7/30/2019 Pressure in Gases & Liquids

1/13

D. Crowley, 2008

7/30/2019 Pressure in Gases & Liquids

2/13

To be able to explain how gases and liquids exert pressure

Friday, January 11, 2013

7/30/2019 Pressure in Gases & Liquids

3/13

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

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

Solids Liquids Gases

7/30/2019 Pressure in Gases & Liquids

4/13

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 thewall more often, increasing its pressure

7/30/2019 Pressure in Gases & Liquids

5/13

Pressure Outside?!

7/30/2019 Pressure in Gases & Liquids

6/13

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

Squash the gas into the space, and it will result in 2x the pressure Squash the gas into the space, and it will result in 4x the pressure

Boyles Law - there must be a fixed amount of gas (i.e. none escapes) andthere must remain a constant temperature

Gas particles randomly hit the side wall

the space, and

the particles will hit

the wall 2x as often(pressure doubles)

the space, and the particles

will hit the wall 4x as often

(pressure quadruples)

7/30/2019 Pressure in Gases & Liquids

7/13

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

Low pressure

High pressure

7/30/2019 Pressure in Gases & Liquids

8/13

The greater the depth, the greater the pressure (the weight of the waterabove 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 thedepth, thegreater the

pressure

The denserthe liquid, the

heavier it is

7/30/2019 Pressure in Gases & Liquids

9/13

Liquids (like solids) cannot be compressed (squashed) as there are nogaps between the particles

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

7/30/2019 Pressure in Gases & Liquids

10/13

How can you stop a car at 70mph using just your foot? How aboutstopping 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 (masterpiston applies the force which is transmitted to the slave piston)

Force of 10N Master Piston10cm2

Slave Piston20cm2

Pressure at master = 10N 10cm2

Pressure = 1N/cm2

P A

F

Force on slave= 1N/cm2x 20cm2

Force = 20N

7/30/2019 Pressure in Gases & Liquids

11/13

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

This is how you can stop a very heavy object, traveling at high speed, bysimply 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 theenergy in)

7/30/2019 Pressure in Gases & Liquids

12/13

Complete the pressure worksheet

7/30/2019 Pressure in Gases & Liquids

13/13

Pressure = ForceArea

Force = Pressure x Area

Pressure = 100

5Pressure = 20N

Force = 20 x 25

Force = 500N

So a force of 100N to the mastergives a force 5x bigger of 500N tothe slave