Understanding Physics Topics is essential for solving complex problems in many fields, including engineering and medicine.
What is Atmospheric Pressure and How is it Measued?
We live on the earth and there is a lot of air above us. The layer of air above the earth is called atmosphere. The air in our atmosphere extends up to about 300 kilometres above the surface of earth. The atmosphere contains a tremendous amount of air. Air has weight, so the atmosphere consisting of tremendous amount of air has enormous weight. The weight of atmosphere exerts a pressure on the surface of the earth and on all the objects on the earth, including ourselves. This pressure is known as atmospheric pressure. We can now say that: Atmospheric pressure is the air pressure which is exerted by the weight of air present in the atmosphere. In other words, atmospheric pressure is due to the weight of air present in the atmosphere above us. Just as the pressure in a liquid acts in all directions, in the same way, atmospheric pressure also acts in all directions (even upwards !).
Magnitude of Atmospheric Pressure
We know that ‘pressure is force per unit area. Now, if we imagine a unit area of earth’s surface and a very tall column ‘filled with air’ standing on it, then the weight of air in this column will be the atmospheric pressure at that place (see Figure). 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 column of air above us is tallest at the sea-level. The atmospheric pressure on the surface of earth (at the sea-level) is 101.3 kilopascals— which is equivalent to the weight of ten elephants pressing on each square metre area ! Thus, the magnitude of atmospheric pressure is very large. As we go up in the atmosphere from 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 is pascal (Pa) but atmospheric pressure is usually measured in the unit of ‘millimetres of mercury’ (mm of mercury) for the sake of convenience in measuring it. Thus, the common unit for expressing and measuring atmospheric pressure is ‘millimetres of mercury’ (mm of mercury). The atmospheric pressure on the surface of earth (at the sea- level) is 760 mm of mercury.
Activity 6
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 press the card hard so that there is no air in the glass tumbler. Keeping the card in position with one hand, we invert the glass tumbler full of 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 (see Figure). 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.
Magdeburg Hemispheres Experiment to Show Large Atmospheric Pressure
The apparatus consists of two hollow copper hemispheres A and B of 51 cm diameter each (see Figure). Each hemisphere has a hook attached to it. One of the hemispheres has also a side tube which can be connected to a vacuum pump. The two hemispheres are tight fitting and become air-tight when joined together. When air is present inside the joined hemispheres, they can be easily separated by pulling with a small force. This is because the air present inside the joined hemispheres also exerts its pressure.
In order to perform the experiment, the two hemispheres are joined together and air is removed completely from the space between them by using a vacuum pump (see Figure). When all the air is removed from inside the hemispheres (or when vacuum is created inside the hemispheres), then the two hemispheres cannot be separated even by pulling with a large force. This is due to the fact that since there is no air inside, the unopposed atmospheric pressure acting over the whole surface of hemispheres from outside presses them very, very hard and does not allow them to be separated.
The effect of atmospheric pressure on the evacuated hemispheres was so great that even two teams of eight horses each pulling in opposite directions could not separate the two hemispheres. This is shown in Figure. This experiment with hemispheres and horses was conducted by a German scientist called Otto von Guericke in the town of Magdeburg in the year 1640. As soon as some air was re-introduced into the evacuated hemispheres, the hemispheres fell apart.
Our Body and Atmospheric Pressure
We have seen from the ‘Magdeburg hemispheres’ experiment that the pressure exerted by atmosphere on the earth and its objects (including us) is very, very large. So, an important question now arises in our mind : If the pressure due to atmosphere is so great, then why are we not crushed by it ? This can be explained as follows : Our body has a liquid called ‘blood’ which flows through blood vessels into each and every cell of our body. Our blood itself exerts a pressure called ‘blood pressure’ which is slightly greater than the atmospheric pressure. Since the atmospheric pressure acting on our body from outside is balanced by the blood pressure acting from inside, we do not get crushed.
Actually, the atmospheric pressure is so finely balanced by our blood pressure that we do not feel any discomfort. We even do not feel the existence of atmospheric pressure at all. We will now describe the effects of low atmospheric pressure on our body. The atmospheric pressure is maximum on the surface of the earth. When we go to high altitudes (say, a high mountain), then the atmospheric pressure decreases. So, at high altitudes, the atmospheric pressure becomes much less than our blood pressure. Since our blood is at a higher pressure than outside pressure, therefore, some of the blood vessels in our body burst and nose bleeding takes place at high altitudes. Thus, nose-bleeding usually occurs in those persons who trek to high mountains (where the atmospheric pressure is much less than our blood pressure).
Applications of Atmospheric Pressure in Everyday Life
We use many simple devices in our everyday life which work on the existence of atmospheric pressure. For example, the devices such as a drinking straw, a syringe, a dropper and a rubber sucker work on the existence of atmospheric pressure (or air pressure) around us. We will now describe all these devices, one by one. Let us start with a drinking straw.
1. Drinking Straw.
The drinking straw is a very thin pipe which is used to drink soft drinks (like Coca-Cola and Pepsi). The drinking straw works on the existence of atmospheric pressure. This can be explained as follows : The lower end of drinking straw is dipped in the soft drink (see Figure). When we suck at the upper end of the straw with our mouth, the pressure of air inside the straw and in our mouth is reduced. But the pressure acting on the surface of the soft drink is equal to atmospheric pressure. So, the greater atmospheric pressure acting on the surface of the soft drink pushes the soft drink up the straw into our mouth (see Figure).
2. Syringe
A glass tube (or plastic tube) with a nozzle and piston for sucking in and ejecting liquid in a thin stream is called a syringe (A syringe may also be fitted with a hollow needle for giving injections).
The syringe works on the existence of atmospheric pressure. When the nozzle of a syringe is dipped in a liquid and its piston is withdrawn, the pressure inside the syringe is lowered. The greater atmospheric pressure acting on the surface of the liquid pushes the liquid up into the syringe (see Figure).
3. Dropper.
The dropper is a short glass tube with a tube rubber bulb at one end and a nozzle at the other end (see Figure). A dropper is used for measuring Out drops of a liquid (such as a liquid medicine). A dropper works on Nozzle the existence of atmospheric pressure. When we press the rubber bulb of the dropper by keeping its nozzle dipped in the liquid, air present in the glass tube and bulb is seen to escape in the form of bubbles. Due to this, the air pressure inside the glass tube and rubber bulb of dropper is very much reduced. When we now release the rubber bulb of dropper, the much greater atmospheric pressure acting on the surface of liquid, pushes the liquid up into the dropper tube.
the rise of liquid (say, water) in a dropper is due to the atmospheric pressure. If we remove the filled dropper from the container of liquid and press its rubber bulb slowly, then the drops of liquid will come out of the nozzle of dropper tube. Just like a dropper, the filling of ink in a fountain pen is also based on the existence of atmospheric pressure.
4. Rubber Sucker.
A rubber sucker is a device made of rubber (or plastic) that sticks firmly to flat and smooth surfaces on pressing. A rubber sucker looks like a small, concave-shaped rubber cup [see Figure (a)], A rubber sucker is also called a ‘suction cup’ because it sticks to a surface by suction (The production of partial vacuum by the removal of air is called suction). When we press the rubber sucker on a flat, smooth surface, its concave rubber cup gets flattened to a large extent, pushing out most of the air from beneath it [see Figure (b)], Since very little air remains inside the flattened rubber sucker, therefore, the air pressure inside the rubber sucker becomes very low (and a partial vacuum is created).
The much greater atmospheric pressure acting on the rubber sucker from outside fixes the rubber sucker firmly on the flat surface [see Figure (b)], Thus, a rubber sucker stays attached firmly to a flat surface due to the atmospheric pressure. In order to pull away the fixed rubber sucker from a surface, we will have to apply a force which is large enough to overcome the atmospheric pressure holding it onto the surface. Rubber suckers are usually used to hold objects together with the help of suction. For example, rubber suckers are used to hold glass ‘table tops’ onto the wooden frames of tables. Rubber suckers are also used for making suction hooks which are fixed on walls, doors and almirahs, etc., to hold various things. A suction hook is a rubber (or plastic) sucker having a plastic (or metal) hook attached to it. A suction hook is shown in Figure.