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Two identical party balloons are shown in the image on the right. One balloon is filled with air to its normal inflated size while the other balloon is inflated with air to roughtly half its normal size. The ends of the balloons are then connected to a pipe with a CLOSED tap at the middle. As the tap is closed, the balloons retains their shape. Now the tap is opened so that air can flow freely either way through the pipe. So what will be the final result? |  |
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Almost every people I have asked replied since the two balloons are identical, air will flow from the bigger balloon to the smaller balloon until the two balloons are equal in size. It is also easy to see that if the balloons are two open container filled with water and joined by a pipe, the water level will indeed equalise when the tap is opened. This line of reasoning is nice and comfortable, but is this true for the two balloons? |  |
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If you had ever blown up a balloon, you sure will remember it is hardest to blow it up first and then it will become much easier afterwards. In blowing up a balloon, the air is actually doing work by expanding the rubber surface. At first the rubber surface is thick(relatively) so more energy is needed. As the rubber surface gets thinner, less energy is needed to expand the surface. What happens is both balloons try to de-flate through the pipe but energy released by the smaller balloon in the process is higher then the energy needed to inflate the bigger balloon. At a result, air is forced into the larger balloon and eventually we ends up with the image at right. This is very hard to believe at first and most people refused to accept the truth even after reading a careful explanation. If you don't, you have my full sympathy. |  |