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| Atmospheric Pressure |
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| The atmosphere is the layer of air around the earth. It extends up to 1000 km above the surface of the earth and has a total weight of 4.5 1018 kg (i.e., five million billion tons). This weight exerts a pressure on the surface of the earth which is called atmospheric pressure. At sea level it is about 105 Pa. We do not normally feel the large atmospheric pressure because the pressure inside our bodies is almost the same as the external pressure and so it is balanced. |
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| At high altitudes the air pressure is less, therefore the breathing is difficult and nose bleeding may occur. Hence, modern aircrafts have pressurised cabins in which air pressure is increased sufficiently to safeguard the passengers and the crew. |
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| Collapsing Can Experiment |
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| The effect of the enormous atmospheric pressure can be demonstrated by removing air from a can using a vacuum pump. The can collapses as the air pressure inside the can becomes less than the atmospheric pressure outside. |
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| Atmospheric pressure is used in many of the things that we do. For example, when we drink water with a straw [figure (a) shown below], while sucking, our lungs expand and air in the drinking straw goes into them. The air pressure in the straw decreases and the atmospheric pressure acting on the surface of water |
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| then forces the water to move up inside the straw and into our mouth. In the same way, atmospheric pressure forces a liquid to move into a syringe when its plunger is withdrawn [figure (b) shown above]. Rubber suckers are used for installing hooks in the kitchen and bathroom. The air between the rubber sucker and the wall is first forced out so that the atmospheric pressure pressing on it can hold it firmly against the wall [figure (c) shown above]. |
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| Atmospheric pressure measurements are important to meteorologists for weather forecasts. The unit of pressure used for meteorological purpose is called the bar. |
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| A bar is the CGS unit of pressure equal to 106 dynes per square centimetre or 105 pascals (approximately 750 mm Hg or 0.987 atmosphere). The millibar (100 Pa) is commonly used in meteorology. |
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