Friday, December 16, 2011

Globes and Zeppelins

Everything I said about density and hydrostatic push is valid for all fluids, gas and liquid. Therefore it can also be applied to the air. The bodies immersed in air also experiment a push towards up that equals the weight of the displaced volume of air, but this push has a low value for objects with high density, as the human body for example, and its effects are not noticeable. This is not the case for low-density objects.

Some gases, like the hydrogen or helium, are less dense than the air. If we fill a bag of an impermeable material with any of these gases, the bag will experiment a push towards up superior to its weight and, if we set it free, it will ascend. This is the principle in which globes and zeppelins go by.

When we talk about the atmosphere, the lower layers of air close to the surface of the earth are denser, partially for the pressure of the superior layers.
We also need to know that the gas density decreases when the temperature of that gas increases, that is if it doesn’t find obstacles to increase volume.

Open and Closed Globes

The Globes that use the mentioned gases (Hydrogen, Helium) have to be necessarily closed, other way the gas would just escape and the globe would crash. But there are ways you can make an Open Globe fly. In this case the gas used to fill the Globe it is the same air, but increasing its temperature above the temperature of the surrounding air.

As long as the temperature of the air contained inside the globe it’s higher than the one outside of it, the globe will ascend, but there is a limitation. We said already that the air density in higher atmosphere layers is low, As the globe ascends, it will eventually find these high-density air layers and to keep ascending it will need to increase its temperature even further. Finally, in higher atmosphere layers, the density of the air is so low he globe won’t be able to ascend anymore no matter how much it increases the temperature of the air inside of it.

The Zeppelin

The Globes, open or closed, depend of the airstreams and wind to come and go. The zeppelin is a kind of closed Globe that has an engine attached to it that moves some propellers. This way it no longer depends on the wind. At the beginning the zeppelins were full of Hydrogen, but this gas is very flammable so it was replaced by Helium, although more expensive it was safer.

The last zeppelin dedicated to passenger transportation was the Hindenburg, of 247 m. of length and capable of lifting 22200 Kg. It was full with Hydrogen, and in May 6th 1937 it went on fire when it was landing on New York.
Source: Source: Ciencia Recreativa (1992) - Planeta Agostini


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