Open Systems

[minnie]

       Trouble is Tinman Mark doesn't very often get things badly wrong.
  Refer to "mathematical analysis of an ideal ZED" started by Mondrasek.
  Webby pulled out all the stops with that one, so did Mondrasek.
               John.

[MarkE]

Once again incorrect.
The PV product dose not fall. P decreases-GV remains the same, and the CA/V increases.
Once again you are missleading people with incorrect information. A liter of water is a liter of water regardless of the size of the bucket it is in.

Tinman when a gas volume does work in the course of expanding then the P*V product at the end of the stroke is always less than the P*V product at the start of the stroke less the amount of work done.

A liter of water is a liter of (essentially) incompressible liquid and in the context of a heat engine cannot be usefully compared to a compressible fluid such as a mixture of hydrogen and oxygen gasses.

You are free to choose one of the two real possibilities here:  1) That the piston moves without doing external work ( we will make the artificial stipulation that there is no friction loss ), in which case the P*V product does not change, but also no external work is performed by the piston.  Or 2) We can perform work with the piston.  In that second case, we transfer energy out of the gas into the external load.  The P*V product IE the energy in the gas at the end of the stroke is less than the P*V product IE the energy in the gas before the stroke.  The difference is ideally equal to the external work performed.  In all real cases, the difference is greater than the external work performed.

Go read this short and very accessible article on the subject:  http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heaeng.html

[MileHigh]

Tinman:

I am just going to jump in here as an outsider to give you some perspective.  I confess I sucked at thermodynamics and all the gas law business in my chemistry classes.  But I think we all intuitively have a sense for PV = nRT.  But where I think you are 'missing it' is the concept of working being done on a gas.  So you have the ideal gas law, Boyle's las, and Charles' law and other stuff.

I get the sense that you are scoffing at that stuff and playing the 'laws' card.  There is always a problem there - the bloody laws are based on common sense, real life, and real life experimentation.  You don't get an 'exit card' for that kind of stuff.  You are putting up a defensive barrier and seemingingly refusing to accept new ideas.  This is very similar to the recent big debate about magnetic fields.

Let me give you my bare-bones simple analogy.

You have a cylinder of air with a piston on one end.  The air inside the piston is at one atmosphere and room temperature.  Imagine some kind of bench apparatus, a clear cylinder one meter long and 20 cm in diameter with a nice plunger mechanism.  You have some gear you can turn by hand to push the piston forward.

Here is what I think you think:  You crank the gear my hand and the cylinder moves half-way up the piston.  Then you think you have gas that is one-half the volume and the pressure has increased and the temperature of the gas still at room temperature.

But that's not the case!!  You cranked the gear and you ended up doing work on the gas.  That work has to go somewhere, where did it go?  The answer is that not only did the gas pressure go up higher than you think as the volume went down, but the temperature of the gas also went up.  The work you did ended up heating up the gas and resulting in 'extra' high pressure and 'extra' temperature.  That's the way it works out.

So supposing you now wait for three hours.  Well, the hot gas is hotter than the room, so eventually it cools back down to room temperature as air convection currents take the excess heat away.   So obviously while the gas was cooling down, the pressure was also lowering because of PV = nRT.

And then finally, what happens if you let the cylinder move back to to the original position?   Now the gas is doing work on the outside world.  If the gas is exporting energy it comes from the pressure, volume, and the temperature.  If the piston actually makes it back to the original starting position, then the gas has to end up colder than the ambient temperature.  Kind of like to get the cylinder back to the starting point, the "gas runs out of 'gas'" and since it is pooped out, it has to give up heat energy to get the cylinder to the starting position and therefore the temperature of the gas goes down.

In essence, that's why you see frost on a big air tank that is emptying.  If you go to the computer store and you buy a can of compressed air and start to use it, it gets really cold in your hand as you use it.

So just some simple non-technical explanations for what it really means when you compress gas with a piston, or the the gas itself moves a piston.

MileHigh

[MileHigh]

Tinman:

Then there is kind of really basic explanation based on simple common sense that ignores all of the laws and top-level energy analysis.

What's a gas?  It's like billiard balls bouncing around forever that never lose energy.  When two molecules hit they bounce off each other and no energy is lost.  Imagine magic ping-pong balls for the air molecules.

So what happens as the cylinder moves forward?  The cylinder moving forward is like a giant swinging ping pong bat.  As it moves forward, the ping pong balls pick up energy from the bat and start moving faster.  Just like hitting a ping pong ball with a bat in real life.

So you end up with the same amount of ping-pong balls in a smaller space, and the ping-pong balls are now more 'energetic' and moving faster.  So what's pressure?   Pressure is the average number of ping pong balls hitting a small area per second.  Obviously the pressure goes up.   So what's temperature?  Temperature is directly related to molecular motion.  Faster and more vigorously moving ping pong balls, the higher the temperature.

Now, if you imagine the cylinder moving backwards.  You can think it through.  The ping pong ball hitting a wall moving away, bounces back with less kinetic energy after the bounce as compared to before the bounce.   So as the 'ping pong bat' moves away, it's almost like it is 'sucking the energy' out of the ping pong balls.  Or you can say the ping pong balls are coughing up kinetic energy to make the pistion move backwards.  So the pressure goes down and the temperature goes down.

What about when a liquid evaporates?   That's simply a liquid ping pong ball getting 'hit out of the park' and taking to the air.   There was a mini ping pong bat in the liquid that had to give up energy to 'whack' the ping pong ball out of the park.   So as a liquid evaporates it's temperature goes down.

So you can see on a microscopic level that when the piston moves forward, that work goes into the gas.  When it moves backward, the gas supplies the work required to move the piston backwards.

MileHigh

[pomodoro]

Profitis, who gets taxed?  Well, the extra energy required to compress the gas comes from power supply. The piston work comes from there, the power supply. Unless someone has proven that pressure has little effect on the power required from electrolyis.  I might be blind but how does the environment give energy to this system?