ACTIVITY DEMONSTRATE THAT A LIQUID EXERTS EQUAL PRESSURE AT THE SAME DEPTH
Take a plastic bottle. Make two small holes of equal size on the two opposite sides of the plastic bottle some distance above the bottom of the bottle. The hole should be exactly the same height as the bottom of the plastic bottle. Now fill the bottle with water. We will observe that the two jets of water coming out of the two holes fall at the same distance away from the base of base of the bottle on its either side. The two jets of water at the depth of two holes in the bottle is equal From this activity we conclude that :
1. A liquid exerts pressure on the walls (or sides) of its container, and
2. A liquid exerts equal pressure at the same depth.
Many times we see a fountain of water rushing out of the leaking joints (or holes) in the pipes of main water supply line in our city. It is due to the very high pressure exerted by water on the sides (or walls) of the pipes that such a fountain of water is formed. So, the formation of fountains of water from the leaking pipes of water supply pipeline tells us that water exerts pressure on the walls of its container (here the walls of water carrying pipes).
So far we have studied that a liquid exerts downward pressure on the base (or bottom) of its container and sideways pressure on the walls of the container. Though it may seem awkward but a liquid exerts pressure in the upward direction also. So, in general we can say that : A liquid exerts pressure in all direction even upwards. Actually, the pressure at any point in a liquid acts equally in all directions (even upwards).
PRESSURE EXERTED BY GASES
The substances such as hydrogen, helium, oxygen, nitrogen, and carbon dioxide, etc., are gases. All gases exert pressure on the walls of their containers. Though pressure is exerted by all the gases but since air is the most common gas, therefore, we will discuss the pressure exerted by air in detail. Air exerts pressure in all the directions. This point will become clear form the pressure exerted by air in detail. Air exerts pressure in all the directions. This can be clearly seen from the discussion below.
Air is a mixed gas. All the gases are made up of tiny particles called molecules which move around quickly in all directions. The fast moving gas molecules collide with the walls of the container (in which they are kept). When the fast moving gas molecules collide with the walls of their container, they exert a force on the walls of the container. This force produces gas pressure (or air pressure). We can now say that : Air pressure arises due to the constant collisions of the tiny molecules of the gases present in air with the walls of the container (or vessel) in which it is enclosed. Now, if a certain mass of air is compressed into a smaller volume (say, by using a pump), then the number of collision of air molecules per unit area Increased, so the pressure exerted by the air increases. And if the container of air has stretchable walls (like a rubber balloon), then the higher pressure exerted by air inflates the container (increases the size of the container and makes it look bigger). This point will become more clear from the following examples.