On 2/10/2011 9:28 PM, Jed Rothwell wrote:
<[email protected] <mailto:[email protected]>> wrote:
What you wrote is true when there is liquid water and steam
together in a container - the combination cannot be heated to a
temperature higher than 100 deg C without raising the pressure.
However once all the liquid has turned to gas there is no longer
any limit to what temperature it can be raised to until the
molecules dissociate into their component elements.
What would keep the molecules in the kettle, assuming the top or spout
is open?
Well the pressure of the atmosphere into which the gas is exiting will
prevent the gas pressure from declining below one atmosphere - allowing
the gas to attain any temperature you chose to heat it to and becoming
less and less dense until it is indeed close to that of a vacuum.
What would prevent the gas density from declining indefinitely until
it is close to a vacuum?
Here we are speaking of two different quantities that relate to a vacuum
- pressure and density. Hot air or helium in a balloon is much less
*dense* than the atmosphere, but is never the less at the same
*pressure* as the atmosphere. Likewise one could imagine *very* hot air
or steam being almost zero *density* (ie close to the density of a
vacuum), but still at the same *pressure* as the atmosphere (ie much
higher pressure than a vacuum).
My understanding is that the temperature does not rise as long as the
volume is free to expand.
If you allow a gas to expand indefinitely into a vacuum pressure then
you are right, it would cool down again until it condensed. But this
does require a mechanism to extract its energy as it expands - such as
getting it to push on a piston and slowly moving the piston out. If you
simply let the gas leak through some sort of throttling valve (like a
long thin kettle spout) into a low pressure or vacuum, then as I recall,
it usually takes a step down in temperature, but once on the low
pressure or vacuum side there is nothing to reduce its temperature
further. In fact if my rusty memory serves, some gases under some
conditions can exit a throttling valve hotter (in terms of temperature -
ie the individual molecules have more kinetic energy) than they were
when on the compressed side - this being because the work done by the
gas traveling through the expander ends up as frictional heat in the
throttling material which gets hot and makes the gas hot also. Sorry my
thermodynamics knowledge is so poor - it always seemed a bit too
steam-aged to fire my imagination and get me to learn it!
