Explaining Charles's law.

Question

7. In a hot-air balloon, the balloonist may have to light the propane torch for a few minutes. Using Charles’s law, explain why he might need to do this.

8. As the temperature of a given sample of gas decreases at constant pressure, the volume of the gas:
a) decreases b) increases c) remains the same

Answer 1

On question 7, the balloonist will have to run the torch for a few minutes because over time the gas in the balloon will cool off, especially at higher altitudes.  In order to keep the balloon floating, he will have to re-heat the air in it a little so it stays warmer and keeps the balloon fully inflated (air inside the balloon will be less dense = balloon will float!).

 

If he doesn't do this the air in the balloon will cool off and the balloon will deflate, and the air inside the balloon will reach the same density as the outside air.  The balloon will sink and crash...  :(

Answer 2

7. Charles law states that for a fixed mass of gas at constant pressure the volume varies directly proportionately as the absolute temperature. Hence the air is heated expanding the balloon, giving rise to an increased uptrust (Archimedes principle) due to the increased amount of air displaced outside the balloon and hence the force on the balloon causes it to rise in the air.

 

8. a) decreases, according to Charles law.