How do Hot Air Balloons Work:Example of Warm Air Rises Up Video Lecture | Class 7 Oxford Science: Chapter Notes, Worksheets & Tests

Sometimes on a nice clear day, we can see
something special in the sky ...
... a hot air balloon!
Have you ever seen one?
Have you ever wondered how a hot air balloon
works?
How does it get up there?
And how does it /stay/ up there?
Well, it’s all about the air!
There’s nothing special inside the balloon
that makes it float.
It’s just the same air that we’re breathing
right now!
So how does /that/ keep the balloon up there?
Well, even though you can’t see it, air
isn’t /nothing/--it’s /something/.
Air is made of matter!
Anything that you can touch, or feel, or that
takes up space, is made of matter.
So air is made of matter, just like water
is made of matter, and so are the clothes
you’re wearing, and the food you eat -- even
your body is made of matter!
So what kind of stuff is matter made of?
It’s made of tiny things called molecules.
These are little particles that are way too
small to see, but they’re there!
So both inside and outside the balloon, there
are many molecules of air, moving around and
taking up space.
But you know what’s fun about molecules?
When they heat up, they start to move differently!
That’s right!
When something gets warm, the molecules that
it’s made of start to move around /faster/,
bouncing around and spreading out.
And that’s the important part: They spread
out.
So let’s think about how that would happen
in a hot air balloon.
When the balloon is on the ground and ready
to go, it has some air inside it.
And that air is the same temperature as the
air /outside/ of the balloon.
So the air molecules are all moving around
at the same speed, and are just as far apart
from each other, both inside and outside of
the balloon.
But /then/, the balloon pilot makes the air
/inside/ the hot air balloon … hot!
You see, these balloons don’t have engines
like airplanes do.
They have big heaters that send flames up
into the balloon!!
So when the pilot turns on the flames, it
heats up the air /inside/ it!
That makes the molecules of air start bouncing
around inside, and they start to get further
apart from each other.
As these molecules spread out, they start
to fill up the balloon even more!
And when those molecules of air /inside/ the
balloon start to take up more space, we say
that the air becomes less dense.
Soon, the air inside the balloon becomes less
dense than the air /outside/.
Now, maybe you remember when we talked about
why some things float, and other things sink.
We learned that things that are less dense
than /water/ will float to the top, while
things that are more dense will sink.
And the same is true with our balloon!
The hot air /inside/ the balloon is less dense
than the cool air /outside/ -- so the balloon
can float up!
In fact, warm air /always/ floats up.
Have you ever noticed that in the winter,
when you have a heater on, the downstairs
of your house is usually a lot cooler than
the upstairs?
That’s because the warm air in your house
is rising to the top!
So, in a hot air balloon, the hot, less-dense
air inside the balloon rises up on top of
the cooler, denser air outside.
And that makes the balloon rise!
Now, how does the balloon get back down?
Well, the pilot opens a flap at the top of
the balloon that lets the hot air out!
As the balloon fills with cooler, denser air,
it begins to sink gently back down to the
ground.
It may be hard to believe that something so
big can be carried by little molecules we
can’t even see.
But that’s what makes hot air balloons -- and
science! -- so amazing!
Do you have a question about anything else
that floats or flies?
Do you want to know how something works?
Then ask a grownup for help, and leave us
a comment below, or send an email to kids@theSciShow.com!
We’ll see you next time!