By learning Physics Topics, we can gain a deeper appreciation for the natural world and our place in it.
Why Does Air Expand On Heating
When air is heated, its volume increases and it occupies a bigger space. We say that air expands on heating.
The fact that air expands on heating can be demonstrated by performing an activity as follows.
(i) Take a boiling tube. The empty boiling tube contains air (though we cannot see this air). Take a balloon, stretch the mouth of the balloon and fix it tightly over the neck of the boiling tube [as shown in Figure (a)], Make the joint of balloon with the neck of boiling tube air-tight by wrapping some adhesive tape over it.
(ii) Take some hot water in a beaker. Place the boiling tube (having rubber balloon fixed to its neck) in the hot water in the beaker. After 2-3 minutes we will see that the balloon gets inflated (as if someone has filled air in it) [see Figure (b)]. This can be explained as follows : When the boiling tube is placed in hot water, the air present in boiling tube gets heated, expands and its volume increases.
The increased volume of air fills the balloon due to which the balloon gets inflated. So, it is the ‘expansion of air on heating’ which inflates the balloon when the boiling tube is placed in hot water. From this activity we conclude that air expands on heating.
If we take out the boiling tube from hot water and allow it to cool, then after some time, the balloon will get deflated (and come back to its original size). This is because the air present in boiling tube and balloon contracts on cooling due to which its volume decreases and balloon gets deflated.
Hot Air (or Warm Air) is Lighter Than Cold Air. Justify
We have just studied that air expands on heating and occupies more space. Now, when the same air occupies more space, it becomes lighter (or less heavy). This means that hot air is lighter than cold air. Now, when air gets heated at a place, it expands, becomes lighter and rises up in the sky. In other words, hot air (or warm air) rises up. We will now describe an activity to show that hot air is lighter (less dense) than cold air and, being lighter, hot air rises upwards.
(i) Take two paper bags (or paper cups) of exactly the same size. Hang the two paper bags in the inverted position (with their open faces pointing downwards) on the two sides of a light wooden stick with the help of short threads [see Figure (a)], Tie a piece of thread in the middle of the stick.
The other end of this thread is tied to a hook in the roof of a verandah. The wooden stick with two paper bags tied to its two ends will hang like a common beam balance. Initially, the wooden stick is perfectly horizontal showing that the two paper bags contain an equal mass of the same air (cold air) [see Figure (a)].
(ii) Put a burning candle below the open mouth of the left side paper bag [see Figure (b)], We will see that after some time, the left side of the wooden stick goes up showing that it has become lighter than the right side [see Figure (b)]. This observation can be explained as follows: When a burning candle is placed below the left side paper bag, the air above the candle flame gets heated.
The hot air, being lighter, rises up and fills the left side paper bag (by displacing the heavier, cold air from it). Since the left side paper bag now contains hot air (which is lighter than cold air present in right side paper bag), therefore, the left end of wooden stick becomes lighter and moves up [see Figure (b)].
The above activity gives us two conclusions about the nature of hot air :
- hot air is lighter (than cold air), and
- being lighter, hot air rises up.
The fact that hot air is lighter (than cold air) and rises up, is utilised in launching hot air balloons (see Figure ). A white, grey or black substance (formed of small particles) emitted from a burning substance is called smoke.
Smoke always rises up. This can be explained as follows : Smoke is produced by the burning of a substance, therefore, smoke is always accompanied by hot air (or warm air). Since warm air is lighter (than cold air), therefore, smoke contained in warm air always rises up. The air inside a house gets warmed by the body heat of all the persons living in the house and also by the burning of fuels (LPG, etc.) in the kitchen.
If we want to buy a house, then we should buy a house having windows as well as ventilators. The windows are for bringing in fresh air from outside into the house. The ventilators (located near the roof of house) take out the warm and stale air from inside the house. The presence of ventilators in the house increases our comfort. In nature there are several situations when air at a place gets heated, becomes lighter and rises up.
When air at a place gets heated and rises up, then the pressure at that place is lowered (or reduced). The cold air from the surrounding areas (being at higher pressure) rushes in to fill the space vacated by hot rising air. This leads to blowing of winds. We will now describe the formation of winds in detail.