Wikijunior:Particles/Gas pressure

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Three states of matter Gas pressure Float or sink?

You may have heard about the experiment in which you suspend a glass of water, and the cardboard under it does not fall down. Why does that happen? And what is the secret of the rubber suckers (the real name is suction cup) that hold up your towels? Read this chapter to find out.

Gas pressure[edit]

Picture this. You have 4 table tennis balls, and you put them in a box of 1m x 1m x 1m. That's pretty big. Now you get a weightlifter to shake it. You will here the sound of the table tennis balls hitting against the wall of the box. Let's assume it's, say, once every ten seconds.

Now you put the balls into a box of 50cm x 50cm x 50cm. This time, you realise that the balls hit against the wall six times every ten seconds! That is because as the volume of the box decreases, there is more chance of the table tennis balls hitting against the wall.

When the table tennis balls hit the wall more, they exert more pressure on it. Pressure is when someone drops a heavy weight on you. Remember that gas particles move freely all the time? When gas particles are put into a container, they hit the walls on the container and exert pressure on them. When you hit put them into a smaller container, they would therefore hit the walls more often as well. The pressure exerted is named 'gas pressure'.

Gas pressure is measured in a unit called 'pascal', or simply 'Pa'. A Bourdon gauge can be used to measure gas pressure. We can also connect a pressure sensor to a datalogger to measure gas pressure.

Air pressure[edit]

Air is also a kind of gas. Air is made of nitrogen, oxygen and other elements. As air is all around us, it also creates gas pressure. Gas pressure from air is called air pressure or atmospheric pressure. Mind you, you cannot actually feel air particles hitting your skin!



If there is no air, there is no atmospheric pressure. In 1664, the Mayor of Magdeburg combined two hemispheres (halves of spheres), then he pumped the air out of the sphere. He tried to use horses to separate them, but it did not work. Why is that?

This can be explained by the diagram on the left. When the hemispheres are filled with air, the air pressure on the inside equals out the air pressure on the outside. When the inside is hollow, there is no pressure so the air presses the hemispheres together, making it impossible to separate. A place without any air is called a vacuum.


A daily life application is the suction cup where you pump out the air while pressing it onto the wall. As the air is pumped out, the air particles outside press against the sucker, causing air pressure. That's why it takes some effort to take it off!


Fill in the blanks.

  1. Gas pressure is caused by gas (1) hitting the wall of its (2).
  2. (3) pressure is also known as air pressure.
  3. The power of a (4) can be shown by the (5) hemispheres.


  1. particles
  2. container
  3. Atmospheric
  4. vacuum
  5. Magdeburg