D. Crowley, 2008

To be able to explain how gases and liquids exert pressure Tuesday, May 19, 2015

 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 SolidsLiquidsGases

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/m 3 ) 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 10N Master Piston 10cm 2 Slave Piston 20cm 2 Pressure at master = 10N ÷ 10cm 2 Pressure = 1N/cm 2 PA F Force on slave= 1N/cm 2 x 20cm 2 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