Open Systems

[MileHigh]

Tinman:

I didn't read your thread, I was only chiming in on the thermodynamics stuff.

Your concept about the amount of heat in the gas remaining the same, and instead it just gets hotter when the volume is smaller, is flawed.

Let's take the same number of moles of gas in two different sized containers.  Let's say the temperature is the same for both gasses.  So we know that P1V1 = P2V2.  So if the second container is 1/2 the size the pressure is double.

How much heat is in each container under these conditions?  Well the mass is the same and the temperature is the same.  You have to measure the amount of heat relative to another temperature, so let's use absolute zero.  You have the same type of gas, and you have the same mass.  So for both cases you have the same amount of heat energy in the gas, by mass.  We are talking about the heat energy that you could extract from the mass of gas in each container.  In other words it's like asking now much ice each container could melt if you put each container in a bucket of ice.  It would be the same.  So your argument about the temperature going up because the same amount of heat has to go into a smaller container is false.  Both containers contain the same amount of heat and they are at the same temperature.  And that's a big clue right there.

Here is a good comparison:  Imagine instead of gas, we use a piece of sponge.  So we do the test in a vacuum.   The sponge gets compressed down to 1/2 size to fit in the smaller container.  Did the temperature of the sponge go up?  The answer is no.  Did the amount of heat in the sponge go up?  The answer is no.  So you compressed the heat of the sponge into a smaller volume and the amount of heat stayed the same and the temperature stayed the same.  The energy went somewhere else, the sponge also acts like a spring and the energy is stored in the compressed spring.

But in a gas there is no mechanical equivalent of a spring.  The only way the gas can store the work done on it by the piston is by having he molecules move faster and store the piston energy as kinetic energy.  Higher kinetic energy equals higher temperature.

Let's so back to the one-meter transparent cylinder with the plunger.  If you compress the piston very rapidly and then let go, the piston will return back to the start position.

So what happens if you compress the piston and then wait three hours for the compressed gas to cool down and then you release the piston?

The answer is that the piston will not make it back to the start position.  It will push back right away when you release it and then say travel 3/4 of the way to the start position.  Then it will almost stop dead.  Then as you watch it, it will slowly creep back over time to get back to the start position.  It might take 10 or 15 minutes before the piston finally makes it back to the start position.

MileHigh

[tinman]

Tinman:

I didn't read your thread, I was only chiming in on the thermodynamics stuff.

Your concept about the amount of heat in the gas remaining the same, and instead it just gets hotter when the volume is smaller, is flawed.

Let's take the same number of moles of gas in two different sized containers.  Let's say the temperature is the same for both gasses.  So we know that P1V1 = P2V2.  So if the second container is 1/2 the size the pressure is double.

How much heat is in each container under these conditions?  Well the mass is the same and the temperature is the same.  You have to measure the amount of heat relative to another temperature, so let's use absolute zero.  You have the same type of gas, and you have the same mass.  So for both cases you have the same amount of heat energy in the gas, by mass.  We are talking about the heat energy that you could extract from the mass of gas in each container.  In other words it's like asking now much ice each container could melt if you put each container in a bucket of ice.  It would be the same.  So your argument about the temperature going up because the same amount of heat has to go into a smaller container is false.  Both containers contain the same amount of heat and they are at the same temperature.  And that's a big clue right there.

Here is a good comparison:  Imagine instead of gas, we use a piece of sponge.  So we do the test in a vacuum.   The sponge gets compressed down to 1/2 size to fit in the smaller container.  Did the temperature of the sponge go up?  The answer is no.  Did the amount of heat in the sponge go up?  The answer is no.  So you compressed the heat of the sponge into a smaller volume and the amount of heat stayed the same and the temperature stayed the same.  The energy went somewhere else, the sponge also acts like a spring and the energy is stored in the compressed spring.

But in a gas there is no mechanical equivalent of a spring.  The only way the gas can store the work done on it by the piston is by having he molecules move faster and store the piston energy as kinetic energy.  Higher kinetic energy equals higher temperature.

Let's so back to the one-meter transparent cylinder with the plunger.  If you compress the piston very rapidly and then let go, the piston will return back to the start position.

So what happens if you compress the piston and then wait three hours for the compressed gas to cool down and then you release the piston?

The answer is that the piston will not make it back to the start position.  It will push back right away when you release it and then say travel 3/4 of the way to the start position.  Then it will almost stop dead.  Then as you watch it, it will slowly creep back over time to get back to the start position.  It might take 10 or 15 minutes before the piston finally makes it back to the start position.

MileHigh

It is your logic that is flawed MH, as you just disipated the heat energy into the enviroment. Push the piston in, and the heat is confined into a smaller volume-then let the piston go straight away, and it will return to the starting position.

[tinman]

@MH&MarkE
If we are to use your ideal gas laws, then in all fairness we are to use an ideal piston/cylinder for this experiment.
The pistion is in the start position-there is no friction in this cylinder/pistion setup. Put a pressure of 5psi gage pressure into the cylender. The cylinder has an ideal insulation. Now push the piston 3/4 of the way down the cylender. The pressure rises, and the temp rises. Leave in the compressed state for 1minute, then let the piston return to the starting point. The pressure will once again be 5psi-all energy has been returned and yet there was a temperature increase.
What caused the temperature increase if all the energy was returned?

[LibreEnergia]

It is your logic that is flawed MH, as you just disipated the heat energy into the enviroment. Push the piston in, and the heat is confined into a smaller volume-then let the piston go straight away, and it will return to the starting position.

This is only true for an adiabatic process, where no thermal energy enters or leaves the system.

Push the piston in and it heats up. Work is converted to heat. The temperature in the system is raised and P1V1 != P2V2.

If you then held the volume constant and allow all the extra heat that was converted from work to transfer back to the environment THEN  P1V1 == P2V2 and the internal energy will also be same as the starting condition.

I don't know why you don't seem to be able to grasp such a simple concept, but by refusing to accept it is shows any other conclusions you have come to about your system to be false.

[MarkE]

@MH&MarkE
If we are to use your ideal gas laws, then in all fairness we are to use an ideal piston/cylinder for this experiment.

Sure, I offered that we ignore friction loss before.  That leaves two possibilities:  The piston movement performs external work, removing more than as much energy in the process, or it does not external work.  Either way your premise fails.

The pistion is in the start position-there is no friction in this cylinder/pistion setup. Put a pressure of 5psi gage pressure into the cylender. The cylinder has an ideal insulation. Now push the piston 3/4 of the way down the cylender. The pressure rises, and the temp rises. Leave in the compressed state for 1minute, then let the piston return to the starting point. The pressure will once again be 5psi-all energy has been returned and yet there was a temperature increase.
What caused the temperature increase if all the energy was returned?

Well if your cylinder is insulated, then your description is not realizable in this world, friction or no friction in the cylinder.  You input energy into the system, and that energy shows up as a higher T in n*R*T.  Unsurprisingly, it also shows up as a higher P*V product.  When you let the piston move out unopposed to its original position, then to your great surprise you would find that the pressure is greater than the 5psi gage that you had at the start.