Definition: The atmospheric pressure at any position is numerically same to the weight of a column of air of unit cross-sectional area extending from that position to the top of the atmosphere.
MEASUREMENT OF ATMOSPHERIC PRESSURE
(i) Mercury Barometer: To calculate the atmospheric pressure experimentally, Torricelli created a mercury barometer in 1643.
Pa = hρg
The pressure given by a mercury column of 1mm height is known as 1 Torr.
1 Torr = 1 mm of mercury column
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![2127_Atmospheric pressure.png](https://www.expertsmind.com/../CMSImages/2127_Atmospheric pressure.png)
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(ii) Water Barometer: Let us consider water is needed in the barometer instead of mercury.
![1797_Atmospheric pressure5.png](https://www.expertsmind.com/CMSImages/1797_Atmospheric%20pressure5.png)
The height of the water column in the tube will be 10 3 m. Such a big tube cannot be occupied easily, water barometer is not feasible.
(i) Liquid goes in elevator: When elevator accelerates upward with acceleration then pressure in the fluid, at depth 'h' may be given by,
P = hρ[g+ao]
and force of buoyancy, B = m(g+ao)
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![114_Atmospheric pressure1.png](https://www.expertsmind.com/../CMSImages/114_Atmospheric pressure1.png)
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(ii) Free area of liquid with horizontal acceleration :tanθ = ao/g
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![1812_Atmospheric pressure2.png](https://www.expertsmind.com/../CMSImages/1812_Atmospheric pressure2.png)
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p1-p2=lpa where p1 and p2 are pressures at point 1 & 2.
Then h1 - h2 = lao/g
(iii) Free surface area of liquid in part of rotating cylinder.
![1324_Atmospheric pressure4.png](https://www.expertsmind.com/CMSImages/1324_Atmospheric%20pressure4.png)
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![164_Atmospheric pressure3.png](https://www.expertsmind.com/CMSImages/164_Atmospheric%20pressure3.png)
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