Why does reducing volume increase pressure?

Class practical

Apparatus and Materials

For each pair of students

• Kinetic model kit, two-dimensional
• Marbles, coloured, 20 - 24, about 1 cm diameter
• Metal tray lined with cork mat

Health & Safety and Technical Notes

Watch out for marbles which may roll accidentally onto the floor.

Read our standard health & safety guidance

Procedure

1. Agitate the tray, listening to the sound of the marbles hitting the sides of the tray.
2. Put a ruler across the tray to reduce the area occupied by the marbles. As it will not be easy to hold the ruler still, it helps to put a book in the tray and to hold the ruler firmly up against it.
3. Now agitate the tray in the same way as earlier. You should hear an increase in noise as the marbles strike the tray walls at a greater rate than before.

Teaching Notes

• This model shows the increase in pressure caused by a decrease in volume.
• If students add a lot more marbles, they can listen to what happens to the pressure (it rises).

This experiment was safety-tested in March 2005