Recreational balloons are made from materials such as, varnished silk, rubber, or other nonporous materials. Scientific balloons are commonly made from rubber. Another material used is neoprene, and these types are commonly filled with helium.
Hot air balloons can float in the air because the air inside the balloon weighs less than the air surrounding it, in the atmosphere. The air inside a hot air balloon is heated by burning a gas to heat the air, which is forced into the balloon. Therefore when the air temperature goes up, air weight goes down, as temperature and air weight are inversely proportional. Therefore when the temperature (T) of a gas (air) inside the balloon increases the density ( D ) of the of the air inside decreases. The gases both inside and out the balloon are the same, the Ideal Gas Law can be used.
Defn; For a gas with a constant molecular weight, the pressure is proportional to the product of the density and the temperature.
P= D x T
This law is a combination of two other laws, Boyle’s law and Charles’ law. These state that pressure is inversely proportional to a fixed volume and proportional to temperature.
Vconstant = T / P
P = T / Vconstant
P = D x T
Dconstant = P / T
T / P = 1 / D constant
Therefore; V is inversely proportional to Density
Therefore; large volume to small density
Small volume to large density
This supports previous comments, for a balloon to float, the volume of air, which is great, has to have a small density.
Hot air balloons therefore stay afloat better when the air surrounding it is cooler, as cool air is dense. The density of air at 0o is 1.29 kg m-3 compared to the density of air at 30o, which is 1.16 kg m-3.
Archimedes’ principle can also show how a hot air balloon can stay a float
Defn; A body completely or partially submerged in a fluid is buoyed up by a force equal to the weight of the fluid
B = Wt fluid displaced
As the Wt is equal to the mass multiplied by the acceleration due to gravity (g).
B = M fluid displaced x g
Therefore mass is also equal to D x V.
B = D fluid displaced x V fluid displaced x g
The volume of air displaced by the balloon is equal to the volume of air inside the balloon.
V fluid displaced = V fluid balloon
The Wt of the air displaced is also therefore equal to the Wt of the air contained in the balloon.
Wt fluid displaced = Wt fluid balloon
The total force applied to the balloon is equal to the buoyant force minus the Wt force.
Fnet = B – Wt
= Wt f b – Wt f d
= M f b x g – M f d x g
= D f b x V b x g – D f d x V b x g
The acceleration due to gravity can be canceled from both sides, also the volume of the balloon and the volume of the displaced air, as they all equal each other.
Therefore; F net = D f d – D f b
The force is therefore +ve when the density of the air in the balloon is less than the density of the air displaced.
D f b < D f d
The balloon can float in the air when the density inside the balloon is less than outside. This has been proven using Achamidies principal and the Ideal Gas Law.
