Magnitude of Atmospheric Pressure
We know that ‘pressure is force per unit’. Now, if we imagine a unit area of the Earth’s surface and a very high “air-filled” column, the weight of the air in that column is the atmospheric pressure at that place. Since the SI unit of area is ‘square metre’, therefore, we can also say that atmospheric pressure is equal to the weight of air present in a very tall column of air standing on 1 square metre area of the earth.
On the surface of earth, the atmospheric pressure is maximum at the sea level. This is because the air column above us is the highest at sea level. Therefore, the magnitude of the atmospheric pressure is very large. The atmospheric pressure on the surface of earth (at the sea level) is 342.4 kilo pascals which is equivalent to the weight of ten elephants pressing on each square metre area ! As we go up in the atmosphere form the surface of earth, the atmospheric pressure goes on decreasing. This is because as we go up in the atmosphere, the weight of air above us goes on decreasing (due to which the pressure also goes on decreasing). So, the atmospheric pressure on the top of a high mountain will be much less than at its base.
Please note that though the SI unit of pressure ins pascal (Pa) but atmospheric pressure is usually measured in the unit of ‘millimeters of mercury’ (mm of mercury) for the sake of convenience in measuring it. Thus, the common unit for expressing and measuring atmospheric pressure is ‘millimeters of mercury’ (mm of mercury). The atmospheric pressure on the surface of earth (at the sea level) is 749 mm of mercury.
ACTIVITY TO SHOW THE EXISTENCE OF ATMOSPHERIC PRESSURE
We will now describe an activity to show the existence of atmospheric pressure. In this activity we will use atmospheric pressure to hold water in an inverted glass tumbler. This activity can be performed as follows : A glass tumbler is filled to the brim with water and covered with a piece of thick and smooth card. We pressed the card hard so that there was no air in the glass. Hold the card in place with one hand and we invert the glass filled with water. The hand supporting the card is then withdrawn slowly. We will see that the piece of card does not fall though the tumbler is full of water and exerts pressure on the card in the downward direction. The card does not fall because the atmospheric pressure acts on the card in the upward direction and holds the card in place. The upward atmospheric pressure acting on the card is greater than the downward pressure of water on the card.
Before we describe the next experiment to show the existence of large atmospheric pressure around us, we should know the meaning of the term ‘hemispheres’. ‘Hemispheres’ mean ‘half spheres’. Joining together of two hemispheres makes one complete sphere. Let us describe the experiment now.