🚀 go-pugleaf

I've been out of physics since 1987, so don't be at all surprised if I
make some dumb mistakes in this post. But I'm sure _someone's_ got it
wrong, so here goes: 

In article <1dbs8vINNau0@agate.berkeley.edu>, aephraim@physics.Berkeley.EDU (Aephraim M. Steinberg) writes:
> In article <1992Nov5.130316.28438@kth.se> e92_anh@vaxkab.lne.kth.se () writes:
>>This will probably sound like a silly question to a lot of you, but WHY
>>does a helium-filled balloon rise upward? Is it because of the
>>difference in air-pressure at the balloon's top and bottom, causing the
>>resulting force to press it upward?
> 
> Yes, amazingly enough.
> 
>>Thus, consider this: If the air surrounding the balloon had exactly the
>>same density on all sides, then the balloon would not move (I think).
> 
> No, actually it would sink, I think, because there is still the effect
> of gravity.

Surely, surely not! We're talking about a standard helium-filled balloon,
aren't we, which is less dense than air. So consider a helium balloon in a
closed room with constant (to a close-enough approximation) pressure
throughout, so that the balloon's volume is independent of its position.
If the balloon rises, then some air falls to replace it, and the
gravitational potential energy of the system has decreased (because the
air is more massive than the balloon). Similarly, for the balloon to fall,
the GPE of the system would have to increase. So given that when a balloon
rises the released GPE is dissipated as heat in the system, the balloon's
ascent is thermodynamically irreversible (and can be driven e.g by thermal
motion of air molecules). Therefore, it seems to me that in the case of
uniform pressure, the gravitational field will still cause the balloon to
rise. 

The real-life case where there's a pressure gradient is trickier, because
one has to justify the claim that the air that falls is more massive than
the balloon allowing for them both changing volume during the process. But
consider a rigid hollow ball, with vacuum inside. This could function as a
balloon, being less massive than the air it displaces, and it won't change
volume -- the above account seems to work again, suggesting that it'll
work for everyday balloons (which don't seem to change volume much at all
in going from, say, ground level to 100ft). 

In sum, I'd have thought that the explanation for why balloons rise is the
same as that for why submarines rise when they blow their ballast tanks --
they're less dense than the surrounding medium, so when they rise the
medium sinks and gravitational potential energy is lost as heat.

Of course, if I've got it all wrong please don't hesitate to tell me so.

Andrew.
-- 
Andrew McRae                       Internet: mcrae@husc.harvard.edu
                                     BITnet: mcrae@HUSC