Subject: Helium lifting ability
From: jim rozen
Date: 22/07/03 19:56 GMT Daylight Time
Message-id:

In article ,
says...

The reason why a balloon floats in still air is because of the
pressure gradient between the top and the bottom of the balloon.

I really don't understand this. Are you talking about
the pressure gradient in the atmosphere?

Yes. Which by definition means that there is also a pressure gradient between
the top and bottom of the balloon, albeit a very minute one. The essential bit
though is only that there be a density gradient. The fact that this is caused
primarily by pressure is not relevant. It could also be caused by temperature.
The balloon rises to a point at which it displaces exactly its own mass. If it
were to rise higher it would displace less than its mass and so sink again. The
reverse would happen if it were to sink from the equilibrium point.


If that were the case then even a ballon filled with air
would float, because it sees the same gradient as one filled
with helium. But air-filled ballons don't float
because the amount of lift from that effect is very,
very tiny.

I'm not sure what you are taking the term "pressure gradient" to be but I think
you are misunderstanding it. It simply means there is a static density gradient
and the original point might have been better made using this term. There is no
way a balloon filled with air can float in air of the same density unless the
material of the balloon is also less dense than air.


Things float because the volume of fluid they displace,
weighs less than their own mass. This is true even in
incompressible fluids, like water for example.

Exactly. But in incompressible fluids objects generally either sink to the
bottom or float to the top. In compressible ones they can find an equilibrium
point from which they rise no higher. Strictly speaking even the tiny amount of
compressibility in liquids means that there is a small density gradient and a
possible equilibrium position for a submerged object but it's a very unstable
equilibrium and requires that the density of the object falls somewhere within
the tiny range of densities of the liquid between the top and bottom.


Dave Baker - Puma Race Engines (www.pumaracing.co.uk)
"How's life Norm?"
"Not for the squeamish, Coach" (Cheers, 1982)