Hydraulic press or Brahma press
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Hydraulic press is used to compress soft materials like cotton, paper, and cloth. These soft materials are placed on the larger piston as shown.
Hydraulic brake
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Atmospheric pressure
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The gaseous envelope surrounding the Earth is called earth's atmosphere, extending to a few kilometres above the surface. The pressure exerted by the atmosphere is called atmospheric pressure. The magnitude of this pressure is nearly 1.01x105N/m2 (Refer Torricelli's experiment) at sea level.
An experiment was conducted by Otto Van Guericke at Magdeburg, where ten horses, unsuccessfully, tried to pull two copper hemispheres (with no air inside) apart.
The presence of atmospheric pressure can be studied through following illustrations
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Activity I
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Remove air from a can by putting a little water into the can and boiling the water in it. The steam forces the air out. Close the opening and keep it below a tap, we find the can crumbling as the pressure inside the can is very small when compared to the pressure outside.
Activity II
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In the above arrangement where the cardboard is intact, water does not come out of the tumbler as the cardboard is balanced due to pressure of liquid in the tumbler and the atmospheric pressure from below.
Activity III
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The fan driven by an electric motor reduces the pressure near the mouthpiece. The pressure due to atmosphere being large, forces air along with dust particles into the long tube and then, into the bag. The fan forces the air out of the sweeper.
Activity IV
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Wet the rim of the plumber's force cup with water and press it against the wall to squeeze out most of the air. We find it difficult to pull the cup. (This simple principle is used to remove any blocks in holes of washbasin)
A Plumber's force cup pushed against a blackboard
A Plumber's force Cup is pushed against the blackboard. What prevents it from being pulled away?
While drinking through a straw, we suck. This expands the lungs and air passes into them. Since atmospheric pressure inside the bottle is greater, the liquid is pushed up through the straw.
Torricelli's experiment
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Torricelli devised a method to measure the atmospheric pressure. He took a 1metre long glass tube of uniform area of cross-section, closed at one end. He filled the whole tube with mercury so that no air or water droplets remained inside. He inverted the glass tube, closing the open end with his thumb, into a trough containing mercury. The mercury dripped down and settled at a height of 76cm from the level of mercury in the trough, as shown.
Torricelli explained that the hydrostatic pressure exerted by the vertical column in the tube balanced the atmospheric pressure, acting on the free surface of the mercury in the meniscus.
On inclining, raising or lowering the trough, the vertical height was found to be 76cm.
Atmospheric pressure = hrg
Units of atmospheric pressure
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- SI unit of pressure is N/m2 or Pascal (Pa)
- Atmospheric pressure is also expressed in mm of Hg or cm of Hg.
- Another unit is torr. [1 torr= 1mm of Hg]
- For meteorological purposes, the unit used is bar.
