f4. c3.3

8/9/2019 F4. C3.3

1/10

UNDERSTANDING GAS

PRESSURE AND

ATMOSPHERIC PRESSURE

8/9/2019 F4. C3.3

2/10

Gas pressure

is the force per

unit area

exerted by the

gas molecules

as they collide

with the wallsof their

container

Atmospheric pressure is the

pressure caused by the weight

of the air above us.

Atmospheric pressure acts

equally in all directions.

Atmospheric pressure varies

with the height of the object

above sea level. It decreases

with the altitude or the height

above sea level. At higher

altitudes, the density and the

temperature of the air are

lower. As a result, the

frequency of collisions of the

molecules is lower. Hence,

atmospheric pressure is lower.

GAS PRESSUREATMOSPHERIC

PRESSURE

8/9/2019 F4. C3.3

3/10

ACTIVITIES TO SHOW EXISTENCE OF

ATMOSPHERIC PRESSURE 1

The explanation for this phenomenon is that theresultant force caused by the atmosphericpressure acts on the surface of the cardboard isgreater than the weight of the water in the glass

Fill the glass to the topwith water and wet rimslightly.

Lie the cardboard onthe top of the glass.

Hold the card firmly inplace and turn the glassover.

Take away your hand.

The cardboard does notfall and the water

remains in the glass.

8/9/2019 F4. C3.3

4/10

ACTIVITIES TO SHOW EXISTENCE OF

ATMOSPHERIC PRESSURE 2

A metal can containingwater is heated untilthe water in itvaporizes.

Allow the steam to exit

from the mouth of thecan.

The can is then cappedand cooled down withtap water.

As the result , the can is crushed and crumpled.

The explanation for this phenomenon is that the

pressure inside the metal can decrease and the

external atmospheric pressure ,which is higher

compresses the metal can.

8/9/2019 F4. C3.3

5/10

APPLICATIONS OF ATM.

PRESSUREWhen we suckWhen we suck

through a straw,through a straw,

the air pressure inthe air pressure in

the straw isthe straw islowered. Then thelowered. Then the

pressure of thepressure of the

atmosphere actingatmosphere acting

on the surface ofon the surface of

the drink in thethe drink in theglass pushes theglass pushes the

water up the strawwater up the straw

and into our mouth.and into our mouth.

(I) DRINKING STRAW(I) DRINKING STRAW

8/9/2019 F4. C3.3

6/10

When the sucker ispressed into place,most of the air behindit is squeezed out. The

sucker is held inposition by thepressure of atmosphereon the outside surfaceof the rubber. If the

seal between the suckerand the surface isairtight , the suckerwill stick permanently.

(II) RUBBER SUCKER(II) RUBBER SUCKER


PRESSURE

8/9/2019 F4. C3.3

7/10


PRESSURE Pulling up the piston

reduces theatmospheric pressureinside the cyclider.The atmospheric

pressure on theliquid surface thenpushes the luquid upinto the syringe.

If we then hold theplunger in place and

lift the syringe out ofliquid , none will fallout. This is again dueto atmosphericpressure

(iii) Syringe(iii) Syringe

8/9/2019 F4. C3.3

8/10

A vacuum cleaner

produces only a partial

vacuum. The fan inside

the cyclinder blows air

out of the vents. Which

less air inside , the air

pressure there drops.

The atmospheric

pressure outside thenpushes air up the

cleaner hose ,carrying

dust and dirt with it.

(IV)VACUUM CLEANER


PRESSURE

8/9/2019 F4. C3.3

9/10

MEASURING ATMOSPHERIC PRESSURE

BY USING FORTIN BAROMETER

The simple barometer Fortinis along glasstube that has

been filled with mercury andthe inverted into a dish ofmercury.

The mercury column rises orfalls according to thepressure of air on the

mercury in the dish. The space above the mercury

column isa vacuum so itexerts no pressure on the topof the mercury column.

If the vertical height ofthe mercury is h cm ,

thefore the atmosphericpressure reading is

h cm mercury .

(i)The Simple Fortin barometer

8/9/2019 F4. C3.3

10/10

HOW DOES THE HEIGHT ,H OF THE

FORTIN BAROMETER VARY?

THE HEIGHT, H WILLTHE HEIGHT, H WILLREMAINS UNCHANGEDREMAINS UNCHANGED

WHENWHEN

(i) the diameter of theglass tube increases

(ii) the glass tube istilted

(iii) the glass tube islowered further into thedish

(iv) the glass tube islifted up from the dish

(v) the quantity ofmercury in the dish isincreased

The height, h will

increases when the

barometer is slowly

submerged in water.THE HEIGHT, H WILLTHE HEIGHT, H WILL

DECREASES WHENDECREASES WHEN

(i) the vacuum space in

the glass tube is filled

with gas(ii) the barometer is

carried out to a

mountain

f4. c3.3

Documents