Atmospheric Pressure

  • Atmospheric pressure on the surface of the earth is the thrust exerted per unit area on the earth surface due to column of air.
  • The existence of atmospheric pressure can be easily demonstrated in laboratory by collapsing tin can experiment.
  • Few effects of atmospheric pressure are
  • Sucking a drink with a straw
  • Filling ink of fountain pen
  • Action of rubber suckers
  • Astronauts wearing special suits
  • Atmospheric pressure at a place is measured by a barometer. The change in the some of the factors like altitude, season, temperature, etc. cause a change in the barometric pressure which indicates a change in the air pressure.
  • The three types of barometers used are
  1. Simple barometer
  2. Fortin’s barometer
  3. Aneroid barometer.
  • Simple barometer was designed by Torricelli in 1643 using mercury as a barometric liquid.
  • The vertical height of mercury column from the mercury surface in trough to the level in tube is a measure of the atmospheric pressure.
  • The barometric height at normal temperature and pressure is 760 mm of mercury.
  • The Fortin barometer is a modified form of a simple barometer. It is used in laboratory to measure the atmospheric pressure.
  • It also uses mercury as a barometric liquid. To measure the atmospheric pressure, the level of mercury in leather cup is raised up or lowered down.
  • Aneroid barometer is device which is used to measure the pressure exerted by the atmosphere without the use of the fluids. It is light and portable and can be carried easily from one place to another. It needs no prior adjustment like Fortin barometer.
  • A barometer is used to measure the atmospheric pressure, for weather forecasting and as an altimeter to measure the height.

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  • Q1

    Give two factors which affect the atmospheric pressure.

    Marks:1
    Answer:

    Two factors which affect the atmospheric pressure are:
    · Temperature: An increase in temperature of air results in the decrease in the atmospheric pressure.
    · Moisture: An increase in moisture in the air results in the decrease in the atmospheric pressure.

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  • Q2

    What is aneroid barometer?

    Marks:1
    Answer:

    Aneroid barometer is a device which is used to measure the pressure exerted by the atmosphere without the use of liquids.

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  • Q3

    How is barometric pressure related to the height?

    Marks:1
    Answer:

    Barometric pressure is inversely proportional to height above the sea level, i.e., the barometric pressure decreases with increase in height or vice-versa.

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  • Q4

    What is the atmospheric pressure in cm of mercury column and in Pascal?

    Marks:1
    Answer:

    The atmospheric pressure at 0oC and at sea level is 76 cm of the mercury column or 1.013 x 105 Pa.

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  • Q5

    What are atmosphere and atmospheric pressure?

    Marks:1
    Answer:

    There is an envelope of air around the world which is known as the atmosphere.
    Atmospheric pressure is the pressure exerted by this air. It is due to the weight of the air above us.

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