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www.GOALias.blogspot.comThe next activity is very interesting.This will make you understand moreabout hot air.Activity 8.6Take two paper bags or empty paper cupsof the same size. Hang the two bags inCAUTIONHandle the burning candle carefully.Fig. 8.7 Hot air rising upthe inverted position on the two ends ofa metal or wooden stick.Tie a piece of thread in the middle ofthe stick. Hold the stick by the thread(Fig. 8.7) as in a balance. Put a burningcandle below one of the bags as shownin the figure. Observe what happens.Why is the balance of the bagsdisturbed?Does this activity indicate that warmair rises up? As the warm air rises up,it pushes the bag above the candle. Doesthe disturbance of the balance suggestthat the warm air is lighter than the coldair?Can you now explain why smokealways rises up?Also, it is important to remember thaton heating the air expands and occupiesmore space. When the same thingoccupies more space, it becomes lighter.The warm air is, therefore, lighter thanthe cold air. That is the reason that thesmoke goes up.In nature there are several situations,where warm air rises at a place. The airpressure at that place is lowered. Thecold air from the surrounding areasrushes in to fill its place. This sets upconvection in air, as you learnt inChapter 4.8.4 WINDCURRENTSARE GENERATEDDUETO UNEVENHEATINGONTHE EARTHThese situations are:(a)Uneven heating betweenthe equator and the polesYou might have learnt in Geography thatregions close to the equator getmaximum heat from the Sun. The airin these regions gets warm. The warmair rises, and the cooler air from the84SCIENCE