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IGCSE Physics

Pressure

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Pressure

Contents

Pressure in liquids

Pressure

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Pressure is exerted whenever a force is applied over an area.

If the same force is applied in each picture, which arm exerts the highest pressure on the board?

1. 2.

What is pressure?

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The arm applies a force to the board via a fingertip.

The force acts over a small area and so produces a high pressure.

1.

High and low pressure

The same force is now acting over a larger area – the palm has a greater surface area than the fingertip.

A lower pressure is produced.

2.

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Pressure is measured in:Newtons per square metre (N/m2), which are also calledpascals (Pa).

Pressure can also be measured in:Newtons per square millimetre (N/mm2);Newtons per square centimetre (N/cm2).

pressure =area

force

P x A

F

Pressure is the force per unit area and is calculated using this formula:

Calculating pressure

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The same force spread over a larger area means a lower pressure.

Which type of pressure?

Which type of shoes would be best for walking over a muddy field – flat soles or heels?

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The boots have flat soles and spread the person’s weight over a large surface area.

These boots exert a low pressure on the ground.

Which type of pressure?

In contrast, the heeled shoes have a smaller surface area and so exert a higher pressure.

These shoes are likely to sink into soft ground.

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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.

Using low pressure

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A force concentrated on a small area means high pressure, e.g. high heeled shoes, needles, ice skates, sharp knives.

The narrow blade of a knife means that it exerts a high pressure and makes it easier to cut fruit and vegetables.

The high pressure of the blade of an ice-skate melts the ice and helps the skater slide across the surface.

Using high pressure

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Pressure

Contents

Pressure in liquids

Pressure

Summary activities

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Pressure in a liquid:

acts in all directions;

increases with depth.

Pressure in a liquid

A liquid can be used to transmit pressure from one place to another.

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high pressure

low pressure

The relationship between pressure and depth is shown by a water bottle with holes along its length.

Pressure (N/m2) = 10 N/kg x depth (m) x density (kg/m3)

The pull of gravity

The greater the depth, the higher

the pressure

The denser the liquid, the heavier it is.

Pressure in a liquid

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Hydraulic systems use the principle that pressure is transmitted throughout a liquid.

Force applied

here

Pressure inside all parts of the hydraulic system is the same

Force transferred

here

Hydraulics

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.

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All hydraulic brake systems (e.g. in a car) use a small master piston and a bigger slave 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.

Hydraulic brake

foot pedal

master piston

slave pistons

hydraulic fluid

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The pressure exerted by the master piston on the hydraulic fluid can be calculated using this equation:

pressure = force applied

area of master piston

Hydraulic brake – pressure equations

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.

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

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The master piston of a car has an area of 5cm2.

Hydraulic brake – calculations

Calculations:

1. At the master piston, P = F = 10 N = 2 N/cm2

A 5cm2

1. If a force of 10N is applied to the master piston, calculate the pressure created in the brake pipes.

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.

2. If the slave piston has an area of 50 cm2, calculate the force exerted on the brake disc.

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Hydraulics activity