PRESSURE EXERTED BY GASES
Take a rubber balloon and fill air into it with mouth (or by using a pump). We will find that on filling in air, the balloon gets inflated (it expands and becomes bigger size). This can be explained as follows : When we put a lot of air in the balloon, then the number of gas molecules in the balloon increases too much. The large number of gas molecules cause too many collisions with he walls of the balloon from inside and create a high air pressure. This high air pressure produced by the gas molecules on the walls of balloon causes it to expand and get inflated. Similarly, when air is pumped into a bicycle tube by using a pump, the bicycle tube gets inflated due to the air pressure exerted by the collisions of gas molecules in air with the inner walls of the rubber tube. This air pressure in the bicycle tube makes the of rubber tube. This air pressure in the bicycle tube makes the bicycle tire feel very hard. Thus, the two examples that gases (like air) exert pressure are :
1. When air is filled into a balloon with our mouth, the balloon with our mouth, the balloon gets inflated (gets bigger in size), and
2. When air is filled into a bicycle tube with a pump, the tube gets inflated and makes the tyre feel ‘hard’.
The air pressure which we have just described is due to the motion of molecules of gases present in air which is enclosed in a container (like balloon or football). We will now describe another type of air pressure called ‘atmospheric pressure’ which is due to the weight of air (or its gases) present in the atmosphere above the surface of earth.
We live on the earth and there is a lot of air above us. The air above the earth is called the atmosphere. Air has a weight, so air composed of a large amount of air has a huge weight. The air in our atmosphere extends up to about 400 kilo metres above the surface of earth. The atmosphere contains a tremendous amount of air. 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 caused by the weight of air present in the atmosphere above us. Just as the pressure in the liquid works in all directions, the atmospheric pressure will also work in all directions (even up!).