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Atmospheric Pressure

Air is present over the surface of the earth up to several hundred kilometers. This great sea of air is called the atmosphere. It is really a mixture of gases consisting of oxygen (21%), nitrogen (77%) and argon (1%). The remaining 1% consists of traces of ammonia, carbon dioxide, helium and other gases as well as water vapour. The weight of this sea of air pressing upon the earth is called the atmospheric pressure. It is a well known fact that all living beings need oxygen from the air for breathing. Hence, air is necessary for life.


Composition of air

Two important properties of air are:


1. Air exerts pressure.

2. Air has weight.

Activity: 1
Make a hole with a nail near the bottom of a tin can. Fill the can with water. Hold the palm of your hand tightly over the top of the can. Water will stop running from the hole. Remove your hand. Water will start running again. What does this prove?

Activity: 2
Take a tumbler or cylinder and fill it completely with water. Cover it with a thick sheet of paper. Keeping the paper in position with one hand, invert the tumbler and withdraw the hand supporting the paper. What do you observe? 
The paper does not fall even though the weight of the water is acting downwards. How does this happen? This is because the atmospheric pressure acting upwards is greater than the liquid pressure acting downwards.




Why doesn’t this enormous force crush us?


This is because the fluid in our body cells and the blood in our veins exert an equal pressure outwards, balancing the atmospheric pressure. At high altitudes where the atmospheric pressure is less, the higher pressure exerted by the blood can cause blood vessels to burst, causing bleeding.



Example : Nose bleeding.


Astronauts wear special suits when they go into space.


The pressure inside the suit is maintained at atmospheric pressure.

Magdeburg Sphere

An experiment to determine how strong the atmospheric pressure is, was performed by a scientist named Otto Von Guericke in the town Magdeburg in 1640. Two tight fitting hollow iron hemispheres each provided with a hook were joined together. The air in space between them was then removed with the help of an exaust pump. The atmospheric pressure outside pressed down on the hemispheres. So great was this pressure that it required two teams of eight horses each pulling in opposite directions to separate the two hemispheres. These spheres were called the Magdeburg spheres.

Blaise Pascal:
Born: 19 June 1623
Birthplace: Clermont-Ferrand, France

Best Known As:
17th century mathematical genius
Died: 19 August 1662

Contribution to Science

Blaise Pascal described the principle that is the basis for the hydraulic press (called Pascal's Law); and he proved that there was a vacuum above the atmosphere.

Sir Isaac Newton

 Born: 4 January 1643
 Birthplace: Lincolnshire, England
 Sir Isaac Newton was an English physicist. He is regarded as one of the greatest scientists in  history.

Contribution to Science

Newton discovered the law of gravitation and the three laws of motion.

 Died: 31 March 1727

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