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Air Pressure

Keith Warne

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Air Pressure• Gas molecules move

rapidly in all directions.

• Collisions exert a force

on objects and the

container.

• The number of

collisions determines

the total force being

exerted.

• Increasing number of

collisions would

INCREASE Pressure.The pressure is given by

the AVERAGE FORCE

PER UNIT AREA.

Standard Atmospheric Pressure

• The pressure that supports 760.0 mm of

mercury is known as STANDARD

ATMOSPHERIC PRESSURE or 1

ATMOSPHERE. (1 atm) taken at sea level.

• Since liquid pressure can be found by

P = gh

= density of liquid = 13 590 kg/m3 (mercury)

g = gravitational field strength = 9.807 N/kg

h = height = 0.7600 m

p = gh = (13590)(0.76)(9.807) = 101 300 Pa Pa =

1atm

H = 760mm

1 Pa = 1 N/m2

ATMOSPHERIC

pressure

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Water Barometer• The pressure that supports 760.0 mm of

mercury is known as STANDARD

ATMOSPHERIC PRESSURE or 1

ATMOSPHERE. (........................) taken at

sea level.

• What would the equivalent height of

water be?

• Since liquid pressure can be found by

P = gh

= density of water = .................kg/m3

g = gravitational field strength = 9.807 N/kg

h = height = ...........?? m

p = gh

ATMOSPHERIC

pressure

WATER!WATER!

H = ?? m

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Atmospheres mm Hg kPa

1 760 100

2 ??? ???

0.5 ??? ???X ??? X ???

X ??? X ???

atm

kPa

Pressure Conversions

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Aneroid Barometer

• An aneroid barometer has

a vacuum chamber that

expands or contracts

with changes in air

pressure.

• The

expansion/contraction

causes a series of levers

to pull on a chain that is

connected to a spindle.

• The spindle turns and

moves the needle

attached to it on the dial,

showing the air pressure. From World Book © 2002 World Book, Inc., 233 N.

Michigan Avenue, Suite 2000, Chicago, IL 60601. All

rights reserved. World Book i llustration by Zorica Dabich

HIGHPRESSURE

HIGH PRESSURE

HIGH PRESSURE

Aneroid Barometer

CHANGE

LOW PRESSURE

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Pressure Demonstrations - Plunger

High Pressure

If the plunger

is forced

down the air is

forced out of

the cavity.

Low Pressure

If the plunger is pulled up –

the pressure inside the cavity

is DECREASED so the

atmospheric pressure on the

outside is GREATER and it

holds the plunger against the

surface.

Air escapes!

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Heated air escapes replaced

by STEAM

Can crushing Experiment

Water boils - atmospheric

pressure balanced by steam

inside.

Steam condenses to water – no gas

inside to balance atmospheric pressure

– ATMOSPHERE CRUSHES CAN!

WATER

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Ball Pressure• A ball needs pressurized

air inside it to enable it to bounce.

• The higher the pressure the better (HIGHER) the ball will bounce.

• If the atmospheric pressure increases a higher pressure will be needed in the ball.

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Aerodynamics & Air Pressure

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Pumping air out of the jar causes the balloon to inflate!

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Hi -

This is a SAMPLE presentation only.

My FULL presentations, which contain loads more slides (with all the gaps filled in) as well as

other resources, are freely available on my resource sharing website:

www.warnescience.net

(paste into your browser if link above does not work)

Have a look and enjoy!

Keith Warne