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1. PRESSURE IN SOLIDS

2. PRESSURE IN LIQUIDS

3. PRESSURE IN GASES

4. MEASURING 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 feetHe’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.

Increasing pressure

Reducing pressure

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

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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.

Mini Lab: Pressure in Liquids

Aim: To show the relationship between pressure and depth.

Hypothesis:

Materials: Can, hammer, nail, ruler, tape and water

Procedure:

Variables: » Independent:» Dependent» Control

Results:

Calculations:

Discussion: Use the data from the lab to show your findings.

P = mass x gravity / area

P = (density x Vol) x gravity / areaP = density x ( Area x height) x gravity / area

Areas cancel out.

P = density x height x gravity

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 hydrometerThis 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 PressureAll 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 liquidsHydraulic 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.

Pressure in Liquids

Answer these questions:

Why does the dam wall need to be thicker at its base?

Why does the boat float?

Why must the diver breathe out as he surfaces?

Answers

The pressure at the bottom is greater so the wall must be stronger.

Upthrust from the water is spread over a large area so the upward force is greater than the weight.

When the diver breathes at depth the air is at a higher pressure. As he surfaces the water pressure decreases so the air in his lungs would expand and burst his lungs.

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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

However, the pressure inside our bodies is similar, so we don’t notice the pressure of the air.

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.

Magdeburg Hemispheres

Magdeburg hemispheres

The pressure of the air outside pushes the cups closed when there is a vacuum inside.

When air enters the pressure outside and inside are the same and they are easily pulled apart

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 Air pressure

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 PressureThe 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

Using pressure – force pump

Plunger out –Negative pressure water pushed up by air pressure.

Plunger in – Pressure exerted on liquid, inlet valve forced closed, so water pressure forces open outlet valve

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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.