What is the atmospheric pressure and how is it determined using Barometer?

Explanation

The atmospheric pressure at any point is equal to the weight of a vertical column of air of unit cross-sectional area extending from that point on the top of the earth's atmosphere.

Determination of atmospheric pressure using Barometer: A long tube closed at one end and filled with mercury is inverted into a trough of mercury. This device is known as mercury barometer. The space above the mercury column in the tube contains only mercury vapour whose pressure p is so small that it may be neglected. Otherwise there is a perfect vacuum, which is called Torricellian vaccum.
The pressure inside the column at point A must equal the pressure at point B, which is at the same level.
'P' at A= Pressure at B = atmospheric pressure= Pa
Pa=pgh _____(1)
where p= density of mercury
h=height of mercury column
In this experiment, it is found that the mercury column in the barometer has a height of about 76 cm at sea level equivalent to one atmosphere (1 atm).
At sea level, atmospheric pressure in the pressure exerted by 0.76cm of mercury column .i.e., h=0.76 m
`rho=13.6times10^3kgm^-3Andg=9.8ms^-2`
`therefore` Atmospheric pressure, Pa=h`rho`g
`=0.76times(13.6times10^3)times9.8`
`=1.013times10^5Nm^-2(or)Pa`
A commom way of stating pressure is in terms of cm or mm of mercury (Hg). A pressure equivalent to 1 mm is called a torr.
1 torr= 133 Pa.