Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: gazza711 on 31/05/2015 19:05:57
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If you took an indestructible vessel into outer space with the lid off,then when in space put the lid back on and brought it back down to sea level.would it float quicker than the equivalent helium filled balloon?
Now,would creating a vacuum balloon at sea level have the same force upwards?
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The vacuum balloon has the problem that the skin would need to be rigid to keep it from being crushed by atmospheric pressure. The buoyant force on a balloon depends on its total mass, so while the empty interior would contribute negligibly to the mass, you would still need to take into account the mass of the skin. It would be difficult to make a skin sufficiently rigid but that was still light enough to float in the air.
The helium in a helium balloon of course is more massive than the same volume of vacuum would be, but the helium exerts pressure on the inside of the balloon to balance the external pressure of the atmosphere. Consequently, a helium balloon requires a much less rigid skin (which tends to mean consequently less massive).
However, if we assume the existence of a sufficiently light and rigid skin, then you are correct that this skin filled with nothing would have a greater buoyant force on it than if the same skin were filled with helium.
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If you were able to fill a balloon with vacuum and keep it at a fixed volume, then yes it would rise more quickly than a helium balloon.
Atmospheric pressure is very high compared to vacuum, and would easily crush most containers of "vacuum" unless the walls are extremely strong. For instance, scientists running experiments at the pressure one would find on the surface of the moon use steel or aluminum boxes with 1/2 in thick walls (for example see this instrument: http://commons.wikimedia.org/wiki/File:Molecular-beam_epitaxy_system_at_LAAS_0516.jpg)
To prevent the balloon from collapsing, one would need to use a very strong balloon that would be substantially more massive than the difference in mass between a vacuum and the surrounding air.
However, there is another way to do something very similar--one can reduce the amount of gas in a given volume without reducing the pressure, if the temperature of the gas is increased.
pressure = (# of molecules of gas) x (absolute temperature) x (universal gas constant) / (volume)
P = n x R x T / V
This is how hot air balloons work. The pressure inside the balloon is the same as the pressure outside the balloon, but the density of gas inside is less than the surrounding air because it is hotter.
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Just to add to what ChiralSPO said. If you fill a vessel with vacuum and another identical vessel with helium, the helium filled vessel will weigh more than the one with vacuum in it. It won't matter whether the vacuum was created in space or at sea level. It will however depend on the pressure of the helium in the vessel, the greater the pressure, the more helium inside, the heavier it weighs.
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You folks made a very simple answer seem very complex to answer. The answer is simply - Yes.
would it float quicker than the equivalent helium filled balloon?
I don't know what you mean by "float quicker" but if you're asking whether it's more buoyant then the answer is yes.
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The answer is yes (with imaginary materials) and no (with real ones because you can't make a container that's light enough and strong enough).