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Pressure in fluids (atmospheric pressure)
The layer of air surrounding the earth is called the atmosphere.
Atmospheric pressure is the pressure exerted by the weight of air on all objects on the earth’s surface.
Atmospheric Pressure depends on altitude.
The density of air above the earth decreases as the altitude increases leading to the decrease of atmospheric pressure at high altitude and the vice versa. At sea level, the atmospheric pressure is 1.0×105Pa.
Though the value of atmospheric pressure is large we do not normally feel it because:
-Blood pressure is slightly greater than atmospheric pressure
-Atmospheric pressure acts equally in all direction.
Experiments to Demonstrate Existence of Atmospheric Pressure.
a) Collapsing Can or Crushing Can Experiment.
If air is removed from the can by a vacuum pump, the can collapses because the air pressure inside becomes less than the atmospheric pressure.
A small quantity of water is boiled in a can until steam forms.
The steam drives out all the air inside the can, hence reducing the pressure inside the can.
The stopper is then tightly fitted onto the can and the heat source removed.
Cold water is then poured over the can. This causes the steam inside to condense producing water and water vapour at very low pressure.
The excess atmospheric pressure outside the can causes it to collapse inwards.
Importance of atmospheric pressure
a) Drinking straw
When sucking, lungs expand and air is driven out from the inside of the straw to the lungs.
This reduces the pressure inside the straw.
Then atmospheric pressure acting on surface of the liquid in the bottle is greater than air pressure in straw and so it forces the liquid up to the mouth.
b) Rubber sucker
When the sucker is moistened and pressed on a smooth flat surface, the air is pushed out. Atmospheric pressure then holds it firmly against the surface.
Suckers are used for attaching car licenses to windscreen and in industry for lifting metal sheets.
Defining pressure in fluids
Fluids refer to gas or liquids. These take up the shape of the container, so the volume of the liquid filling a cylindrical container is equal to the volume of that cylindrical container.
Then from the definition of pressure:
