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Describe a Simple Experiment to Illustrate that Air Exerts Pressure.
We live on the earth and there is a lot of air around us. Air is a mixture of colourless gases, so we cannot see air. The layer of air above the earth is called atmosphere. The atmosphere contains a tremendous amount of air. About 99 per cent of atmosphere is made up of just two gases : nitrogen and oxygen. The amount of water vapour in the atmosphere depends on temperature and humidity.
The earth’s atmosphere extends to a height of several hundred kilometres above the earth but most of the air lies within about 10 kilometres of the earth’s surface. The atmosphere exerts a pressure called atmospheric pressure due to the weight of air contained in it. The atmospheric pressure decreases with height.
Even the small changes in atmospheric pressure can bring about considerable changes in the weather on earth. All the phenomena such as winds, storms, and cyclones, etc., occur in the earth’s atmosphere. In this Chapter we will study the formation of winds, storms and cyclones, and their effects.
Before we go further, we will discuss the two important properties of air which are : air exerts pressure, and air expands on heating (and becomes lighter).
Air Exerts Pressure
The continuous physical force exerted on an object (or against an object) by something in contact with it, is called pressure. Air exerts pressure. Actually, air exerts pressure in different ways under different situations. For example :
- Air enclosed in a container (like a balloon or bicycle tube) exerts pressure,
- Air in the atmosphere around us exerts pressure, and
- Moving air (called wind) exerts pressure.
We will now describe the pressure exerted by air under different conditions in detail, one by one.
Air is a mixture of gases. Air is made up of tiny particles called ‘molecules’ which move around quickly in all directions. The moving air molecules (or gas molecules) are so small that we cannot see them. If we put air in a closed container (say, a balloon), then the fast moving air molecules ‘collide’ with the walls of the container and exert a force on the walls of the container from inside.
This force produces pressure. Now, if we put more air into the container, then the container will have more air molecules in it. Due to more air molecules, the number of collisions of air molecules with the walls of the container will increase and hence the pressure exerted by air will also increase. The fact that air exerts pressure can be demonstrated by the following activity.
Take a rubber balloon and fill air into it with mouth. We will find that on filling air, the balloon gets inflated (it expands and becomes bigger in size) (see Figure). This can be explained as follows : When we put air in the balloon, then the number of air molecules in the balloon increases.
These air molecules cause collisions with the rubber walls of balloon and create an ‘air pressure’. The air pressure acting on the thin rubber walls of the balloon from inside causes the balloon to expand and get inflated. Just like balloon, a football also gets inflated when air is filled in it. The football gets inflated because the air filled in it exerts pressure.
Let us take the example of bicycle tube now. The bicycle tube is a rubber tube (which is enclosed inside the tyre of the bicycle). We have to fill air in the bicycle tube to inflate it. This air is filled by using an ‘air pump’ called ‘bicycle pump’. When we fill air into the bicycle tube, then the air molecules inside the tube collide with the walls of the tube and exert air pressure.
This air’pressure exerted from inside, inflates the bicycle tube. So, it is the pressure exerted by air filled in a bicycle tube which keeps the tube ‘tight’ and makes the bicycle tyre feel ‘hard’. If we go on filling more and more air into bicycle tube with a pump, then ultimately the air pressure in the bicycle tube will increase too much due to which the bicycle tube may even burst ! Thus, the two observations (or experiences) which tell us that air exerts pressure are as follows :
- When air is filled into a balloon with our mouth, the balloon gets inflated. This observation shows that air exerts pressure.
- When air is filled into a bicycle tube with a pump, the bicycle tube gets inflated and makes the bicycle tyre feel hard. This observation also shows that air exerts pressure.
From the above discussion we conclude that when air is filled in a closed container (like a balloon or a bicycle tube), it exerts pressure. This air pressure is due to the motion of the molecules of gases present in air which is enclosed in the container. We will now describe another type of air pressure called ‘atmospheric pressure’ which is due to the weight of air present in the atmosphere 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 (or air) above us exerts a pressure on the surface of earth and on all the objects on the earth. 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 above us. The magnitude of atmospheric pressure is very large. As we go to higher altitudes, the atmospheric pressure goes on decreasing (because the weight of air above us goes on decreasing). Atmospheric pressure acts in all directions : downwards, sideways and even upwards!
We will now describe the crushing a tin can activity to show the existence of air pressure (or atmospheric pressure) around us. This activity will also show that the magnitude of atmospheric pressure is very large. Tin can is a vessel made of a thin and flexible metal sheet. An empty tin can has air inside it as well as outside it.
So, the air pressure (or atmospheric pressure) acting on the walls of an empty tin can from inside and outside is equal and opposite, and balance each other. Due to this, an empty tin can (which contains air) does not get crushed by the large atmospheric pressure exerted by the air around us. We will now perform an activity to show what happens when air from inside the tin can is removed.
To Show that Air Around Us Exerts Pressure
Take a tin can and put some water in it [see Figure (a)]. Heat the tin can by using a burner to boil the water so that steam is formed. This steam expels all the air from inside the tin can so that when water is boiling, there is no air inside the can, the whole can is filled with steam [see Figure (a)].
A tight cork is now fitted in the mouth of the tin can and heating is stopped. We now pour cold water from a tap on the hot tin can. On pouring cold water, the tin can collapses inwards as if a large force acting on it from outside has crushed it [see Figure (b)]. These observations can be explained as follows:
(i) When the water is boiling, steam is formed in the tin can [see Figure (a)]. Steam has pressure. The steam pressure acting from inside the tin can balances the air pressure (or atmospheric pressure) acting on all the sides of the tin can from outside and hence the tin can does not get crushed.
(ii) When a tight cork is put on the mouth of the tin can containing steam and then cold water is poured over it, then the hot steam inside the tin can gets cooled, condenses and forms water and water vapour at very low pressure. Thus, on cooling, the pressure inside the tin can decreases too much and it cannot balance the large air pressure (or atmospheric pressure) acting on the tin can from outside.
So, the large air pressure (or atmospheric pressure) outside the tin can crushes the tin can inwards [see Figure (b)], The crushing of tin can having very low pressure inside it shows the existence of a large air pressure (or large atmospheric pressure) around it. So, this activity confirms that air around us exerts pressure.
A yet another type of force (or pressure) is exerted by air when it starts moving. Moving air is called wind. Moving air (or wind) exerts a force or pressure on the objects on which it strikes. An object which is moving is said to possess kinetic energy. The force (or pressure) exerted by the moving air (or wind) is due to the kinetic energy possessed by it. Some of the observations from our daily life which tell us that moving air (or wind) exerts pressure are as follows.
(i) If the air around us is not blowing, we will find that the leaves of trees, flags and banners, etc., do not move at all, they remain standstill. But when the air blows, then the leaves of trees, flags and banners start fluttering (moving with irregular motion). It is due to the pressure exerted by moving air (or wind) that the leaves of trees, flags and banners flutter when the wind is blowing (see Figure).
A long strip of cloth used to display advertisements is called a banner and a large board used to display advertisements is called a hoarding. Holes are usually made in hanging banners and hoardings so that high speed wind may pass through them easily without damaging them or bringing them down with its huge force or pressure.
(ii) While riding a bicycle, we find that it is easier to move the bicycle forward if the wind is coming from our back side. It is easier to ride a bicycle in the direction of blowing wind because the blowing wind exerts a force (or pressure) on us in the same direction in which our bicycle is moving and makes our bicycle move faster.
On the other hand, we find it very difficult to ride a bicycle against the direction of wind (see Figure). This is because in this case the blowing wind exerts a force on us in the direction opposite to the motion of our bicycle. And since the wind opposes our motion, we have to pedal very hard to keep the bicycle moving against the wind.
(iii) While rowing a boat in the lake, we find it easier to row the boat forward if the wind is coming from behind us. This is because the blowing wind exerts pressure on the boat (and on the persons sitting in it) in the direction of their motion and helps it move forward. On the other hand, it becomes much more difficult to row a boat against the direction of blowing wind because in this case the wind exerts a pressure opposite to the direction of motion of boat. And since the wind opposes the motion of boat, we have to apply much more force to row the boat against the direction of wind.
(iv) When we are flying a kite, then the wind coming from our back side helps. This is because the wind coming from our back side, strikes the kite and exerts pressure on the kite -to make it fly higher and higher. We can, however, not fly the kite against the direction of wind. This is because in this case the pressure of blowing wind will make the kite fall to the ground.
All the above observations (or experiences) show that moving air or wind exerts a force (or pressure) on all the objects which come in its way. If the wind blows in the same direction in which the object is moving, it makes the movement of object easier. On the other hand, if the wind blows in opposite direction to the motion of the object, it makes the movement of the object difficult.
Before we go further, we should know the meaning of the terms ‘expansion of air’ and ‘contraction of air’. The term ‘expansion of air’ means ‘increase in the volume of air’, and the term ‘contraction of air’ means ‘decrease in the volume of air’. Air expands on heating and contracts on cooling.
In other words, when air is heated, its volume increases and it occupies a bigger space. And when air gets cooled, then its volume decreases and it occupies a smaller space. We will now discuss the expansion of air on heating in detail.