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Buoyant Vacuums?
Name: Steve T.
Status: other
Age: 20s
Location: N/A
Country: N/A
Date: 2001-2002
Question:
is it possible to take advantage of the fact that a
vacuum is lighter than helium by creating an envelope that was engineered
to be very light while also strong enough to resist acute internal
strain? could purging a container like this cause it to float?
Replies:
Steve,
The major difficulty with such a device is the extreme amount of pressure
exerted by the atmosphere. Standard air pressure is 14.7 pounds per square
inch (1 atmosphere). A surface the size of a window (3 ft by 4 ft) feels a
force of more than 25,000 pounds from the atmosphere. Fortunately, the
windows on your house feel this same pressure from both sides: the air
pressure inside your house keeps the air pressure outside your house from
crushing it. A device with a vacuum inside has NO pressure inside. The
only materials that can handle such pressure without support from air inside
are strong metals, and then only when very thick. The weight of all the air
molecules above us makes such a device highly unlikely.
Helium and hot air are much more reasonable because they do provide the
necessary pressure. The gas contained in a helium or hot air balloon pushes
with a full "atmosphere" of pressure while being less dense than air. As
they rise, the air gets less dense. This is why they stop rising once
reaching a certain height.
Dr. Ken Mellendorf
Physics Instructor
Illinois Central College
It would work, but the practical difficulty is to make the container light
enough and still strong enough not to collapse under the atmospheric
pressure.
Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
Yes. Obviously you would have to fabricate some very light structural
support to take advantage of this idea. The lightest structural support
I can think of would be a bag full of very warm hydrogen, which is not
exactly helpful in this context.
Tim Mooney
Buoyancy (floating) occurs, not because of the material of object, but by the
surrounding medium. Archimedes' principle states that an object is buoyed up
by a force equal to the weight of the volume of the DISPLACED MEDIUM. So an
aluminum foil "boat" floats on water, but sinks if the same foil is
"crumpled". And the "boat" does not float in air because the volume of air
displaced is insignificant compared to the weight of the "boat". The
engineering feat of making a "balloon"
strong enough to withstand atmospheric pressure is not practical, at least
at present.
Vince Calder
The concept is correct. If a sufficiently strong "container" could be made it
would be lighter when evacuated than when filled with helium. The forces to
collapse the evacuated container would be tremendous, however, with about 14.7
pounds of force per square inch of surface when the container is at sea level.
Greg Bradburn
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Update: June 2012
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