On Wed, 31 Dec 2003 16:51:21 -0800, Loren Amelang
wrote:

On Mon, 29 Dec 2003 23:04:25 GMT, ATP wrote:

I don't understand why this is not clear to everyone here. Which has more
energy-one liter of 200 psi gas or two liters of 100 psi gas? Can you do
any work going from one state to the other? In one direction you can,

True. There is one catch in this example (and a lot of similar ones in this
thread) though... If you start with one liter of 200 psi gas, and let it
expand through an air motor into a receiving tank until you have two liters
of 100 psi gas, you can extract some work, but you don't end up with those
two liters in the receiving tank.

To accomplish this task you must use a one-liter receiving tank. At the end
of the demonstration, one liter is still in the original source tank,
because the flow stopped when the receiving tank reached 100 psi. Putting
that second liter of air into the source tank cost you more than putting
the first liter in, and more than putting a constant 100 psi against the
motor until the receiving tank was at 100 psi.

But the additional energy invested in the first case isn't lost. At
the end of the first experiment you still have 2 liters of 100 PSI
air remaining at the end of the experiment. In the second case you
have only 1 liter of 100 PSI air remaining. Further, if you
discharged thru the motor from the 200 PSI tank to the receiving tank
you got twice as much work because the intial delta-P was 200 PSI
rather than 100 since the receiver starts at 0. Net result: you
get 4 times as much work: twice as much while discharging to 100 PSI
and twice much afterwards as you discharge 2 liters (rather than 1)
from 100 PSI to 0.