On the table there are 2 special bell jars: you can make them a ‘vacuum’. In other words, you suck out all the air. The arrow on the pressure gauge is then approximately at ‑1 bar.
In one bell jar you can hear the bell ringing when there is air inside. Sucked the air out? Then you’ll no longer hear a thing.
That’s because sounds are actually vibrations. These vibrations need air, water or any other substance to move around. If you suck all the air out of the bell jar, the vibrations can no longer move. The bell still rings, but the vibrations no longer reach your ear.
In the other bell jar you can see the flag fluttering when there is air inside. Sucked the air out? Then it doesn’t move.
The flag flutters because the fan blows air particles towards it. They collide with the flag and that’s why it moves. If there are no more air particles, nothing collides and it doesn’t move.
You may still be wondering whether there is any air left in the bell jars? In fact, a few air particles are always left behind. So it’s not a perfect vacuum, but fortunately it’s good enough to hear and see the effect.
Vacuum for food
Want to make food keep longer? Then you can vacuum pack it. Then, there is (almost) no air, and therefore no oxygen, in the packaging. This prevents the growth of bacteria that need oxygen to survive and reproduce.
But do be careful, because there are also bacteria that don’t need oxygen. So always keep your food cool.