Hot Air Balloons - How they work, how they comply with laws of thermodynamics, and how to create a home-made balloon

Essay by SolidusHigh School, 10th gradeA+, May 2007

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Hot air balloons might seem magical to some people because of its ability to float in the sky, but the process that allows them to take flight is actually very uncomplicated. The balloons are prime examples of the implementation of gas laws. The physics are based on the simple principle that warm air rises in cooler air. Hot air has less mass per unit of volume.

The relevant gas law in use is Charles’s Law, which is V1/T1=V2/T2. The law states that raising the temperature (T1) will also increase the volume (V2), in order to keep the equation true. Generally, air is heated through the use of propane burners. When ignited, the volume of the gas increases but the balloon cannot expand much, so air will escape out of the bottom of the balloon. Eventually, enough air will escape that the balloon will become lighten enough to take flight. Lowering air density in a balloon without losing air pressure can be accomplished with heat.

After all, pressure must remain constant in the equation V1/T1=V2/T2.

Hot air balloons have to be extremely large in order for it to work. A cubic foot of air is calculated to weigh 28 grams. If you heat a cubic foot of air by 100 degrees Fahrenheit, it will weigh 21 grams. This means that each cubic foot in a hot air balloon can lift 7 grams. Therefore, lifting a 100-pound girl will require 1,000 cubic feet of hot air.

Descent can be achieved with two possible methods. One can simply let the air cool down, or one can open a small hole to allow more air in. The latter is the more popular and preferred method because it is a lot quicker than waiting for the air to cool. These methods make sense because it...