Pressure in liquids

All these effects are due to the pressure in liquids.

The dam

You can see from the diagram that the dam is much thicker at the bottom than it is at the top. This is because the pressure of the water is much greater deeper down and the dam needs to be thick at the bottom so that it is strong enough to withstand this larger pressure.






Experiments
You can investigate the pressure in liquids by the following simple experiments:
1. The pressure in a liquids increases with depth
Get a can or a large plastic bottle with a series of holes drilled in the side one above the other. Cover the holes with your fingers or a piece of sticky tape and then fill the container with water.
Now remove the tape. The water gushes out but comes out faster from the holes nearer the bottom where the water is deeper and so the pressure of the water is greater.
Just imagine doing the experiment with a high-density liquid. The pressure would be much greater.







2. The pressure in a liquids acts equally in all directions
Take a plastic bag and fill it with water. Make some holes in the bag and then squeeze it quickly. The water squirts out equally fast from all the holes, even the ones near the top of the bag.





These simple experiments show you three things:



The pressure at a depth in a liquid can be shown to be given by the formula:



where g is the acceleration due to gravity (10m/s²)

The pressure of the air on the water surface is about 100 000 N/m² so the total pressure is 200 000 N/m². In other words the pressure 10 m below the water surface is twice what it is at the surface.






The pressure at a certain depth in the liquid is the same at all points at that depth.

Pressure at A = Pressure at B = Pressure at C = Pressure at D = Pressure at E

Notice that it is only the depth of the liquid that matters; not the angle of the tube, the cross sectional area of the liquid column or the area of the liquid surface.

Water finds its own level, if we open the tap in the tube shown in Figure 5 the levels will become equal.






Knowledge of water pressure is very important in the supply of water to our homes.