Atmospheric Pressure


 
 
Concept Explanation
 

Atmospheric Pressure

Atmospheric pressure is defined as the pressure exerted on an object by the weight of the air above it. The atmospheric pressure on the Earths surface at sea level is about one hundred thousand pascal, i.e., 100 kPa.

Measuring atmospheric pressure: An instrument called barometer is used to measure atmospheric pressure. It consists of long and thin tube filled with mercury and closed at one end. The open end of the tube is placed in a small trough full of mercury. The pressure exerted by air on the mercury of trough is able to hold certain height of the mercury column. When the air pressure is low, the mercury will move down, and when the air pressure increases the height of the mercury column increases. We can measure the pressure by the weight of mercury column in mm of Hg. Note: At sea level, the atmospheric pressure is 760 mm of Hg and is taken to be equal to 1 atmospheric pressure, i.e., 760 mm of Hg = atmospheric pressure at sea level.

Variation of Atmospheric Pressure:

  • The atmospheric pressure changes from place to place time to time. This is due to changes in temperature and quantity of water vapour in the air. The density of moist air is less than that of dry air. The density also decreases with an increase in temperature. Hence, the atmospheric pressure also decreases.
  • Atmospheric pressure at a place is due to the weight of the air above it, therefore the pressure will decrease as we go higher above the sea-level. Thus, the atmospheric pressure decreases with increasing altitude.
  • Aim: To show the presence of atmospheric pressure.

    Materials needed: A glass tumbler (with a smooth edge at the mouth, and withour a rim), a piece of stiff cardboard ( a little bigger than the mouth of the tumbler), and water. (It would be convenient to perform this activity over a wash basin or the kitchen sink.)

    Method:

  • Fill the tumbler with water to the brim.
  • Cover the tumbler with the cardboard piece
  • Place the palm of your hand over the piece of cardboard, and quickly invert the tumbler
  • Slowly remove your hand supporting the piece of cardboard as shown
  • Observation: You will observe that the cardboard piece will not fall.

    Conclusion: Atmospheric pressure provides enough force to support a full glass of water.

     

    Sample Questions
    (More Questions for each concept available in Login)
    Question : 1

    We use a straw (narrow pipe) to drink juice from a glass. Theis is possible due to presence of:

    Right Option : B
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    Explanation
    Question : 2

    A rubber sucker stickes to a surface because of:

    Right Option : D
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    Explanation
    Question : 3

    Which of the following parameters decreases as we go up ?

    Right Option : C
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    Explanation
     
     


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