If you have seen what is moving up and down in the image above, you will notice that it is a person enjoying a ride in a hot air balloon. While hot air balloons are not the best type of transportation, it is a fun way of traveling in the air. In fact, a lot of people describe hot air balloon rides as one of the most enjoyable experiences they have experienced in life. Hot air balloon rides can be a lot of fun, but do you know how they work?
Observations:
Have you ever seen a hot
air balloon rise off the ground? Well if you have, you probably know
that it takes a lot of work to get a hot air balloon off the ground.
The first thing that happens is that the hot air balloon crew team attaches
a burner system to the basket.
What does the burner do?
Just as it sounds, a burner flame heates the air in the balloon envelope. Yet, at this point, it is only being attached. Next, the crew attaches the balloon envelope and begins to lay it on the ground. Once it is evenly layed out, the crew starts to inflate it by using a powerful fan that is placed at the base of the envelope. When the crew realizes that there is enough air in the balloon, the crew turns on the burner and blasts falme into the envelope mouth. Since we know that if we increase the temperature of a gas that the gas will hit the walls of its contianer.
So how exactly does increasing the temperature affect gas molecules? Well, one of the assumptions of the kinetic-molecular theory states that the average kinetic energy of gas particles depends on the temperature of the gas. Since all the particles of a specific gas have the same mass, their kinetic energies of gas particles increase with an increase in temperature.
In other words, at a higher temperature, the gas molecules are zipping and bouncing around the sides of the balloon a lot faster! As a result, they collide with the sides of the balloon more frequently and with more force. This increased pressure causes the volume of the flexible balloon to increase. This causes the gas molecules to travel a farther distance to reach one end of the balloon to the other. Therefore, the amount of times that the gas particles actually hit the the sides of the balloon decreases. Since there are fewer collisions, it evens out the greater collision force at the higher temperature so in the end, the pressure stays constant.
Since pressure
is constant, the main variables
that help explain why hot air balloons work are temperature
and volume. The relationship between these
two variables is also known as Charles's Law.
If you would like to learn about this relationship by doing an at home
activity, click here: Activity:
How does temperature affect air? More or less, Charles's
law states that as the temperature
a gas increases, the volume of the
gas also increases.
Essentially the main reason why hot air balloons work is due to the fact that hot air is lighter than cool air because it has less mass per unit volume. If you think about the overall balloon, this makes sense. First of all, the increase in temperature causes the molecules to move faster and to expand over the same volume (inside the envelope). Yet, for the hot air, their will be less mass per unit volume than in cooler air. Therefore, the density of the gas decreases as temperature increases which makes it rise into the air because the balloon's density is lower than the density of the air around surrounding it.
Another phenomena that deals with changes in density is How does the Cartesian Diver Work?
In order for the balloon to stay in the air, the gases and air inside the envelope must be reheated. This is due to the fact that if temperature of the air and gas inside the hot air balloon decreases, the total volume of the gas will increase. This will then cause the density of the gas to increase and possibly become more dense than the air surrounding it and sink!
If the pilot wants to return to the ground, he or she has to vent the parachute valve and allow some of the hot air to escape from the envelope which will decrease the temperature and compress the gas molecules and allow the balloon to sink to the ground.
If helium is a gas that is low in
density
, why is it not used to make hot air balloons work? First of all,
let's make sure that we understand what helium is. Many students, such
as those in the students' ideas
page believe that helium is hot air. Well, it is not air which therefore
means that it is not hot air! The main reason why helium is not used
is due to the fact that it costs more than other gases that could be used
to make hot air balloons work.
Now that you know how air balloons work,
you can do the following activity and build your own mini hot air balloon
by clicking below.
NOTE: Please do this activity under adult supervision!
Gases Web of Ideas Phenomena Lessons Students' Ideas Chemistry Web of Ideas