WHAT IS ATMOSPHERIC PRESSURE?
You know that there is air all around us. This envelop of air is known as the atmosphere. The atmospheric air extends up to many kilometers above the surface of the earth. The pressure exerted by this air is known as atmospheric pressure. We know that pressure is force per unit area. If we imagine a unit area and a very long cylinder standing on it filled with air, then the force of gravity on the air in this cylinder is the atmospheric pressure (Fig. 11.17).
Figure 11.17: ( Pressure of atmosphere in cylinder )Atmospheric pressure is the force of gravity on air in a column of unit area
But, how large or small is the atmospheric pressure? Let us get an idea about its magnitude
Activity 11.11:
* Take a good-quality rubber sucker. It looks like a small rubber cup (Fig. 11.18).
* Press it hard on a smooth plane surface. Does it stick to the surface?
* Now try to pull it off the surface.
* Can you do it?
Figure 11.18: Rubber sucker pressed on the surface
When you press the sucker, most of the air between its cup and the surface escapes out. The sucker sticks to the surface because the pressure of the atmosphere acts on it. To pull the sucker off the surface, the applied force should be large enough to overcome the atmospheric pressure. This activity might give you an idea about the magnitude of atmospheric pressure. In fact, it would not be possible for any human being to pull the sucker off the surface if there were no air at all between the sucker and the surface. Does it give you an idea of how large the atmospheric pressure is?
The force due to air in a column of the height of the atmosphere and area 15 cm × 15 cm (Fig. 11.19) is nearly equal to the force of gravity on an object of mass 225 kg (2250N). The reason we are not crushed under this force of gravity is that the pressure inside our bodies is also equal to the atmospheric pressure and balances the pressure from outside.
Figure 11.19: Pressure of atmosphere on your head
More to know
Otto von Guericke, a German scientist of the 17th century, invented a pump to extract air out of a vessel. With the help of this pump, he demonstrated dramatically the force of the air pressure. He joined two hollow metallic hemispheres of 51 cm diameter each and pumped air out of them. Then he employed eight horses on each hemisphere to pull them apart (Fig. 11.20). So great is the force of air pressure that the hemispheres could not be pulled apart.
Figure 11.20: Horses pulling the hemispheres
Source: This topic is taken from NCERT TEXTBOOK