What is Atmospheric Pressure?

Published on Jul 15, 2016

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Our earth is enveloped in an atmosphere which extends several kilometers above the surface of the planet. The air that surrounds us this way has the right mixture of gases which makes our planet suitable for sustaining life. You cannot see air around you, but you can feel the air. Can you also feel the weight of the air?

Since ancient times it was believed that air was weightless and it was accepted as a fact till the early 17th century. Then Evangelista Torricelli, an Italian physicist, changed the perception with rational explanations.

Empirical evidence

In various experiments conducted during the first half of the 17th century, scientists observed that if a tube filled with water was allowed to be emptied into a large and wide basin which already had water, all of the water from the tube did not flow into the basin. Some water remained in the tube up to a certain height: exactly 10.3m every time. This was attributed to the “power of vacuum”. Many of the scientific minds of the century believed vacuum held the water and kept it at that height.

Torricelli’s skepticism

Torricelli refused to believe such an explanation. He argued that it was actually the weight of the atmosphere or air that pushed down on the water present in the basin which in turn prevented the water inside the tube from falling below 10.3m. Air exerts a force on the water in the basin. This force is nothing but the weight of the air. This force limits the water in the tube from reaching a level equal to that of the water in the basin.

What is atmospheric pressure?

Pressure is nothing but the force applied on a surface divided by the total area of the surface over which the force is applied. The pressure that the atmosphere exerts at sea level is denoted as 1 atm and it is called atmospheric pressure. The SI unit of pressure is Pascal, shortened as Pa. 1 atm is equal to 101.325 kilo-Pascal or kPa. So in the experiment described above, the atmosphere exerts a pressure of 1atm over the surface of water in the basin. In the region of the tube, since the tube is covered from the top, as the water flows into the basin and the volume previously occupied by water is unable to be filled by air, a region of vacuum is created. Thus, in the region of the tube, there is no pressure being applied by the atmosphere over the water column. This allows water to remain in the tube up to a height of 10.3m.

Why is water column exactly 10.3 meter high?

The pressure exerted by a fluid is given by the formula:

P = ρgh

where, ρ is the density of the fluid
g is acceleration due to gravity, and
h is the height of the fluid

After substituting the appropriate values for density of water, acceleration due to gravity and setting ‘P’ on the left-hand side as 1atm, the value for ‘h’ comes out to be 10.3m.

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