Objects that are denser than water (> 1g/cm3) tend to sink and objects less dense than water float. The same principle applies to air, which is also a fluid. Typical air is made up of 78% Nitrogen and 21% Oxygen. If you have a gas that is less dense than air, it will rise. This is exactly what happens when you fill up a balloon with helium. Helium is a very light gas and has a much lower molar weight and density than typical air. For this reason helium balloons fly off into the sky if you let go of them. We also know that due to the kinetic theory of matter that when we heat a substance and raise its temperature, its volume expands and density goes down. Conversely, cooling a substance lowers its temperature and volume and increases its density.
If we heat the air inside of a balloon it will expand to fill the balloon. The air inside the balloon has higher energy and is moving faster than the air outside of it. It uses this energy and motion to expand and make the balloon expand . As the balloon gets bigger (its volume is increasing) the density of the air inside the balloon decreases and becomes lower than the air outside of it and if the density lowers enough it will float or rise up into the sky. You might have heard the phrase "warm air rises" and this is the reason why. The particles in warm air are more spread out and it is less dense than cold air.
Another way of looking at it is using Buoyant force and weight. The weight *mass times gravity) of the entire balloon is pulling it towards the ground and the buoyant force of air (all fluids exert a buoyant force not just water!) is equal to the mass of air displaced multiplied by the acceleration due to gravity on the surface of the earth. The more air you displace as the balloon expands when the air in it is heated, the greater the buoyant force. Once the buoyant force exceeds the weight of the balloon, it will fly upwards.
Purpose: to create a mini-hot air balloon and achieve flight using what we know about science.
Materials
· 4-6 Straws
· Scissors
· Tape
· 3 Candles (break in half!)
· Aluminum foil
· Long Handled Lighter or Bunsen Burner
· High Density waste basket bag (6-8um)
· Newspaper or cardboard (makes clean-up easier if you put it down first!)
Procedure:
1) Insert the ends of the straws into one another to make two equal length straw poles (2-3 straw each). Make them into a plus sign (+) using as little scotch tape as possible (keep the weight low).
2) Take the three candles and cut them in half with the scissors. Dig the wick out of the bottom half. You now turned three candles into six and it will help speed up/ensure the flotation/flying process.
3) Cut a very small square of aluminum foil (~5cm by 5cm) and use the lighter or burner to melt the bottom end of the candles a little bit and then press it down onto the aluminum foil and hold it for 5 seconds. The wax should dry and it will stick to the foil. Repeat for the rest of the candles.
4) Tape the aluminum foil candle holder to the straw base. It should look something like this.
5) Then you want to take the plastic bag (typical grocery store bags are too heavy!!!) and tape it (use as little tape as possible (2-3 centimeters per corner is enough!) to the four ends of the straw. Just put half of the tape on the very end of the bag and then the other half onto the straw (don't crinkle the bag, we want it to open as big as possible when the candles are lit).
6) When ready two people should hold the bag open and in the air (make sure its fully open) and one person lights the candles. If you want, use a Sharpie to write the name of your balloon on it before lighting it. Once lit, eventually it should float off on its own. You can tie a string to it and pull it down before it reaches the ceiling or have someone stand on a high lab table preventing it from going out of reach.
LAB WRITEUP: One page typed in MLA format explaining how hot air balloons work with a picture attached of your mini hot air balloon floating. You should explain density and thermal expansion thoroughly in your write-up.