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Vacuum Container and Buoyancy
Name: Casper
Status: student
Grade: other
Country: Denmark
Date: N/A
Question:
Hello, Maybe this is a stupid question, but I have never
received a satisfying answer, so I will try my luck here, worst case
scenario is being called stupid, so why not. My question is: Is it
theoretical possible to create a container that can handle the pressure of a
high 'enough' level of vacuum to become zero-weight or even float like
Helium? Or how close is it possible to get to the zero weight of the
container. My thought is based on Helium balloons, and my thought is that a
high level of vacuum has to be better than the expensive Helium.
Replies:
Hi Casper,
The key principle to consider is "buoyancy" -- helium balloons float
in air, and air has a particular density (that depends on temperature
and pressure). If you build a container whose average density is less
than that of air (in other words, its mass is lower than the mass of
an equal volume of air), the container will float. You can also create
buoyant lift by simply heating air (such as with a hot air balloon).
Hotter air expands, and therefore has lower density, than colder air.
Your first question about 'is it possible' to build a lighter-than-air
craft, I would have to say 'yes', although I am not aware of such a
device actually having been built using vacuum rather than a
lighter-than-air gas such as hydrogen or helium. I would add that its
weight cannot be zero, though, as the container must have some weight
(blimps are lighter than air, but are still very heavy in terms of
actual weight - they just have a very large volume). The reason the
vacuum route may not have been pursued I would guess is because of
your second question. Cost is an interesting twist. It is very
expensive to build a strong, rigid container that is also well-sealed
enough to hold vacuum, and much easier to build, basically, a big sack
that you can fill with a lighter-than-air gas. If your goal is to be
cost-effective, the "big sack of gas" is a pretty cheap option.
Hope this helps,
Burr Zimmerman
Casper,
That is not a stupid question, that is a great question. You
question is about Archemedes' Principle, where the evacuated
container displaces air and produces a buoyancy force equal to the
weight of the displaced air. The theory here is sound. If you could
produce a lightweight container which can withstand the pressure
without collapsing then this would work. And in fact this idea was
proposed in 1670 by Father Francesco de Lana, an Italian Jesuit
priest, who suggested using very thin copper to make lightweight
evacuated spheres. Such spheres would collapse under the pressure,
however. Even today we do not have any materials strong enough to
withstand such pressure while remaining light enough so that the
buoyancy force exceeds the gravitational force. You would need to
fill the interior of such an object with another gas to provide the
outward pressure necessary to resist the inward pressure of the
atmosphere. Hydrogen weighs only 1/10th that of air and is the ideal
candidate although it does have safety concerns (it is highly flammable).
John Strong
Be assured, neither you nor your question is "stupid". Many have asked
this question, and confusion takes hold. I hope I do a little bit clearer.
The principle of buoyancy is this: An object is exposed to a buoyant
force that is equal to the WEIGHT of the VOLUME of the fluid that the object
displaces when placed on/in the liquid. Now this requires some thought
because "what is equal" is the weight (force) that weighs the same amount as
the same amount of fluid (volume) displaced. That is a little tricky.
Possibly an example will help.
Suppose you float an aluminum "boat" on water. If the aluminum is
crumpled into a ball, the "boat" will sink because you have decreased its
volume. But, if the aluminum foil is shaped into a "boat" so that it
occupies a larger volume, the "boat" will float. If you have a closed boat,
it does not matter what is in the boat, what the "boat" is made of -- only
its shape. What is important is the weight of the volume of water that is
occupied by the "boat".
If you could make a balloon, the volume whose weight exceeds the volume
of air displaced by the surrounding air, it will sink. If the balloon's
weight is less than the weight than the amount of fluid displaced, it will
float. So, properly designed, the "boat" could be lead or concrete. The
important issue is, "The weight of the volume of fluid displaced by the
object.
Vince Calder
Hi Sophia,
This is a reasonable question. If it were possible to construct a lightweight
and strong container and remove the air from it, the container would float like
a balloon does. That is true. Unfortunately, the density of air is quite small,
so that the weight of the container would need to be very small for it to float.
Because the pressure of air is fairly high, there are no practicable materials
that are light and strong enough to withstand the pressure and still float. That
is why it is necessary to use an internal gas (helium, hydrogen) to support the
pressure to make the container (blimp or balloon) practicable.
This is different from floating in water. Water is dense, and has large but
manageable pressure for containers in the ocean (like a submarine). Standard
steel construction is adequate to make a vessel that is buoyant in water in
reasonable depths.
Robert Erck
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Update: June 2012
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