Ask a Teacher

1.The pressure in a liquid is not the same at all depths. The pressure exerted by a liquid changes with depth in the liquid. Actually, the pressure exerted by a liquid increases with increasing depth inside the liquid. The pressure exerted by a liquid is small just under the surface of the liquid. But as we go  deeper in a liquid, the pressure of liquid increases. We will now describe an activity to show that the pressure of a liquid depends on its depth.

Take a tall vessel having three short and thin tubes A,B and C fitted at different depths from the top of the vessel. The three tubes are of equal diameters and corks are fitted into them. The vessel is filled with water and then all the corks are removed quickly. On removing the corks, the water from the uppermost tube A is found to travel the shortest distance from the base of the vessel, the water from  middle tube B goes a little farther away where as the water from the lowermost tube C shoots out farthest of all. This can be explained as follows:

The depth of water near tube A is small so the water comes out from tube A with smaller pressure and falls nearer the bottom of the vessel. The depth of water near tube B is greater, so the water comes out with greater pressure from tube B and falls farther away from the base of the vessel. The depth of water near tube C with the greatest pressure and goes farther from the vessel. This means that as the depth of water increases from A to B to C, the pressure of water gradually increases.

From the above activity we conclude that as the depth of water in the vessel increases,the pressure exerted by water also increases.In general,we can say that the pressure of a liquid increases with depth.

2.Given below is an activity to show that a liquid exerts pressure on the walls of its container (or sides of a container)

Take a plastic bottle. Fix a small glass tube a little above the bottom of the plastic bottle. We can do this by heating one end of the glass tube slightly over a burner and then inserting this hot end quickly into the wall of the plastic bottle. We should seal the joint of the glass tube with plastic bottle by using molten wax so that water does not leak from the joint. Tie a thin sheet of rubber 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 the glass tube gets stretched and bulges out. The bulging out of rubber sheet tied to the glass tube fixed in the wall of plastic bottle demonstrates that water present in plastic bottle exerts pressure on the wall of the bottle(or 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 in the plastic bottle to increase its depth, we will see that the bulge in the rubber sheet increases. This indicates that the pressure exerted by water increases with increasing depth. From this activity we conclude that:

(i)A liquid exerts pressure on the walls (or sides) of its container, and

3.Through an activity we can show that a liquid exerts equal pressure at the same depth.

Take a plastic bottle. Make two holes of equal size on the two opposite sides of the plastic bottle some distance above the bottom of the bottle. The holes should be at exactly the same height from 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 the bottle on its either side. The two jets of water can fall at equal distance on the two sides of the bottle only if the pressure of water at the depth of two holes in the bottle is equal. From this activity we can conclude that: