Page 133 - Science Coursebook

9.6 Pressure in gases and liquids

Particles and pressure
We can understand the pressure of liquids and gases
another way, using the particle model which you
learnt about in Unit 5 of Stage 7.

The diagram shows the particles of a gas in a box.
The particles move around rapidly, bouncing off the
walls of the box.

Each collision of a particle with the wall causes a
tiny force on the box. There are vast numbers of
particles inside the box, and they are moving around
very rapidly. This means that there are many, many
collisions each second and all these tiny forces add up
to cause the pressure on the walls of the box.

The particles of a liquid cause pressure in the same How the particles of a gas cause pressure.
way. However, the particles are much closer together
than in a gas, so they collide with the walls of their
container more often, creating greater pressure.

Activity 9.6

Pressure and particles

The pictures show particles of a gas
in a box. These pictures will help
you to answer two questions:

• What happens to the pressure of
a gas if more gas is squashed into
a container?
• What happens to the pressure
of a gas if it is squashed into a
smaller volume?

Be prepared to explain your ideas to
the rest of the class.

Summary
• Liquids and gases can cause pressure.
• The pressure of a gas or liquid is caused by its weight
pressing downwards.
• The particles of a gas or liquid collide with the walls of its
container; this causes the pressure.

9 Forces in action 131