Leslie Tate, age 9, of Delmar, New York, for his question:
What makes hot air rise?
A fire in the grate heats a patch of air. The warm air heats up the grate, the burning logs and other things it touches. Some of it spreads out with a cozy glow into the room. But a lot of it streams up the chimney and as it rises, it carries up plumes of smoke and blazing flames.
A hot radiator heats the air around it. This air spreads out and warms up the cooler air in the room. We can feel it happen. The warm air spreads around in a certain way and sometimes you can see some of this happen. Watch the air above the radiator. Sometimes it seems to wobble and the wall behind the radiator seems to move in wavy patches. These strange waves are rising streams or currents of warm air. Later they will spread out to mix and mingle with the cooler air in the room.
It took the experts ages to figure out why this happens. The problem was tough because the air is invisible. We cannot see it. It is made of gas molecules that are much, much too small for our eyes to see. There are billions of these tiny molecules in just a thimbleful of air. They float around separately with plenty of space between each other. And they are speedy little fellows.
The air molecules in a room dash around at more than 1000 miles an hour. But they do not go in straight lines. Each little molecule zips and zags. And it smashes and bashes into other molecules thousands of times every second. As the air gets warmer, the air molecules speed up they travel faster and get bashed in more traffic accidents. Heat gives them more energy to speed faster. They use this energy to spread out far and wide.
As the air warms up, the separate molecules rush farther apart and the air must expand, or spread:, out to get more room. There is more space between the gas molecules and the warm air becomes less heavy. So warm air is lighter than cool air and it needs more room. It has to expand and, when it can, it spreads out in all directions. But often there is cool, heavier air around it. Then it goes up like a rising balloon.
Balloons filled with hot air lifted men up on their very first flights. Almost 200 years ago, a big balloon painted with gaudy pictures took two Frenchmen on a flight over the City of Paris. The big, bright bag was filled with hot, smoky air from a fire. The two brave men climbed into a round passenger boat and as the balloon filled with light air, it rose and lifted t6tri safely up and over the city. Thousands of people watched and agreed that the flight was a wonderful sight.
This famous flight in a hot air balloon was made on November 21, 1783. Since then we have learned to tame hot air for other duties. Hot, steamy air expands from a boiler with terrific force. In a steam engine its pushing force can be guided to turn the wheels of a locomotive and pull a whole train. An automobile has a different engine, but its strength also comes from hot, expanding air. The hot air that explodes in the cylinders comes from burning gasoline and its strength is guided to push along the automobile.