Pressure

Pressure is the amount of force working on a unit of area.

When a force is working on a small area it exerts a large pressure.

•Atmospheric pressure is caused by gases in the atmosphere.•It is estimated that there is 500 million tonnes of gas in the atmosphere.

Pressure in Gases

The air that surround the earth, theatmosphere, is made of gases that

havemass and exert pressure.

Measuring Atmospheric Pressure

• Atmospheric pressure is measured using an instrument called a barometer.

• Atmospheric pressure decreases as we go up into the atmosphere.

Barometer is used to measure pressure.

Normal atmospheric pressure supports 76cm of mercury in a barometer

Atmospheric pressure is not constant and changes according to temperature and moisture in the atmosphere

Mercury Barometer

• It is used by measuringthe height of mercury in the tube.

• Normal atmospheric pressure can support acolumn of mercury76 cm high (1013hectopascals).

TorricellianVacuum

hAtomophericPressure

Barometer

Aneroid Barometer

The pressure causes the lid of the metal box to move up or down.

Scale

Needle Spindle

Lever

Vacuum Box

An Altimeter is a barometer used to measure how high in the atmosphere we are (they are used in aeroplanes).

Exp: To demonstrate air (atmospheric) pressure.

Method:1. Place a small amount of

water in the bottom of a metal can.

2. Boil the water in the can until the steam drives all the air out.

3. Take care to avoid burning and seal the can tightly.

4. Allow time for the can to cool.

Result: The can collapses.

Conclusion:The pressure of the air causes the can to collapse.

SteamCap on

Air Pressure

Exp: To demonstrate atmospheric pressureMethod:

1. Fill a glass to the brim with water.2. Cover the top of the glass

completely with a piece of cardboard.

3. Holding the cardboard in place turnthe glass upside down.

4. Remove your hand.

Result: The cardboard stays in place and

the water will stay in the glass.Conclusion: Atmospheric pressure is pressing upwards and keeping the water in place.

Glass Jar

Water

Air pressure

Isobars are pressure lines on a map.

Atmospheric Pressure and the Weather

• High pressure – settled, dry weather

• Low pressure – wind and rain

What are fronts?

• A warm front is the edge of a warm air mass. • It looks like this on a weather chart: • Ahead of a warm front there is usually a wide

belt of rain. • Behind the warm front the temperatures

increase and the rain gradually eases.

• A cold front is the edge of a cold air mass. It can move twice as fast as a warm front.

• It looks like this on a weather chart: • Ahead of a cold front there is a narrow rain

belt. • Behind a cold front the weather is brighter but

the temperature is cooler.

• An occlusion brings weather similar to a warm front.

• It looks like this on a weather chart: • It usually forms when a warm front catches up

with a cold front.

Iosobars• The lines on a weather chart are

called isobars.• Isobars join places of equal pressure.• The isobars circle areas of high or

low pressure.

Pressure in a liquid increases with depth

Pressure in a liquid increases with depth

Everyday Uses of Pressure

Presure in liquids andpistons can be used to

exertgreat force:• Breaking systems on

articulated trucks.• Lifting gear on fork-lift

trucks.• Lifting the bucket on

a JCB.

Pressure Cooker• Food can be cooked faster in a pressure cooker since

the boiling point of water increases with pressure

• A lady weighs 800 N and is standing in a pair of stiletto heels, each of area 2 cm2.

• An elephant weighs 27000 N and the total area of its feet is 1800cm2.

• Which exerts more pressure on the ground – the lady in stilettos or the elephant?

• [2010]• The diagram shows a tank full of water. The

mass of the water in the tank is 48 000 kg. • Calculate the approximate pressure that it

exerts on the base of the tank. • Give the units of pressure with your answer

The Earth’s atmosphere seen from space is the thin curve at the top of the photo.

Name the force that holds the atmosphere to the Earth.

This force gives the atmosphere weight and causes atmospheric pressure.

Define pressure and give the unit for pressure.Why does atmospheric pressure decrease with height?

• The diagram is an Atlantic weather chart.• Use the chart to predict two weather

conditions that you might expect for Ireland.• Explain why low atmospheric pressure causes

one of the weather conditions that you have given.

• A block of wood of mass 3kg has a length of 15cm and a height of 4cm.

• What is its weight?• Find the pressure exerted by the block if:

• 1. it is lying flat• 2. it is standing on its edge

• A car of mass 1000kg is standing on four tyres. Each tyre is in contact with 65cm2 of the ground. What pressure is exerted on the ground?