Pressure

Pressure in solids An elephant will exert less pressure than a person wearing high heels. This is because the weight of the elephant is spread over a larger surface area. Sir jumps quickly to his feet He’s got the point (-he’s got a scar!) The pressure acting on his seat, Is force per unit areaaaaaaaaagh!

Pressure is Force per Unit Area Force ÷ Area = Pressure Measured in N/m2 or N/cm2 N/m2 = 1Pa (pascal)

Pressure in solids This man can lie on a bed of nails because by having many nails he has increased the surface area so his pressure is less.

Reducing Pressure These are all examples of where pressure in reduced by increasing surface area

Increasing pressure

Pressure in solids It acts in the direction of the force only. The smaller the area the higher the pressure. The larger the area the smaller the pressure

Calculating pressure

Pressure in liquids

Pressure in liquids at depth Pressure in liquids acts in all directions. The greater the depth the greater the pressure, as the mass (and therefore the force exerted) is greater.

Pressure is exerted throughout the liquid and in all directions. The deeper the water the greater the pressure because the weight of the water is greater

Calculating pressure at depth Pressure = 10 x depth x density (N/m2 ) ( N/kg) (m) (kg/m3) The hydrometer This is used to measure the density of milk and other liquids. It floats to different depths in different liquids, depending on their densities

Upthrust and Pressure All liquids exert an upthrust because the pressure inside the liquid increases as you go deeper. This means that the pressure on the bottom of an object is greater than on the top, and so there is a resultant force upwards

Pressure in liquids Liquids will find their own level

Pressure in Liquids Higher density higher pressure.

Using pressure in liquids Hydraulic machines It was easier to push on the small syringe that on the big syringe. This is because a force acting over small area creates a high pressure. This pressure is then transmitted through the fluid and acts over a large area. This results in a larger force on the plunger of the large syringe.

Using pressure in liquids

The drivers foot pushes the piston to exert pressure on the liquid The drivers foot pushes the piston to exert pressure on the liquid. This pressure is transmitted to the pistons on each side of the large disc on the axle. The pressure makes the pistons squeeze the disc like the brakes on a bicycle to slow down the car. Exactly the same pressure is applied to the other brakes on the car. If the pistons have twice the area of the master piston, they will exert twice the force that the driver applied with her foot. The force is magnified by the increased area of the pistons.

Using Pressure in Liquids The moving arms on this mechanical digger use hydraulic systems

Hydraulic Jack

Pressure in Gases We are living at the bottom of a ‘sea’ of air called the atmosphere, which exerts a pressure on us (just as the sea squeezes a diver). Atmospheric pressure is 100 000N/m2

Collapsing Can Experiment Before the pump is switched on, molecules are hitting the outside and inside with equal pressure. After the pump is switched on, there are almost no molecules inside the can and the pressure of the molecules outside the can crushes it.

Gases can be Compressed ...Pressure increases

Exploding Marshmallow At first the atmospheric pressure on the outside of the balloon balances the air pressure inside ....

...when you pump out the air A vacuum is created in the bell jar air pressure in the balloon / marshmallow is higher than the pressure in the bell jar so the balloon expands

Using pressure Sucking creates a negative pressure in the straw. The high air pressure acting on the surface of the liquid pushes the liquid up the straw.

Using Air Pressure The higher pressure outside the sucker holds it in place. As you push the sucker down air is forced out of the cup creating a negative pressure

Measuring Pressure Bourdon Gauge: The higher pressure makes the tube straighten out slightly, and this movement is used to turn a pointer

Measuring Pressure Aneroid Barometer: This uses a flexible metal can which has had the air taken out of it. A strong spring stops the atmosphere from completely crushing the can. If the air pressure increases, the top of the can is squeezed down slightly. If the air pressure decreases, the spring pulls up the top of the can. This small movement is magnified by a long pointer

Measuring Pressure The Mercury Barometer: The column of mercury is held up by air pressure. As the air pressure varies from day to day (depending on the weather), the height of the Mercury varies. The distance to measure is shown in the diagram. A height of 760 mm is called Standard Atmospheric Pressure

PRESSURE SUMMARY Pressure is the force per unit area It is measured in N/m2 ( Pascals), N/cm2 , Bar. The larger the area the more the force is spread so the less the pressure. The smaller the area the less the force is spread, so the greater the pressure. In liquids and gases the fluids will flow from an area of high pressure to an area of low pressure until the pressure is equal throughout. Pressure in a liquid is greater at depth. Pressure in liquids and gases are exerted equally throughout. The same pressure at equal depths.