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๐ฌ Think about it
Air is invisible and seems to weigh nothing — yet press a rubber sucker to a wall and it clings stubbornly. A whole ocean of air presses on you right now with the weight of a small car, and you don't even feel it. How can empty-looking air push so hard? Let's find out.
What is the atmosphere?
- The atmosphere is the envelope of air surrounding the Earth.
- It contains nitrogen, oxygen, argon, carbon dioxide and other gases.
- It extends many kilometres above the Earth's surface.
Example:
The air you breathe right now is part of this atmosphere pressing all around you.
Does air exert pressure?
- Yes — air pushes on objects, like a sheet covering a paper plate.
- A larger covering sheet is harder to lift, though its weight is the same.
- So air exerts a force, and force per unit area is pressure.
- The pressure exerted by the air around us is called atmospheric pressure.
Does air exert pressure in all directions?
- Blow into a balloon and it inflates in every direction.
- The air inside pushes equally on all the balloon walls.
- So air, like liquids, exerts pressure in all directions.
Example:
A balloon stays round, not flat, because air pressure pushes out evenly on every part of it.
In this Activity, we will cover a paper plate with sheets of different area to feel the force of air on them.
Materials needed
A paper plate, a stick, and two identical chart-paper sheets (about 70 cm by 56 cm).
Procedure
1. Invert the paper plate, attach a stick to it, and place it on a plain surface.
2. Fold one sheet twice, make a centre hole for the stick, and place it over the plate. Lift using the stick and feel the effort.
3. Replace it with the second, unfolded sheet (with a centre hole) and lift again.
4. In which case is lifting easier — folded (small area) or unfolded (large area)?
2. Fold one sheet twice, make a centre hole for the stick, and place it over the plate. Lift using the stick and feel the effort.
3. Replace it with the second, unfolded sheet (with a centre hole) and lift again.
4. In which case is lifting easier — folded (small area) or unfolded (large area)?
Observation
More effort is needed to lift the plate with the large unfolded sheet than with the small folded one, even though the sheet's weight is the same.
Explanation
Air pushes down on the covering sheet, and this force is greater over a larger area. Force per unit area is pressure, so air exerts atmospheric pressure on objects.
โ Summary
- Bigger sheet, harder to lift
- Air pushes on the sheet
- This is atmospheric pressure
Why does a rubber sucker stick to a surface?
- Pressing the sucker pushes out most of the air inside it.
- This lowers the air pressure inside the sucker.
- The higher outside air pressure holds the sucker against the surface.
- To pull it off, you must overcome this pressure difference.
In this Activity, we will press a rubber sucker onto a surface and feel how strongly the atmospheric pressure holds it.
Materials needed
A good-quality rubber sucker and a smooth flat surface.
Procedure
1. Press the rubber sucker firmly against a smooth flat surface (Fig. 6.11).
2. Do you realise that it sticks to the surface?
3. Now, try to pull it off. Do you find it difficult to pull it off?
2. Do you realise that it sticks to the surface?
3. Now, try to pull it off. Do you find it difficult to pull it off?
Observation
The sucker sticks to the surface, and it is difficult to pull off.
Explanation
Pressing the sucker pushes out most of the air inside it, reducing the pressure within. The higher surrounding air pressure holds it against the surface, so a strong pull is needed to overcome the pressure difference.
โ Summary
- Pressed sucker sticks
- Inside air pressure reduced
- Outside pressure holds it on
How large is atmospheric pressure, and why aren't we crushed?
- The air column over a 15 cm by 15 cm area pushes with about 2250 N (the weight of a 225 kg mass).
- That is a huge force, yet we are not crushed.
- The pressure inside our bodies equals the atmospheric pressure outside.
- These balance, caused by fluids and gases moving inside our tissues and organs.
โ
Test Yourself
-
What is the atmosphere?
View Answer
The envelope of air surrounding the Earth, extending many kilometres up. -
What is atmospheric pressure?
View Answer
The pressure exerted by the air around us. -
Why does a sucker stick to a surface?
View Answer
The air inside is reduced, so the higher outside air pressure holds it on. -
Why aren't we crushed by atmospheric pressure?
View Answer
The pressure inside our bodies equals the outside pressure, so they balance. -
In how many directions does air exert pressure?
View Answer
In all directions, which is why a balloon inflates evenly.
Important Definitions
- Atmosphere — The envelope of air surrounding the Earth.
- Atmospheric pressure — The pressure exerted by the air around us.
