D. Crowley, 2008

To be able to explain how gases and liquids exert pressure

Tuesday, April 18, 2023

Remember, there are 3 states of matter – solids, liquids, and gases (only gases can be compressed (squashed))

In liquids and gases the particles are moving around – as they bump into the surface the force of the particles hitting it causes pressure

Solids Liquids Gases

Pressure is caused by the force of particles hitting the side of the container

Why does tyre pressure increase when you pump more air into it?

Particles in gases are normally spaced far apart

When a tyre is pumped up, the air gets compressed (squashed) inside – this means there are more gas particles trapped inside

As there are more particles inside the tyre, particles hit the sides of the wall more often, increasing its pressure

Pressure Outside?!

Squashing a gas into a smaller space causes the pressure to increase in proportion to how much you squash it

Squash the gas into ½ the space, and it will result in 2x the pressure

Squash the gas into ¼ the space, and it will result in 4x the pressure

Boyle’s Law - there must be a fixed amount of gas (i.e. none escapes) and there must remain a constant temperature

Gas particles randomly hit the side wall

½ the space, and the

particles will hit the wall 2x as

often (pressure doubles)

¼ the space, and the particles will hit the wall 4x

as often (pressure quadruples)

Pressure in a liquid acts in all directions Pressure in a liquid increases with depth – can you explain why?

Low pressure

High pressure

The greater the depth, the greater the pressure (the weight of the water above compresses the water below)

Low pressure

High pressure

Pressure = gravity (10N/kg) x depth (m) x density (kg/m3)

Pull of gravity (on

Earth)

Greater the depth, the greater the

pressure

The denser the liquid,

the heavier it is

Liquids (like solids) cannot be compressed (squashed) as there are no gaps between the particles

As liquids cannot be compressed, they can be used to send forces from one area to another – these are known as hydraulic systems

How can you stop a car at 70mph using just your foot? How about stopping a motorbike at this speed with two fingers?!

Pressure is transmitted through a liquid

Hydraulic systems can be used to increase the size of a force (master piston applies the force which is transmitted to the slave piston)

Force of 10NMaster Piston

10cm2

Slave Piston20cm2

Pressure at master = 10N ÷ 10cm2

Pressure = 1N/cm2

P A

F

Force on slave= 1N/cm2 x 20cm2

Force = 20N

The slave piston has a larger area than the master piston, so the force exerted by the slave piston on the brakes is greater than the force exerted at the master (i.e. you get much more force from slave when you gently press the master)

This is how you can stop a very heavy object, traveling at high speed, by simply using your foot / your fingers

The larger force at the slave piston comes at a price – it only moves the slave piston half the distance (so the energy out is the same as the energy in)

Complete the pressure worksheet

Pressure = Force ÷ Area

Force = Pressure x Area

Pressure = 100 ÷ 5

Pressure = 20N

Force = 20 x 25

Force = 500N

So a force of 100N to the master gives a force 5x bigger of 500N to the slave