7th Class Science Winds, Storm and Cyclones Air Around us

Air Around us

Category : 7th Class

*         Air Around us


Every empty space is filled with air. Air is a mixture of gases. It contains 78% of nitrogen, 21% of oxygen, and small amount of other gases. Air also contains water vapor, which comes from the evaporation of water. Atmosphere of the Earth is extended up to several kilometers above the surface. The percent of gases and dirt particle in air, including water vapor, vary slightly according to the altitude and location. For example, air contains small amount of water vapor in desert as well as in low temperate zone. The pressure exerted by atmosphere on Earth is called the atmospheric pressure. Atmospheric pressure is caused by the weight of air in atmosphere. A pressure measuring device/called barometer is used for the measurement of atmospheric pressure.

An air filled car tier is heavier than an empty tier, it proves that air has weight. A balloon inflates when air fills inside of it, therefore, air exerts pressure.


          Fig. (1)                                                        Fig. (2)  

In figure (1), water filled bottle is punched. The cap of bottle is screwed tightly. Empty part of bottle is filled with air and extra air cannot enter through the cap as it is airtight. Thus water drops stop flowing from the holes. Flow of drops of water stops as the water inside the bottle does not get extra pressure from outer air. In figure (2), big hole causes the heavy flow of water from the bottle. Air continues. to pass through the big holes due to outer air pressure.


Fig. (1) A deflated balloon                            Fig. (2) An inflated balloon  

Air expands when heat is applied, which is clear by the above pictures.

A water containing bottle gets heated and a balloon on the mouth of the bottle is inflated. From the above experiment, it is clear that when heat is applied on a water containing jar, the upper part of jar that contains air gets heated and expands causing the balloon to inflate by hot air. A heater below the bottle gives heat to the steel pot. Upper part of both the bottle is empty. Air blows from high pressure area to low pressure. Warm air moves up. Cold air fills the gap made by the movement of hot air upwards. The pressure of air can be observed while riding a bicycle. When air blows from north to south and a bicycle rider riding the bicycle towards south gets extra support from air and while riding the bicycle in opposite direction, air pushes the bicycle rider in opposite direction.


Look at the following pictures



In the picture (1), two paper boxes of equal weight have shown. In picture (2), one paper box is lifted upward on applied heat. This experiment proves that, cold air is heavier than warm air. The box with cold air is not lifted upward.





       Which one of the following statements is correct?

                Statement 1: Air exerts pressure on us.

Statement 2 : Air does not exert pressure on us.

(a) Statement 1

(b) Statement 2

(c) Both statements are correct

(d) Both statements are incorrect

(e) None of these


Answer: (a)



       Which one of the following statements is correct about air?

(a) Air is made of gasses

(b) Warm air rises up

(c) Air blows from high pressure aria to low pressure area

(d) All of these

(e) None of these


Answer: (d)

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