Air Expands on heating


Observe that air expands on heating.

The Theory

The air around us has many molecules in it, freely moving around. These molecules move randomly. As the air heats up or its temperature increases, these molecules gain energy, making them move faster and apart. Hence, air expands on heating and contract on cooling.

The air molecules are not as close to each other as in a solid. There are large spaces between them, and hence it is less dense. The same number of molecules will occupy more space/volume as the air expands on heating. Thus, warm air becomes lighter and rises upward. This is the principle behind the working of hot air balloons. The hot air inside the balloon expands and becomes lighter, keeping the balloon inflated and lifting it through the denser and cooler surrounding air.

Different parts of the earth receive different amounts of sunlight, which causes uneven heating of the air. The air from the equator gets heated up more and rises. This reduces the air pressure in the region, and the cold air from the poles rushes towards the equator to fill its place. This moving air is called wind.

The land breeze and sea breeze are winds caused by uneven heating of land and water, resulting in the uneven heating of air above them. In comparison to water, the ground always warms and cools more quickly. In coastal regions, the air above the land warms up much more rapidly during the day than the water in the sea. This hotter and lighter air above the ground rises. The cooler air from the sea rushes towards the land to take its place. This is called sea breeze. Land cools down more quickly at night, whereas the water remains warm. Due to its relative heat, the air above the water rises and the air above land rushes towards the sea, known as the land breeze.

If we place a balloon on the mouth of an empty bottle or a test tube and place it in hot water, the air inside the test tube becomes hot. As the air expands, the air molecules will collide with the walls of the balloon, inflating it. When the same test tube with the balloon is transferred to cold water, the air contracts decreasing the pressure on the walls of the balloon. Thus the balloon will deflate.

Learning Outcome:

· Students witness and learn about the expansion of air on heating.

· Students understand the concepts of wind-generated due to the uneven heating of air