Best answer so far. Thanx.
--
Cheerio,
Graham
> -----Original Message-----
> From: [log in to unmask] [mailto:[log in to unmask]]
> Sent: 27 April 2005 17:44
> To: [log in to unmask]
> Subject: Re: Helium question
>
> Quoting "Balin Graham (F81606) Harewood CO12 5AD"
> <[log in to unmask]> on Wed 27 Apr 2005 10:05:22 BST:
>
> > Why does it? If the helium atom can move at > escape velocity, how do
> they
> > end up, even in very low density, in a particular region of the
> atmosphere,
> > unless held there by gravity?
> >
> > --
> > Cheerio,
> >
> > Graham
>
> The helium atoms have a distribution of velocities; some of them will
> have
> velocities which are less than the escape velocity. To escape, they must
> acquire a velocity greater than the escape velocity by colliding with
> other
> atoms (or molecules) whoch give them a greater velocity, then get going in
> the
> right direction (up!) and then avid colliding with any other atom/molecule
> until they have made their escape. Off hand I cannot remember what the
> mean
> free path for an atom in the earth's atmosphere is, but it is not very
> far. So
> all of this takes time. AT the same time, there is a steady dribble of
> Helium
> atoms either released within the earth's atmosphere (some of it from party
> balloons) and some coming in from space. The result is that there is a
> dynamic
> equilibrium between the loss of helium from the atmosphere, and the rate
> at
> which new atoms appear.
>
> Mike Wells
>
>
>
>
> > > -----Original Message-----
> > > From: Adrian Midgley [mailto:[log in to unmask]]
> > > Sent: 25 April 2005 22:18
> > > To: [log in to unmask]
> > > Subject: Re: Helium question
> > >
> > > On Mon, 2005-04-25 at 18:10, Balin Graham (F81606) Harewood CO12 5AD
> > > wrote:
> > > > That would not explain hydrogen and helium already being components
> of
> > > the
> > > > exosphere...
> > >
> > >
> > >
> > > > are only
> > > > present at extremely low densities.
> > >
> > > explains it.
> >
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