IGCSE Physics Pressure 1 of 37 20 Boardworks

IGCSE Physics Pressure 1 of 37 20 © Boardworks Ltd 2005 2004

Contents Pressure in liquids 1 20 2 of 37 © Boardworks Ltd 2005 2004

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? 1 20 3 of 37 © Boardworks Ltd 2005 2004

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. 1 20 4 of 37 A lower pressure is produced. © Boardworks Ltd 2005 2004

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/m 2), which are also called pascals (Pa). Pressure can also be measured in: Newtons per square millimetre (N/mm 2); Newtons per square centimetre (N/cm 2). 1 20 5 of 37 © Boardworks Ltd 2005 2004

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? 1 20 6 of 37 © Boardworks Ltd 2005 2004

Which type of pressure? 1 20 7 of 37 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. © Boardworks Ltd 2005 2004

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. 1 20 8 of 37 © Boardworks Ltd 2005 2004

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. 1 20 9 of 37 © Boardworks Ltd 2005 2004

Contents Pressure in liquids Summary activities 1 10 ofof 20 37 © Boardworks Ltd 2005 2004

Pressure in a liquid: l acts in all directions; l increases with depth. A liquid can be used to transmit pressure from one place to another. 1 11 ofof 20 37 © Boardworks Ltd 2005 2004

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/m 2) = 10 N/kg x depth (m) x density (kg/m 3) The pull. The greater the depth, The denser the liquid, of gravity the higher the heavier it is. the pressure 1 12 ofof 20 37 © Boardworks Ltd 2005 2004

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 transferred here Force applied here Pressure inside all parts of the hydraulic system is the same 1 13 ofof 20 37 © Boardworks Ltd 2005 2004

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. 1 14 ofof 20 37 © Boardworks Ltd 2005 2004

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. 1 15 ofof 20 37 © Boardworks Ltd 2005 2004

Hydraulic brake – calculations The master piston of a car has an area of 5 cm 2. 1. If a force of 10 N is applied to the master piston, calculate the pressure created in the brake pipes. 2. If the slave piston has an area of 50 cm 2, calculate the force exerted on the brake disc. Calculations: 1. At the master piston, P = F = 10 N = 2 N/cm 2 A 5 cm 2 2. At the slave piston, F = P x A = 2 N/cm 2 x 50 cm 2 = 100 N So, the force exerted on the brake disc is ten times greater than the original force applied to the master piston. 1 16 ofof 20 37 © Boardworks Ltd 2005 2004