ACTIVITY TO DEMONSTRATE THAT A LIQUID EXERTS PRESSURE ON THE WALLS OF ITS CONTAINER (OR SIDES OF ITS CONTAINER)
Take a plastic bottle (like a water bottle or a soft drink bottle). Secure a small glass tube over the bottom of the plastic bottle. We can do this by slightly heating one end of the glass tube onto the burner and then quickly inserting the hot end into the wall of the plastic bottle. We should seal the joint between the glass tube and the plastic bottle with molten wax so that the water does not leak from the joint. Tie a thin sheet of rubber (like that from a rubber balloon) tightly on the open end of glass tube. Now, fill half of plastic bottle with water.
We will observe that on filling water, the rubber sheet tied to the mouth of glass tube gets stretched and bulges out. The rubber sheet attached to the glass tube fixed to the wall of the plastic bottle protrudes to indicate that water present in the plastic bottle exerts pressure on the bottle wall (or the side of the bottle). It is the sideways pressure exerted by water which inflates the thin rubber sheet forming a bulge. If we pour more water into the plastic bottle to increase its depth, we will see an increase in the bulge in the rubber sheet. This indicates that the pressure exerted by the water increases as the depth increases. From this activity we conclude that:
1. A liquid exerts pressure on the walls (or sides) of its container, and
2. The pressure exerted by a liquid on the walls (or sides) of its container increases with increasing depth.
Keeping the pipe vertical, let us pour some water in the pipe from the top end. We will find that on pouring water in the pipe, the rubber sheet tied to its bottom stretches and bulges out. The rubber sheet protrudes to indicate that water poured in the pipe exerts pressure on the bottom of the container. This is because the bottom of container (pipe) in this case is made of a flexible, thin rubber sheet which can get stretched by the pressure exerted by water to from a bulge (The hard bottoms of containers like metal vessels or glass vessels, however, do not get stretched or bulge by the pressure of water kept in them).
Let us now pour some water in the plastic pipe so that the height of ‘water column’ in the pipe increases. We will find that as the height of water column increases, the bulge in the rubber sheet also increases, showing that the pressure of water on the bottom of its container (rubber sheet) has increased. In fact, greater the height of water column in the pipe, greater will be the bulge in its rubber sheet bottom (showing the greater pressure exerted by water). From this activity we conclude that:
1. A liquid exerts pressure on the bottom of its container, and
2. The pressure exerted by a liquid depends on the height of the liquid column (above the bottom of the container).