (elastic force) is broken into components parallel (tangential) and perpendicular (radial) to the surface , it can be seen (see diagram below) that radial component of the smaller balloon is greater than the radial component of the larger balloon. The result is that the internal pressure is greater in the smaller balloon than in the larger balloon. Therefore, the smaller balloon will force air into the larger one.

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