Compressed air pushes
Blowing up a balloon means packing a large amount of air into a limited volume — that is, compressing it. A U-tube can show that this produces a pressure.
Activity 15.2
- Pour water into a U-tube. The water levels in its two arms X and Y will be equal.
- Blow up a balloon and tie its neck with a thread, in a knot that can easily be undone.
- Then connect it to arm X of the U-tube and tie it there with another thread.
- Now slowly loosen the first knot on the balloon.
Figure to be added
The observation
Once the knot is removed the water level in arm X falls while the level in arm Y rises.
What the difference in levels tells us
All points at the same level in a liquid are at the same pressure. So the levels in the two arms being equal before the balloon was connected showed that the pressures applied above those two surfaces were equal — atmospheric pressure on both sides.
After the balloon is connected, the level in arm Y goes above the level in arm X. That shows the pressure at the water surface in X is now greater than the pressure at the water surface in Y.
The conclusion
The pressure in arm X rose because the compressed air in the balloon applied a pressure to that arm. That is, a compressed gas produces a pressure.