Concepts

Class 8
Chapter 11 Class 8 - Force And Pressure

## What is atmosphere?

The layer of air around us is called atmosphere

It extends to many kilometers above earth

## What is atmospheric pressure?

It is the air pressure exerted due to weight of air present in atmosphere

Explanation

We know that layer of air around us is atmosphere

It extends to many kilometers above earth and contains many gases like oxygen,nitrogen etc

All these gases have weight and they exert pressure on all directions

This is called atmospheric pressure

## Is Atmospheric Pressure Same all over the earth?

No

It is less in mountains and highest on sea levels

Reason

Since mountains are high,there is less column of  air above it in atmosphere ,so there is less weight of air and less atmospheric pressure

At sea levels,there is large column of air above,which has higher weight,hence higher air pressure

### Why are we not crushed under atmospheric pressure

It is because pressure inside our body is equal to atmospheric pressure

Explanation

Our body has blood which exerts pressure called blood pressure

This blood pressure balances atmospheric pressure

However ,when we go to high altitudes(mountains),the  atmospheric pressure becomes much less than blood pressure ,so we experience nose bleeding

## Examples of Atmospheric Pressure

### Example 1 - Rubber Sucker

When we stick a rubber sucker on a plane surface,it sticks to surface

When we try to pull it,it does not come off easily

Reason

It is because when we stick the rubber sucker,most of air between sucker and surface escapes out

Hence,there is very less air from inside

Sucker keeps on sticking because of atmospheric pressure exerted from outside

To remove the sucker, we have to use force larger than atmospheric pressure

### Example 2 - Drinking Straw

When straw is put in water and air is sucked out from other end

It causes pressure inside the straw to reduce

However,there is atmospheric pressure on surface of water

This forces water to come out from the  straw

### Example 3 - Syringe

It has a nozzle and piston

When syringe is dipped in water and piston withdrawn

Pressure inside syringe is lowered

However,there is atmospheric pressure on surface of liquid

It forces liquid inside the syringe

### Example 4 - Dropper

It also has a nozzle and rubber bulb

When bulb is pressed, air present in tube and bulb escapes in the form of bubbles.

Due to this, Pressure inside bulb decreases

However, there is atmospheric pressure on surface of liquid.

When we release the bulb, the water moves inside the tube.

Water moves from higher pressure (from water kept) to lower pressure (inside the glass tube).

Note - Atmospheric Pressure is exerted from all side

## Atmospheric Pressure is Exerted from all sides

Many people believe that since atmosphere exists above us,it  exerts atmospheric pressure only from upside

But it is not the case

Actually air pressure is exerted from all sides

### Example 1 Magdeburg Hemisphere Experiment

This experiment was undertaken in city of Magdeburg by German Scientist called Otto Van Guericke

He took 2 hemispheres made of copper

Both hemispheres tightly fit into each other

One of hemisphere was attached to a pipe which was attached to vacuum pump

Air was struck out from vacuum pump

Hence due to absence of air from inside the hemispheres,the two hemispheres could not be separated even by  2 teams of 8 horses

This was because  The atmospheric pressure was exerted from all sides which held the 2 hemispheres together

### Example 2 Holding Water in Inverted Glass Tumbler

We fill a tumbler fully with water

We put a card over it in such a way that there is no air in glass tumbler

If we invert water,card does not fall

This is due to atmospheric pressure exerted from below

There is no matching air pressure inside the tumbler to make card fall

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