Air Pressure

6.1 Understanding Air Pressure

• The kinetic theory of gases

states that gas consists of tiny

particles.

• These particles are constantly

in random motion and collide

against the walls of the

container.

• These collisions produce the

gas pressure.

Atmospheric Pressure

Atmospheric pressure

• The thick layer of air surrounding the Earth is called

the atmosphere.

• The air particles in the atmosphere collide with one

another and collide against the surfaces of all things.

• These collisions result in air

pressure or

atmospheric pressure.

GAS PRESSURE AND ATMOSPHERIC PRESSURE

Atmospheric pressureAtmospheric pressure

• Atmospheric pressure decreases with the

altitude or the height above sea level.

Show existence of atmospheric pressureShow existence of atmospheric pressure

• The cardboard does not fall and the

water remains in the glass even

though it’s not supported by

anything.

• Explanation:

The force caused by the

atmospheric pressure acts on the

surface of the cardboard is greater

than the weight of the water in the

glass.

Show existence of atmospheric pressureShow existence of atmospheric pressure

• When a can filled with hot

water is closed and is cooled

down rapidly by pouring cold

water on it, it will crash

instantly.

• Explanation:

When the air inside the can is

cooled, its pressure decreases.

The high atmospheric pressure

outside exerts a great force on

the can and causes it crashes.

Instrument Used to Measure Atmospheric PressureInstrument Used to Measure Atmospheric Pressure

-Fortin Barometer

-Aneroid Barometer

-Simple Barometer

Aneroid BarometerFortin Barometer Simple Barometer

• Atmospheric pressure is 76 cm mercury at sea

level. The unit of air pressure is atmosphere

and the air pressure near sea level is one

atmosphere.

Low pressure or high pressure Low pressure or high pressure

• If in some area the pressure is higher than it is

in the surrounding area, we say that it is an

area of high pressure.

Air pressure

GAS PRESSURE AND ATMOSPHERIC PRESSUREGAS PRESSURE AND ATMOSPHERIC PRESSURE

• Gas pressure is the force per unit area

exerted by the gas molecules the gas molecules as they collide

with the walls of their container.

• Atmospheric pressure is the pressure caused

by the weight of the air weight of the air above us.

• The factors which affect the air pressure are:

(a) Volume of air

(b) Temperature of air

Volume of the container

• When the volume of the container is reducedvolume of the container is reduced, the air

particles become closer.

• The air particles collide more frequently among

themselves and against the walls of the container.

Hence, the air pressure increasesair pressure increases or vice versa.

Temperature

• When the temperature is increasedtemperature is increased, the air

particles gain more energy and move faster.

• The rate of collisions between the particles

and against the walls increases.

• This increases the air pressureincreases the air pressure or vice versa.

Application of air pressure

• Drinking straw

• Rubber sucker

• Syringe

• Vacuum cleaner

• Lift pump

Drinking Straw

• When the water is sucked,

the air inside the straw is

sucked out.

• The air pressure inside the

straw is reduced.

• The higher air pressure

outside the straw forces

the water to enter the

straw.

Syringe

• When the piston is pulled, the air pressure in the syringe decreases.

• The higher air pressure outside forces the liquid into the syringe.

• When the piston is pushed, the air pressure in the syringe increases .

• The higher pressure in the

syringe forces the liquid to

flow out.

Rubber sucker

• The higher air pressure

at Y presses the pump

and it sticks firmly on

the surface of wall.

• The air pressure that

presses on it is enough

to support light

objects.

Y

• When the plunger is

pulled upwards, the

higher pressure in Z

forces the blockage

out.

ZZ

Plunger / Suction pump

• The air pressure at P

is higher.

• It pushes the water

up the siphon to

beaker Q.

Siphon

6.3 Gas under High Pressure

Gas cylinder

• When gases are compressed under high

pressure, they turn into liquids.

• This property is used in the storage of gas in

gas cylinders.

• A larger volume of

gas can be stored in

the form of liquid.

Aerosol cans

• Similarly, perfumes, insecticides and paints

can be stored in the form of liquids in aerosol

cans.

Safety precautions when using gas

under high pressure.

(a) Gas cylinders should be kept away from heat

sources.

(b) Gas cylinders should be placed vertically.

( c) Gas cylinders should be kept in rooms with

good ventilation.

(d) Always check the gas cylinders for leakage.

(e) Light a match first before turning on the gas.

Safety precautions when using gas

under high pressure.