M-ST 1: Pressure in a small soap bubble is greater than in a large bubble.

PIRA: Unknown

Equipment: divided blow tube with clamp to shut off one path part of the time, soap solution in flat dish; two similar spherical rubber balloons, glass tube flared at both ends. These can be found in the soap bubble box in Set 11, Cabinet 1, Shelf 4.

Procedure: Dip the bell ends of the divided tube in the bubble solution and blow on the single end to form bubbles about an inch in diameter. Then with the clamp shut off the path to one bubble and increase the size of the other to about two inches indiameter. Remove the clamp and hold a finger over the open end. The small bubble will decrease in size while the large one increases. Unfortunately, the scale of the demonstration is small, but this can be improved by shadowing the bubbles on a distant wall with a divergent beam of light from a point source or virtual point source. Do not project an image as the heat will destroy the bubbles before the experiment is complete. Blowing larger bubbles defeats the demonstration because too much time is necessary for the effect to be visible since pressure differential is small and flow impedance large.

The same principle can be demonstrated with two rubber balloons connected by a tube. If the balloons have the right size opening, this opening stretched over the flared tube end furnishes adequate seal. With two hands surrounding the larger balloon compress it against the table top until it is slightly smaller than the other balloon. Then release it and it will continue to pump up the other balloon without help. (You can tell by the feel whether you have gone far enough to bring this about.) Then repeat the experiment by forcing the air back the other way. The rubber does increase tension as it is stretched, but over quite a range this increase is not rapid enough to defeat the behavior being like that of the soap film where the tension is constant. You will note considerable hysteresis in the behavior of rubber balloons, so it is best to work with moderate dispatch.