Post on 04-Apr-2018
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D. Crowley, 2008
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To be able to explain how gases and liquids exert pressure
Friday, January 11, 2013
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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
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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
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Pressure Outside?!
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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)
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Pressure in a liquid acts in all directions Pressure in a liquid increases with depth can you explain why?
Low pressure
High pressure
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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
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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
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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
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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)
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Complete the pressure worksheet
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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