PURPOSE: To demonstrate surface tension in a counterintuitive way.
DESCRIPTION: Use two identical balloons. Blow up one balloon on the tube and clamp it. Then blow up the other balloon to a different size and slip it onto the other end of the tube. Q: When you remove the clamp, what will happen?: (a) the small balloon will get smaller and the large one larger, (b) the two balloons will become equal, or (c) they will stay the way they are. A: The small balloon will blow up the larger one, and get smaller, due to surface tension effects. The rubber is thicker in a smaller balloon, and thus produces greater surface tension.
SUGGESTIONS: Let the students vote before doing the experiment. Hold the smaller balloon on top as you release the clamp; then suggest that gravity pulls the air down into the lower balloon.
REFERENCES: (PIRA 2A10.51) See Demonstration Reference File for relevant papers. See also F3-03: SURFACE TENSION - SOAP BUBBLES.
EQUIPMENT: Two balloons with connecting tube and clamp.
SETUP TIME: None.
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