Going up!
Do you know that some people use a bag of hot air to fly? They really
do. The “bag” is a huge hot-air balloon. The balloon can lift people
because hot air behaves in a special solid or a liquid. So heated air
expands much more than does a heated solid or liquid. The molecules of
hot air push farther and farther apart until only a few molecules take
up a great deal of space.
The hot air that fills a balloon has many fewer molecules than the
colder air outside the balloon. So, the hot air actually weighs much
less than the colder air. Because the hot air is lighter, it rises. It
pushes up inside the balloon—and when the push is strong enough, it
lifts the balloon and keeps it floating in the heavier, colder air
outside.
The hot air that fills a balloon has many
fewer molecules than the colder air outside
the balloon. So, the hot air actually weighs
much less than the colder air. Because the
hot air is lighter, it rises. It pushes up inside
the balloon-and when the push is strong
enough, it lifts the balloon and keeps it
floating in the heavier, colder air outside.
Heating the air in your house won’t make the house float away. But it
does make the air rise. The warmer, lighter air from a stove or furnace
flows upward toward the ceiling, spreading heat through the rooms. Then,
as the air loses heat energy, it cools down. As it sinks toward the
floor, other warm air takes its place.
