Re: 'The' or 'a'?


Mark Brader wrote:
> George Waginger asks about:
> > > The gas can be expanded up to *a volume where its
> > > pressure virtually reaches the environmental pressure.
> > >
> > > The gas can be expanded up to *the volume where its
> > > pressure virtually reaches the environmental pressure.
>
> I agree with those who said "correct either way". The first wording
> allows for the possibility that there is more than one such volume,
> but there isn't, so the effective meaning is the same.
>
> John O'Flaherty:
> > What do you mean by 'the gas can be expanded'? If you give it room, it
> > expands by itself, to fill all the available room. ...
>
> I take it that the gas is in a closed container whose volume can be
> externally controlled -- for example, by withdrawing a piston from a
> cylinder. Or it could be being released into a larger container.

Then the right way to say it would be 'the gas can be allowed to
expand', I think. It still leaves the question of what ambient the gas
is at a lower pressure than. The only way it could 'virtually reach'
some pressure, not reaching it, would be in a dynamic case where the
expansion of the containment catches the gas by surprise (it's usually
pretty alert). Then, some of the gas might be at a higher pressure
because of its inertia, compared to the pressure right at the piston
that's being withdrawn. Unless the system he's talking about is
something really peculiar.
--
john

.

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