Atmospheric pressure at work
We put atmospheric pressure to work every day — without noticing. Here are three examples.
(i) Drinking through a straw
Figure to be added
When you put your mouth to the end of the straw and suck, the air in the tube goes into your mouth and the pressure inside the tube falls. At the liquid surface outside the tube the pressure is still atmospheric pressure. So atmospheric pressure pushes the liquid in the glass up into the straw.
A common mistake
The liquid does not climb the straw because we "pull it up". All we do is lower the pressure inside the straw — what pushes it up is the atmospheric pressure outside.
(ii) Emptying a tank by siphon
Figure to be added
The method
At the start the tube is filled with water and one end closed with a finger so that the water cannot escape, and that end is lowered until it is close to the bottom of tank A. When the finger is then removed, water begins to flow into tank B.
Why the water flows
At the end of the tube at A, the pressure = the pressure of the liquid column above it in the tank + atmospheric pressure.
The end of the tube at B is open to the atmosphere, so there the pressure = atmospheric pressure alone.
The greater pressure therefore pushes the water from A, where the pressure is higher, to B, where it is lower.
(iii) How a rubber sucker works
Figure to be added
When a rubber sucker is pressed against a glass surface, most of the air between the sucker and the surface is driven out. The pressure inside is then less than atmospheric pressure, so the atmospheric pressure outside holds the sucker pressed against the surface.
One pattern in all three
Lower the pressure on one side — atmospheric pressure does the rest. In the straw it pushes the liquid up, in the siphon it drives the water along, and in the sucker it holds the cup against the wall.
A sucker works properly only if no air can travel between its rim and the glass surface.