PressureThe idea of pressure

High pressure, low pressurePressure calculations

Pressure in gases and liquids

The idea of pressure

• If a force is concentrated on small area, we say that the force is creating a high pressure.

• If the force is spread over a bigger area, the pressure will be less.

• example:• If you lie on a bed, the force of your weight is spread

over a large area.(shallow dip)• If you stand on the bed, your weight will be

concentrated on a much smaller area.(much deeper dip)

High pressure, low pressure

• Pressure is caused when a force acts on an area.• A big force acting on a small area creates a high pressure.• Example:• 1. A knife is better at cutting when its blade is sharp. Then the

force pushing on it will be concentrated on a small area to create a high pressure.

• 2. A snowboard is wide and does not sink into the snow.• 3. The Inuit people wear wide snowshoes for the same

reason.• Then the pressure will be low and they will not sink into the

snow.

Pressure calculation

• P=F/A, pressure=force/area• Example:• Force of 8N pressing on an area of 4m2.• P=8N/4m2=2N/m2• The force of 2N is acting on each square metre

of the surface.• UNITS of pressure is the pascal (Pa)• 1 Pa is the same as 1N/m2 (examples on p.128)

Pressure in gases and liquids• Balloon-when the balloon has been blown up, the

pressure of the air inside presses outwards.• Diving deep-divers have to be careful if they want to

dive to a great depth in the sea. The weight of the water above presses down on them.

• The pressure of the atmosphere, The air is less dense at the top of the mountain, its pressure is less. We call the pressure of air atmospheric pressure.

• Atmospheric pressure is greatest at sea level, because we have the weight of the whole atmosphere above us, pressing down.

Particles and pressure

• We can understand the pressure of liquids and gases another way, using the particle model.

• Particles of a gas in a box, moving around very rapidly, make many collisions each second and all these tiny forces add up to cause the pressure on the walls of the box.

Summary

• Liquids and gases can cause pressure.• The pressure of a gas or liquid is caused by its

weight pressing downwards.• The particles of gas or liquid collide with the

walls of its container; this causes the pressure.