IGCSE Physics
Pressure
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Contents
Pressure
Pressure
Pressure in liquids
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What is pressure?
Pressure is exerted whenever a force is applied over an area.
1.
2.
If the same force is applied in each picture, which arm exerts
the highest pressure on the board?
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High and low pressure
1.
2.
The arm applies a force
to the board via a fingertip.
The same force is now acting
over a larger area – the palm
has a greater surface area
than the fingertip.
The force acts over a
small area and so
produces a high pressure.
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A lower pressure is produced.
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Calculating pressure
Pressure is the force per unit area
and is calculated using this formula:
pressure =
force
F
area
P x A
Pressure is measured in:
Newtons per square metre (N/m2), which are also called
pascals (Pa).
Pressure can also be measured in:
Newtons per square millimetre (N/mm2);
Newtons per square centimetre (N/cm2).
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Which type of pressure?
The same force spread over a larger area means a lower
pressure.
Which type of shoes would be best for walking over a muddy
field – flat soles or heels?
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Which type of pressure?
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The boots have flat soles
and spread the person’s
weight over a large surface
area.
In contrast, the heeled
shoes have a smaller
surface area and so
exert a higher pressure.
These boots exert a low
pressure on the ground.
These shoes are likely
to sink into soft ground.
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Using low pressure
A force spread over a large area means low pressure,
e.g. skis and snowboards.
The large surface area
of the board means the
skier exerts very little
pressure on the snow.
This means he slides
over the top of the snow
and does not sink into it.
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Using high pressure
A force concentrated on a small area means high pressure,
e.g. high heeled shoes, needles, ice skates, sharp knives.
The high pressure of the
blade of an ice-skate melts
the ice and helps the skater
slide across the surface.
The narrow blade of a knife
means that it exerts a high
pressure and makes it easier
to cut fruit and vegetables.
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Contents
Pressure
Pressure
Pressure in liquids
Summary activities
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Pressure in a liquid
Pressure in a liquid:
 acts in all directions;
 increases with depth.
A liquid can be used to transmit
pressure from one place to
another.
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Pressure in a liquid
The relationship between pressure and depth is shown
by a water bottle with holes along its length.
low pressure
high pressure
Pressure (N/m2) = 10 N/kg x depth (m) x density (kg/m3)
The pull
The greater the The denser the liquid,
of gravity depth, the higher
the heavier it is.
the pressure
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Hydraulics
Hydraulic systems use the principle that pressure is
transmitted throughout a liquid.
They are used to transfer movement from one part of a
machine to another without linking the parts mechanically.
All hydraulic systems use two pistons linked via a pipe
carrying a special oil called hydraulic fluid.
Force
applied
here
Force
transferred
here
Pressure inside all parts of the
hydraulic system is the same
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Hydraulic brake
All hydraulic brake systems (e.g. in a car) use a small
master piston and a bigger slave piston.
hydraulic fluid
slave pistons
foot pedal
master piston
The master piston is used to apply a force. This puts the
liquid under pressure. The pressure is transmitted to the
pistons on all four wheels of the car.
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Hydraulic brake – pressure equations
The pressure exerted by the master piston on the
hydraulic fluid can be calculated using this equation:
pressure =
force applied
area of master piston
The pressure is transmitted to the slave pistons and so the
force exerted by the slave piston can be calculated using:
pressure =
force exerted
area of slave piston
force exerted = pressure x area of slave piston
The slave piston has a larger area than the master piston.
So, the force exerted by the slave pistons on the brakes is
greater than the force exerted by the driver on the brake pedal.
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Hydraulic brake – calculations
The master piston of a car has an area of 5cm2.
1. If a force of 10N is applied to the master piston, calculate
the pressure created in the brake pipes.
2. If the slave piston has an area of 50 cm2, calculate the
force exerted on the brake disc.
Calculations:
1. At the master piston, P = F = 10 N = 2 N/cm2
A
5cm2
2. At the slave piston, F = P x A = 2 N/cm2 x 50cm2 = 100 N
So, the force exerted on the brake disc is ten times greater
than the original force applied to the master piston.
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Hydraulics activity
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