What are the physics behind a helium balloon? Why does it rise?
The physics behind the hot air balloon is based on Archimedes Principle:
a law stating that a body totally or partially immersed in a fluid is subject to an upward force equal in magnitude to the weight of the fluid it displaces
(Oxford dictionary).
So, therefore, the hot air balloon which is immersed in the fluid of the earth’s atmospheric gases will experience a force in an upward direction to counteract the weight of the balloon. In helium balloons, the helium inside is less dense than the air surrounding it. Helium is lighter per volume so gravity will have less of an effect on it so will rise.
The pressure above the balloon is less than the pressure of the air below, the difference in pressure means an unbalanced force upwards called buoyancy. Buoyancy is experienced on every object although usually so small so doesn’t have an effect. The pressure difference is very small, although, sufficient to yield Archimedes Law. A balloon doesn’t require very strong forces so the change in pressure can cause an unbalanced force upwards.
1 cubic metre of helium will lift 0.9888116 kilograms. So, to work out how much helium you need, divide the payload and weight of balloon by 0.9888116 to get the total number of cubic metres of helium required. However, this will be at equilibrium so in order for the balloon to rise you’ll need slightly more helium. The excess helium added is called positive lift, the more positive lift the faster the balloon will go.
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